Ventilation, Mechanical

Background

 The Drinker and Shaw tank-type ventilator of 1929 was one of the first negative-pressure machines widely used for mechanical ventilation. Better known as the iron lung, this metal cylinder completely engulfed the patient up to the neck. A vacuum pump created negative pressure in the chamber, which resulted in expansion of the patient's chest. This change in chest geometry reduced the intrapulmonary pressure and allowed ambient air to flow into the patient's lungs. When the vacuum was terminated, the negative pressure applied to the chest dropped to zero, and the elastic recoil of the chest and lungs permitted passive exhalation

Ventilation of the patient was accomplished without the placement of a tracheostomy or an endotracheal tube. Nevertheless, this mode of ventilation was cumbersome and led to patient discomfort. In addition, it limited access to the patient by health care providers. Because the negative pressure created in the chamber was exerted on the abdomen as well as the chest, the cardiac output tended to decrease from pooling of venous blood in the lower torso.

Today, negative-pressure ventilation is used in only a few situations. The cuirass, or shell unit, allows negative pressure to be applied only to the patient's chest by using a combination of a form-fitted shell and a soft bladder. It provides a suitable and attractive option for patients with neuromuscular disorders, especially those with residual muscular function, because it does not require a tracheostomy with its inherent problems.

Concepts that the military developed during World War II to deliver oxygen and gas volume to fighter pilots operating at high altitude were incorporated into the design of the modern positive-pressure ventilator. With the development of safe endotracheal tubes with high-volume, low-pressure cuffs, positive-pressure ventilation replaced the iron lung.

Intensive use of positive-pressure mechanical ventilation gained momentum during the polio epidemic in Scandinavia and the United States in the early 1950s. In Copenhagen, the patient with polio and respiratory paralysis who was supported by manually forcing 50% oxygen through a tracheostomy had a reduced mortality rate. However, this heroic intervention required the continuous activity of 1400 medical students recruited from the universities. The overwhelming manpower needed, coupled with a decrease in mortality rate from 80% to 25%, led to the adaptation of the positive-pressure machines used in the operating room for use in the ICU.

Positive-pressure ventilation means that airway pressure is applied at the patient's airway through an endotracheal or tracheostomy tube. The positive nature of the pressure causes the gas to flow into the lungs until the ventilator breath is terminated. As the airway pressure drops to zero, elastic recoil of the chest accomplishes passive exhalation by pushing the tidal volume out.

Classifications of Positive-Pressure Ventilators

Modern ventilators are classified by their method of cycling from the inspiratory phase to the expiratory phase. That is, they are named after that parameter that signals the termination of the positive-pressure inspiration cycle of the machine. The signal to terminate the inspiratory activity of the machine is either a preset volume (for a volume-cycled ventilator), a preset pressure limit (for a pressure-cycled ventilator), or a preset time factor (for a time-cycled ventilator).

Volume-cycled ventilation is the most common form of ventilator cycling used in adult medicine because it provides a consistent breath-to-breath tidal volume. Termination of the delivered breath is signaled when a set volume leaves the ventilator.

Indications for Mechanical Ventilation

Many factors affect the decision to begin mechanical ventilation. Because no mode of mechanical ventilation can cure a disease process, the patient should have a correctable underlying problem that can be resolved with the support of mechanical ventilation. This intervention should not be started without thoughtful consideration because intubation and positive-pressure ventilation are not without potentially harmful effects.

Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to sustain life. In addition, it is indicated as a measure to control ventilation in critically ill patients and as prophylaxis for impending collapse of other physiologic functions. Physiologic indications include respiratory or mechanical insufficiency and ineffective gas exchange.

Common indications for mechanical ventilation include the following:

• Bradypnea or apnea with respiratory arrest
• Acute lung injury and the acute respiratory distress syndrome
• Tachypnea (respiratory rate >30 breaths per minute)
• Vital capacity less than 15 mL/kg
• Minute ventilation greater than 10 L/min
• Arterial partial pressure of oxygen (PaO₂) with a supplemental fraction of inspired oxygen (FIO₂) of less than 55 mm Hg
• Alveolar-arterial gradient of oxygen tension (A-a DO₂) with 100% oxygenation of greater than 450 mm Hg
• Clinical deterioration
• Respiratory muscle fatigue
• Obtundation or coma
• Hypotension
• Acute partial pressure of carbon dioxide (PaCO₂) greater than 50 mm Hg with an arterial pH less than 7.25
• Neuromuscular disease

The trend of these values should influence clinical judgment. Increasing severity of illness should prompt the clinician to consider starting mechanical ventilation.

Initial Ventilator Settings

Mode of ventilation

After deciding to start positive-pressure ventilation with a volume-cycled ventilator, the clinician must now select the safest initial mode of machine operation.

In most circumstances, the initial mode of ventilation should be the assist-control mode, in which a tidal volume and rate are preset and guaranteed. The patient can affect the frequency and timing of the breaths. If the patient makes an inspiratory effort, the ventilator senses a decrease in the circuit pressure and delivers the preset tidal volume. In this way, the patient can dictate a comfortable respiratory pattern and may trigger additional machine-assisted breaths above the set rate. If the patient does not initiate inspiration, the ventilator automatically delivers the preset rate and tidal volume, ensuring minimum minute ventilation. In the assist-control mode, the work of breathing is reduced to the amount of inspiration needed to trigger the inspiratory cycle of the machine. This trigger is adjusted by setting the sensitivity of the machine to the degree of pressure decrease desired in the circuit

The pressure, volume, and flow to time waveforms for assist-control ventilation.

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The pressure, volume, and flow to time waveforms for assist-control ventilation.

Assist-control differs from controlled ventilation because the patient can trigger the ventilator to deliver a breath and, thereby, adjust their minute ventilation. In controlled ventilation, the patient receives only breaths initiated by the ventilator at the preset rate

The pressure, volume, and flow to time waveforms for controlled ventilation.

The pressure, volume, and flow to time waveforms for controlled ventilation.

Although the work of breathing is not eliminated, this mode gives the respiratory muscles the greatest amount of rest because the patient needs only to create enough negative pressure to trigger the machine. An added advantage is that the patient can achieve the required minute ventilation by triggering additional breaths above the set back-up rate.

In most cases, a minute ventilation that provides a reasonable pH based on the respiratory rate is determined by the patient's chemoreceptors and stretch receptors. The respiratory center in the central nervous system receives input from the chemical receptors (arterial blood gas tensions) and neural pathways that sense the mechanical work of breathing (mechanoreceptors). The respiratory rate and respiratory pattern are the result of input from these chemoreceptors and mechanical receptors, which allow the respiratory center to regulate gas exchange. In the assist-control mode, this process is accomplished with the minimum work of breathing.

A second possible advantage of this mode of mechanical ventilation is that cycling the ventilator into the inspiratory phase maintains normal ventilatory activity and, therefore, prevents atrophy of the respiratory muscles.

A potential disadvantage of the assist-control mode is respiratory alkalosis in a small subset of patients whose respiratory drive supersedes the chemoreceptors and mechanical receptors. Patients with a potential for alveolar hyperventilation and hypocapnia in the assist-control mode include those with end-stage liver disease, those in the hyperventilatory stage of sepsis, and those with head trauma. These conditions are typically identified with the first arterial blood gas results, and the assist-control mode of ventilation can then be changed to an alternate mode.

Another possible disadvantage is the potential for serial preset positive-pressure breathes to retard venous return to the right side of heart and to affect global cardiac output. Nevertheless, the assist-control mode may be the safest initial choice for mechanical ventilation. It may be switched to another option if hypotension or hypocarbia are evident from the first arterial blood gas results.

Tidal volume and rate

For a patient without preexisting lung disease, the tidal volume and rate are traditionally selected by using the 12-12 rule. A tidal volume of 12 mL for each kilogram of lean body weight is programmed to be delivered 12 times a minute in the assist-control mode.

For patients with chronic obstructive pulmonary disease (COPD), the tidal volume and rate are slightly reduced to the 10-10 rule to prevent overinflation and hyperventilation. A tidal volume of 10 mL/kg lean body weight is delivered 10 times a minute in the assist-control mode.

In acute respiratory distress syndrome (ARDS), the lungs may function best and volutrauma  is minimized with low tidal volumes of 6-8 mL/kg. Tidal volumes are preset at 6-8 mL/kg of lean body weight in the assist-control mode. This ventilatory strategy is called lung-protective ventilation. These lowered volumes may lead to slight hypercarbia. An elevated PCO₂ is typically recognized and accepted without correction, leading to the term permissive hypercapnia. However, the degree of respiratory acidosis allowable is a pH not less than 7.25. The respiratory rate of the ventilator may need to be adjusted upward to increase the minute ventilation lost by using smaller tidal volumes.

Double-checking the selected tidal volume

After a tidal volume is selected, the peak airway pressure necessary to deliver a single breath should be determined. As the tidal volume increases, so does the pressure required to force that volume into the lung. Persistent breath-to-breath peak pressures greater than 45 cm water are a risk factor for barotrauma . The tidal volume suggested by the above rules may need to be decreased in some patients to keep the peak airway pressure less than 45 cm water

The components of mechanical ventilation inflation pressures. Paw is airway pressure, PIP is peak airway pressure, Pplat is plateau pressure.

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The components of mechanical ventilation inflation pressures. Paw is airway pressure, PIP is peak airway pressure, Pplat is plateau pressure.

Some researchers have suggested that plateau pressures should be monitored as a means to prevent barotrauma in the patient with ARDS. Plateau pressures are measured at the end of the inspiratory phase of a ventilator-cycled tidal volume. The ventilator is programmed not to allow expiratory airflow at the end of the inspiration for a set time, typically half a second. The pressure measured to maintain this lack of expiratory airflow is the plateau pressure. Barotrauma is minimized when the plateau pressure is maintained at less than 30-35 cm water  Monitoring the peak and plateau pressures allows physicians to make clinical judgments on the progress of their patient

The effects of increased airway resistance (A) and decreased respiratory system compliance on the pressure-time waveform.

The effects of increased airway resistance (A) and decreased respiratory system compliance on the pressure-time waveform.

Sighs

Because a spontaneously breathing individual typically sighs 6-8 times each hour to prevent microatelectasis, some investigators once recommended that periodic machine breaths that were 1.5-2 times the preset tidal volume be given 6-8 times per hour. However, the peak pressure often needed to deliver such a volume was high enough to predispose the patient to barotrauma. At present, accounting for sighs is not recommended if the patient is receiving tidal volumes of 10-12 mL/kg or if the patient requires positive end-expiratory pressure (PEEP). When a low tidal volume is used, sighs are preset at 1.5-2 times the tidal volume and delivered 6-8 times an hour if the peak and plateau pressures are within acceptable limits.

Initial FIO₂

The highest priority at the start of mechanical ventilation is providing effective oxygenation. For the patient's safety after intubation, the FIO₂ should always be set at 100% until adequate arterial oxygenation is documented. A short period with an FIO₂ of 100% is not dangerous to the patient receiving mechanical ventilation and offers the clinician several advantages. First, an FIO₂ of 100% protects the patient against hypoxemia if unrecognized problems occur as a result of the intubation procedure. Second, using the PaO₂ measured with an FIO₂ of 100%, the clinician can easily calculate the next desired FIO₂ and quickly estimate the shunt fraction.

The degree of shunt with 100% FIO₂ can be estimated by applying this general rule: The measured PaO₂ is subtracted from 700 mm Hg. For each difference of 100 mm Hg, the shunt is 5%. A shunt of 25% should prompt the clinician to consider the use of PEEP.

Inadequate oxygenation despite the administration of 100% oxygen should lead to a search for complications of endotracheal intubation (eg, mainstem intubation) or positive-pressure breathing (pneumothorax). If such complications are not present, PEEP is needed to treat the intrapulmonary shunt pathology. Because only a few disease processes can create an intrapulmonary shunt, a clinically significant estimated shunt should narrow the potential source of hypoxemia to the following conditions:

• Alveolar collapse - Major atelectasis
• Alveolar filling with something other than gas - Lobar pneumonia
• Water and protein - ARDS
• Water - Congestive heart failure
• Blood - Hemorrhage

Positive end-expiratory pressure

PEEP is a mode of therapy used in conjunction with mechanical ventilation. At the end of mechanical or spontaneous exhalation, PEEP maintains the patient's airway pressure above the atmospheric level by exerting pressure that opposes passive emptying of the lung. This pressure is typically achieved by maintaining a positive pressure flow at the end of exhalation. This pressure is measured in centimeters of water.

PEEP therapy can be effective when used in patients with a diffuse lung disease that results in an acute decrease in functional residual capacity (FRC), which is the volume of gas that remains in the lung at the end of a normal expiration. FRC is determined by primarily the elastic characteristics of the lung and chest wall. In many pulmonary diseases, FRC is reduced because of the collapse of the unstable alveoli. This reduction in lung volume decreases the surface area available for gas exchange and results in intrapulmonary shunting (unoxygenated blood returning to the left side of the heart). If FRC is not restored, a high concentration of inspired oxygen may be required to maintain the arterial oxygen content of the blood in an acceptable range.

Applying PEEP increases alveolar pressure and alveolar volume. The increased lung volume increases the surface area by reopening and stabilizing collapsed or unstable alveoli. This splinting, or propping open, of the alveoli with positive pressure improves the ventilation-perfusion match, reducing the shunt effect.

After a true shunt is modified to a ventilation-perfusion mismatch with PEEP, lowered concentrations of oxygen can be used to maintain an adequate PaO₂. PEEP therapy may also be effective in improving lung compliance. When FRC and lung compliance are decreased, additional energy and volume are required to inflate the lung. By applying PEEP, the lung volume at the end of exhalation is increased. The already partially inflated lung requires less volume and energy than before for full inflation.

When used to treat patients with a diffuse lung disease, PEEP should improve compliance, decrease dead space, and decrease the intrapulmonary shunt effect. The most important benefit of the use of PEEP is that it enables the patient to maintain an adequate PaO₂ at a low and safe concentration of oxygen (<60%), reducing the risk of oxygen toxicity

Because PEEP is not a benign mode of therapy and because it can lead to serious hemodynamic consequences, the ventilator operator should have a definite indication to use it. The addition of external PEEP is typically justified when a PaO₂ of 60 mm Hg cannot be achieved with an FIO₂ of 60% or if the estimated initial shunt fraction is greater than 25%. No evidence supports adding external PEEP during initial setup of the ventilator to satisfy misguided attempts to supply prophylactic PEEP or physiologic PEEP.

Many clinicians use the least-PEEP philosophy, which recommends using the lowest positive pressure that provides an adequate PaO₂ with a safe FIO₂. Another manner of selecting the optimal PEEP is based on identifying the low inflection point on the volume-pressure curve generated breath to breath by using modern mechanical ventilators. PEEP should be set 1-2 cm of water pressure above this measured low inflection point to obtain the optimal PEEP

Because PEEP basically resets the baseline of the pressure-volume curve, the peak and plateau pressures will be affected. The clinician should pay close attention to the status of these pressure measurements

Determination of the lower inflection point to estimate the best (optimal) positive end-expiratory pressure (PEEP) from the pressure-volume hysteresis curve.

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Determination of the lower inflection point to estimate the best (optimal) positive end-expiratory pressure (PEEP) from the pressure-volume hysteresis curve.

Summary of initial ventilator setup

Initial settings for ventilation may be summarized as follows:

• Assist-control mode
• Tidal volume set depending on lung status  
  • Normal = 12 mL/kg ideal body weight
  • COPD = 10 mL/kg ideal body weight
  • ARDS = 6-8 mL/kg ideal body weight
• Rate of 10-12 breaths per minute
• FIO₂ of 100%
• Sighs rarely needed
• PEEP only as indicated after first arterial blood gas determination, ie, shunt greater than 25%
• Inability to oxygenate with an FIO₂ less than 60%

Adjustments and Withdrawal

Prone positioning

Prone positioning has been used in patients with ARDS and severe hypoxia and improves FRC, postural drainage of secretions, and ventilation-perfusion matching. Moving the intubated patient from the supine position to the prone position requires a coordinated effort from the nursing staff, respiratory therapists, and physicians to prevent inadvertent extubation or loss of various lines and tubes. Prone positioning may improve oxygenation in greater than 50% of such patients, but no survival benefit has been documented.

Sedation, protocols, and prophylaxis

Most patients receiving mechanical ventilation need sedation given by means of continuous infusion or scheduled dosing to help with anxiety and psychological stress inherent with this intervention. Daily interruption of sedation, when clinically allowable, decreases the number of days of mechanical ventilation. This sedation holiday helps the patient become reoriented and prevents the unintended prolonged effects of sedation. Such interruptions also help in assessing the patient for the appropriateness of weaning and hasten the transition to spontaneous respiration.

Studies have demonstrated that protocols driven by respiratory therapy safely decrease the number of ventilator days. These protocols allow the respiratory therapists to begin spontaneous breathing trials (SBTs) when they consider the patient a candidate for weaning.

Elevating the head of the patient's bed by greater than 30° decreases the risk of ventilator-associated pneumonia (VAP). Likewise, rates of VAP can be decreased with implementation of GI prophylaxis with histamine-2 blocking agents or proton-pump inhibitors, as well as deep vein thrombosis prophylaxis. Each of these measures should be undertaken in all patients receiving mechanical ventilation unless a contraindication is present.

PEEP adjustment

A PEEP level of less than 10 cm water rarely causes hemodynamic problems in the absence of intravascular volume depletion. The cardiodepressant effects of PEEP are often minimized with judicious intravascular volume support or cardiac inotropic support. Although peak pressure is related to the development of barotrauma, arterial hypotension is related to the mean airway pressure that may decrease venous return to the heart or decrease right ventricular function.

A PEEP level greater than 10 cm water is generally an accepted indication to monitor cardiac output by using a Swan-Ganz catheter. However, if the patient remains clinically stable with an adequate urine output, then hemodynamic monitoring may not be necessary. When PEEP greater than 10 cm water is necessary, the left atrial filling pressure can be estimated after an adjustment is made for the effect of the PEEP on the transducer of the catheter. The equation commonly used is LAP = PCWP - (PEEP/3), where LAP is left atrial pressure and PCWP is pulmonary capillary wedge pressure.

Withdrawal of PEEP from a patient should not be attempted in most clinical situations until the patient has achieved satisfactory oxygenation with an FIO₂ of 40% or less. Formal weaning from PEEP is then undertaken by reducing the PEEP in 3- to 5-cm of water decrements while the hemoglobin-oxygen saturations are monitored. An unacceptable decrease in the hemoglobin-oxygen saturation should prompt the clinician to immediately reinstitute the last PEEP level that provided good hemoglobin-oxygen saturation.

When to withdraw mechanical ventilation

Weaning or, as some physicians prefer, "liberation from mechanical ventilation," is an important issue. Unnecessary delays in the withdrawal of mechanical ventilatory support increase the patient's risks for complications and increase the length of ICU stay and hospital costs. However, premature withdrawal from the ventilator can also be deleterious.

Weaning should be considered when the event that precipitated the patient's need for mechanical support is adequately addressed. Patients should be evaluated each day to determine if they are a candidate for weaning. Patients who may be able to support their own ventilation and oxygenation can often be recognized by assessing objective measurements or by asking the following questions:

• Is the process responsible for the patient's respiratory failure resolving or improving?
• Is the patient hemodynamically stable? Is the patient free of active cardiac ischemia or unstable arrhythmias, and vasopressor support absent or minimal?
• Is oxygenation adequate with a PaO₂ of greater than 60 mm Hg with an FiO₂ of less than 40% and a PEEP of less than 5 cm water?
• Are mental and neuromuscular statuses appropriate with the patient on minimal or no sedation? Does the patient have adequate strength of the respiratory muscles?
• Are the acid-base status and electrolyte status optimized?
• Is the patient afebrile?
• Are the patient's adrenal and thyroid functions adequate to allow for weaning?

Numerous weaning parameters can be used to help predict successful extubation. However, no weaning protocol is 100% accurate in predicting successful weaning and extubation. These weaning parameters must be tailored for each clinical scenario.

For instance, if the rapid, shallow breathing index (the respiratory rate/tidal volume, or frequency/tidal volume [f/Vt]) is less than 105, the patient is likely to be weaned from mechanical ventilation. The investigators who derived this number examined primarily middle-aged patients. However, data from follow-up studies of patients older than 70 years suggest that a slightly higher rapid, shallow breathing index of less than 130 may be acceptable.

These parameters give no insight into whether a patient can protect his or her airway or clear secretions. Clinical judgment and experience play a large role in the physician's decision to withdraw mechanical ventilatory support. If a patient cannot be extubated and/or if the results of the rapid, swallow breathing test are not satisfactory, the reason for the failure must be evaluated and treated.

Parameters commonly used to assess a patient's readiness to be weaned from mechanical ventilatory support include the following:

• Respiratory rate less than 25 breaths per minute
• Tidal volume greater than 5 mL/kg
• Vital capacity greater than 10 mL/k
• Minute ventilation less than 10 L/min
• PaO₂/FIO₂ greater than 200
• Shunt (Qs/Qt) less than 20%
• Negative inspiratory force (NIF) less than (more negative) -25 cm water
• f/Vt less than  105, or less than 130 in elderly patients

How to withdraw mechanical ventilation

Weaning from mechanical ventilation is intended to shift the work of breathing from the ventilator back to the patient over time. An issue separate from discontinuing ventilator support is determining if the patient can maintain his or her airway and be extubated safely. The weaning process must ensure the patient's safety while avoiding undue delay that might increase the risk of VAP.

The 3 general approaches to weaning are synchronized intermittent mandatory ventilation (SIMV), pressure-support ventilation (PSV), and an SBT.

In SIMV, breaths are either a mandatory ventilator-controlled breath or a spontaneous breath with or without pressure support. The original intent of SIMV was to let the patient's respiratory muscles rest during the mandatory breaths and to work during the spontaneous breaths  Weaning is accomplished by decreasing the number of mandatory breaths, gradually increasing the workload of the respiratory muscles. Weaning is typically done by 2 breaths every 1–2 hours. The patient's heart rate, respiratory rate, and oxygen saturation indicate his or her ability to accept the work of breathing.

The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV).

The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV).

Evidence now suggests that the respiratory muscles are not able to rest during the mandatory breaths and that this mode may actually result in muscle fatigue and prolonged mechanical ventilation. Findings from randomized trials suggest that SIMV weaning delays extubation compared with PSV and SBT and that it should not be the primary mode of weaning in most patients. However, SIMV weaning does ensure that the patient receives some ventilatory support, and it may be favored in institutions where the staffing level of respiratory therapists is not optimal.

In PSV weaning, all breaths are spontaneous and combined with enough pressure support to ensure that each breath is a reasonable tidal volume. The pressure support lowers the work of breathing for the patient. Weaning is performed by gradually decreasing the amount of pressure support and by transferring an increased proportion of the work to the patient. This transfer is continued until the pressure support approaches 5-6 cm water. When the patient can tolerate this level of ventilatory support, extubation is usually successful. Studies have demonstrated that PSV weaning reduces the number of days on mechanical ventilation compared with SIMV alone. PSV can be used in conjunction with SIMV when a patient is weaned from mechanical ventilation . The coupling of these 2 modes is an especially attractive option in frail patients with underlying chronic illnesses.

The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV) with pressure-support ventilation.

The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV) with pressure-support ventilation.

The preferred method of weaning is the SBT. This is an attempt to gauge how the patient might do if he or she is immediately removed from the ventilator. This method is also referred to as the sink-or-swim trial. The key is to withdraw ventilatory support while oxygenation is continued.

The simplest form of SBT is the T-piece trial. The patient is disconnected from the ventilator, and the endotracheal or tracheostomy tube is hooked to a flow-by oxygen system, usually from the wall. The transition from the ventilator tubing to the new tubing attached to the wall oxygen outlet requires extra work and patient monitoring by the respiratory therapist.

The same assessment can be made by using the continuous positive-airway pressure (CPAP) mode while the patient is still connected to the ventilator. This is a relatively common method of assessing the patient's ability to do the work of breathing by himself or herself. Variations on this theme include adding a small amount of pressure and using a CPAP of 5 cm water or a CPAP of 0 but with a PSV of 5-6 cm water to offset the resistance from the artificial airway. To the authors' knowledge, no controlled studies have shown any superiority in assessing the outcomes of weaning between these approaches.

In some studies, approximately 80% of patients receiving mechanical ventilation do not require prolong weaning. This observation explains why SBT is both useful and practical. This approach has had the most success with weaning in randomized controlled trials. Therefore, it is a preferred approach to removing patients from mechanical ventilation.

The SBT should last 30-90 minutes. At the end of the SBT, the patient should be evaluated for possible extubation, as his or her blood pressure, respiratory rate, heart rate, and gas exchange are also considered. An SBT should be performed only once a day. Several SBTs a day offer no benefit.

Complications of Mechanical Ventilation

Complications can occur at any stage of mechanical ventilation and are sometimes life threatening.

Complications of intubation

Complications that can occur during placement of an endotracheal tube include upper airway and nasal trauma, tooth avulsion, oral-pharyngeal laceration, laceration or hematoma of the vocal cords, tracheal laceration, perforation, hypoxemia, and intubation of the esophagus. Inadvertent intubation of the right mainstem bronchus is reported in 3-9% of all intubations in adults. Aspiration rates are 8–19% in intubations performed in adults without anesthesia. Sinusitis, tracheal necrosis or stenosis, glottic edema, and VAP may occur with prolonged use of endotracheal tubes.

Additionally, the following guidelines from the American Association for Respiratory Care may be helpful: . Additionally, Solsona et al reported that observation of intercostal retraction after adding dead space may help in the detection of susceptibility to extubation failure.

Ventilator-induced lung injury

With ventilator-induced lung injury, the alveolar epithelium is at risk for both barotrauma and volutrauma.

Barotrauma

Barotrauma refers to rupture of the alveolus with subsequent entry of air into the pleural space (pneumothorax) and/or the tracking or air along the vascular bundle to the mediastinum (pneumomediastinum). The true prevalence of barotrauma is difficult to establish, and reports suggest a rate  of 6-25%. Large tidal volumes and elevated peak inspiratory and plateau pressures are risk factors. Studies in patients with ARDS demonstrated that the severity of the underlying lung pathology is a better predictor of barotrauma than the observed peak inspiratory pressure. Even so, peak inspiratory pressures of less than 45 mm Hg and plateau pressures of less than 30-35 mm Hg are recommended.

The inspiratory-to-expiratory ratio can be adjusted by increasing the inspiratory flow rate, by decreasing the tidal volume, and by decreasing the ventilatory rate. Attention to the inspiratory-to-expiratory ratio is important to prevent barotrauma in patients with obstructive airway disease (eg, asthma, chronic obstructive pulmonary disease).

Also see Barotrauma and Mechanical Ventilation.

Volutrauma

Volutrauma refers to the local overdistention of normal alveoli. Volutrauma has gained recognition over the last 2 decades and is the impetus for the lung protection ventilation with low tidal volumes of 6–8 mL/kg. CT studies have demonstrated that ARDS has a heterogeneous pattern of lung involvement. Abnormal consolidated lung is dispersed within normal lung tissue. When a mechanical ventilation breath is forced into the patient, the positive pressure tends to follow the path of least resistance to the normal or relatively normal alveoli, potentially causing overdistention. This overdistention sets off an inflammatory cascade that augments or perpetuates the initial lung injury, causing additional damage to previously unaffected alveoli. The increased local inflammation lowers the patient's potential to recover from ARDS. The inflammatory cascade occurs locally and may augment the systemic inflammatory response as well.

Another aspect of volutrauma associated with positive ventilation is the shear force associated with the opening and closing effects on collapsible alveoli. This has also been linked to worsening the local inflammatory cascade. PEEP prevents the alveoli from totally collapsing at the end of exhalation and may be beneficial in preventing this type of injury. Since volutrauma was recognized, a lung-protective ventilation strategy is recommended in all patients with ARDS or acute lung injury.

Oxygen toxicity

Oxygen toxicity is a function of increased FIO₂ and its duration of use. Oxygen toxicity is due to the production of oxygen free radicals, such as superoxide anion, hydroxyl radical, and hydrogen peroxide. Oxygen toxicity can cause a variety of complications ranging from mild tracheobronchitis, absorptive atelectasis, and hypercarbia to diffuse alveolar damage that is indistinguishable from ARDS.

No consensus has been established for the level of FIO₂ required to cause oxygen toxicity, but this complication has been reported in patients given a maintenance FIO₂ of 50%. The clinician is encouraged to use the lowest FIO₂ that accomplishes the needed oxygenation.

The medical literature suggests that the clinician should attempt to attain an FIO₂ of 60% or less within the first 24 hours of mechanical ventilation. If necessary, PEEP should be considered a means to improve oxygenation while a safe FIO₂ is maintained. When PEEP is effective and not contraindicated because of hemodynamics, the patient can often be oxygenated while the risks of oxygen toxicity are limited.

Ventilator-associated pneumonia

VAP is a life-threatening complication with mortality rates of 33-50%. It is reported to occur in 10-25% of patients given mechanical ventilation. The risk of VAP is highest immediately after intubation. VAP is estimated to occur at a rate of 3% per day for the first 5 days, 2% per day for next 5 days, and 1% per day thereafter. VAP occurs more frequently in trauma, neurosurgical, or burn units than in respiratory units and medical ICUs.

VAP is defined as a new infection of the lung parenchyma that develops within 48 hours after intubation. The diagnosis can be challenging. VAP should be suspected when a new or changing pulmonary infiltrate in seen in conjunction with fever, leukocytosis, and purulent tracheobronchial secretions. However, many diseases can cause this clinical presentation. Examples include aspiration pneumonitis, atelectasis, pulmonary thromboembolism, drug reactions, pulmonary hemorrhage, and radiation-induced pneumonitis. Qualitative and quantitative cultures of protected brush and bronchoalveolar lavage specimens may help with the diagnosis, but the utility of these techniques is still debated.

Microorganisms implicated in VAP that occurs in the first 48 hours after intubation are flora of the upper airway, including Haemophilus influenza and Streptococcus pneumonia. After this early period, gram-negative bacilli such as Pseudomonas aeruginosa; Escherichia coli; and Acinetobacter, Proteus, and Klebsiella species predominate. Staphylococcus aureus, especially methicillin-resistant S aureus (MRSA), typically becomes a major infective agent after 7 days of intubation and mechanical ventilation. Most of the medical literature recommends initial therapy with broad-spectrum antibiotics that cover pathogens resistant to multiple drugs until the sensitivities of the causative organism are identified. Knowledge of organisms that cause VAP in the individual ICU and the pattern of antibiotic resistance is imperative. Choices of antibiotics should be tailored to the microorganisms and the antimicrobial resistance observed in each ICU.

Intrinsic PEEP, or auto-PEEP

Intrinsic PEEP or auto-PEEP is a complication of mechanical ventilation that most frequently occurs in patients with COPD and/or asthma who require prolonged expiration. These patients may have difficulty in totally exhaling the ventilator-delivered tidal volume before the next machine breath is delivered. When this problem occurs, a portion of each subsequent tidal volume may be retained in the patient's lungs, a phenomenon sometimes referred to as breath stacking  If this goes unrecognized, the patient's peak airway pressure may increase to a level that results in barotrauma, volutrauma, hypotension, patient-ventilator dyssynchrony, or death.

The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

Manometry performed by using an esophageal balloon to record changes in pleural pressure is the most accurate way to recognize intrinsic PEEP. However, this technology is not available at most institutions. Therefore, clinicians must anticipate this complication and carefully monitor the measured peak airway pressure. When intrinsic PEEP is diagnosed, the patient should temporarily be released from mechanical ventilation to allow for full expiration. The ventilator can then be adjusted to shorten inspiration by decreasing the set tidal volume or by increasing the inspiratory flow rate.

Cardiovascular effects

Mechanical ventilation always has some effect on the cardiovascular system. Positive-pressure ventilation can decrease preload, stroke volume, and cardiac output. Positive-pressure ventilation also affects renal blood flow and function, resulting in gradual fluid retention. The incidence of stress ulcers and sedation-related ileus is increased when patients receive mechanical ventilation. In fact, mechanical ventilation is a primary indication for GI prophylaxis. Positive pressure maintained in the chest may decrease venous return from the head, increasing intracranial pressure and worsening agitation, delirium, and sleep deprivation.

Alternative Modes of Mechanical Ventilation

In the last 2 decades, several modes of ventilation have emerged from the successful marriage of the ventilator and computer technology. Staying abreast of emerging ventilator modifications can be a formidable and ongoing challenge for physicians.

Dual-control ventilation modes were designed to combine the advantages of volume-control ventilation (guaranteed minute ventilation) with pressure-control ventilation (rapid, variable flow at a preset or limited peak airway pressure). These dual-control modes attempt to increase the safety and comfort of mechanical ventilation. Although this new technology seems promising, no findings from randomized trials indicate improved patient outcomes (including mortality).

Dual-control, breath-to-breath, pressure-limited, time-cycled ventilation

This mode has been called pressure-regulated volume-control (PRVC), adaptive pressure ventilation, auto-flow, volume-control +, or variable-pressure control ventilation according to various commercial ventilators. This mode is under the dual control of pressure and volume. The physician presets a desired tidal volume, and the ventilator delivers a pressure-limited (controlled) breath until that preset tidal volume is achieved. The breath is essentially like a conventional pressure-controlled ventilation breath, but the ventilator can guarantee a predetermined minute ventilation.

Breath to breath, the inspiratory pressure is automatically adjusted down or up according to the patient's lung compliance and/or resistance to deliver a preset tidal volume. The ventilator monitors each breath and compares the delivered tidal volume with the set tidal volume. If the delivered volume is too low, it increases the inspiratory pressure on the next breath. If it is too high, it decreases the inspiratory pressure. This adjustment gives the patient the lowest peak inspiratory pressure needed to achieve a preset tidal volume. The advantage of this mode is that it gives the physician the opportunity to deliver minimum minute ventilation at the lowest peak airway pressures possible .

The pressure, volume, and flow to time waveforms for pressure-regulated volume-controlled ventilation.

The pressure, volume, and flow to time waveforms for pressure-regulated volume-controlled ventilation.

Dual-control breath-to-breath, pressure-limited, flow-cycled ventilation

This mode has been called volume-support ventilation (VSV) or variable-pressure-support according to which ventilator is used. This mode is a combination of PSV and volume-control ventilation. Like PSV, the patient triggers every breath, controlling his or her own respiratory frequency and inspiratory time. This mode delivers a breath exactly like conventional PSV, but the machine can guarantee minute ventilation. The pressure support is automatically adjusted up or down according to the patient's lung compliance and/or resistance to deliver a preset tidal volume.

This mode is similar to the dual-control breath-to-breath, pressure-limited, time-cycled ventilation except that it is flow cycled, which means that the patient determines the respiratory rate and inspiratory time. The mode cannot be used in a patient who lacks spontaneous breathing effort.

Volume support has also been marketed as a self-weaning mode. Therefore, as the patient's effort and/or compliance or resistance improve, pressure support is automatically titrated down without the need for input from a physician or therapist.

A number of potential problems can arise. If the patient's metabolic demand increases, raising the tidal volume, the pressure support decreases to provide less ventilatory support when the patient needs it most. The clinician must be aware that, as the level of pressure support drops, mean airway pressure decreases; this effect may possibly result in hypoxemia. The other concern is that the tidal volume must be correctly set to the patient's metabolic needs. If the tidal volume is set too high, weaning is delayed. If it is set too low, the work of breathing may be more than what the patient can reasonably accomplish.

Automode and variable support or variable-pressure control

This mode is basically the combination of the 2 modes described above. If the patient has no spontaneous breaths, the ventilator is set up in the PRVC mode. However, when the patient takes 2 consecutive breaths, the mode is switched to VSV. If the patient becomes apneic for 12 seconds, the ventilator switches back to PRVC mode.

Automode and variable support or variable-pressure control was designed for automatic weaning from pressure control to pressure support depending on the patient's effort. This ventilatory mode can also be used in conventional volume control and volume support; again, the mode depends on the patient's effort. To the authors' knowledge, no randomized trials have been conducted to evaluate this automode, and no evidence suggests that this type of weaning is more effective than conventional weaning.

Dual control within a breath

This mode has been called volume-assured pressure support or pressure augmentation according to various manufacturers. This mode can switch from pressure control to volume control within a single specific breath cycle. After a breath is triggered, rapid and variable flow creates pressure to reach the set level of pressure support. The tidal volume that is delivered from the machine is monitored. If the tidal volume equals the minimum set tidal volume, the patient receives a typical pressure-supported breath, which makes this mode essentially like volume support. However, if the tidal volume is less than the set tidal volume, the ventilator switches to a volume-controlled breath with constant flow rate until the set tidal volume is reached.

One study compared volume-assured pressure support with simple assist-control volume support and showed a 50% reduction in the work of breathing, lowered airway resistance, and lowered intrinsic PEEP. However, because of its complexity, this mode is rarely used.

Automatic tube compensation

This mode is specifically used for weaning and is designed to overcome the resistance of the endotracheal tube by means of continuous calculations. These calculations deal with known resistive coefficients of the artificial airway (size and length), tracheal pressures, and measurement of instantaneous flow. These calculations allow the ventilator to supply the appropriate pressure needed to overcome this resistance throughout the entire respiratory cycle. To the authors' knowledge, no studies have proven that this mode is any better than SBTs.

Proportional assist ventilation⁴

This mode was designed to decrease the work of breathing and improve patient-ventilator synchrony. The mode adjusts airway pressure in proportion to the patient's effort. Unlike other modes in which the physician presets a specific tidal volume or pressure, proportional assist ventilation (PAV) lets the patient determine the inspired volume and the flow rate. This mode requires continuous measurements of resistance and compliance to determine the amount of pressure to give. The support given is a proportion of the patient's effort and is normally set at 80%. This support is always changing according to patient's effort and lung dynamics. If the patient's effort and/or demand are increased, the ventilator support is increased, and vice versa, to always give a set proportion of the breath. The patient's work of breathing remains constant regardless of his or her changing effort or demand (see Media File 12).

The pressure, volume, and flow to time waveforms for proportional-assist ventilation.

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The pressure, volume, and flow to time waveforms for proportional-assist ventilation.

This mode can be used only in patients with spontaneous respiratory efforts. PAV has promise, but the US Food and Drug Administration (FDA) has not approved it for commercial use.

Airway pressure–release ventilation

Bilevel, or biphasic, ventilation is a relatively new mode of ventilation that has recently gained popularity. The ventilator is set at 2 pressures (high CPAP, low CPAP), and both levels are time cycled. The high pressure is maintained for most of the time, while the low pressure is maintained for short intervals of usually less than 1 second to allow exhalation and gas exchange to occur. The patient can breathe spontaneously during high or low pressure (see Media File 13). This mode has the benefit of alveolar recruitment. Its disadvantage is that the tidal volume is variable. The clinician must be constantly aware of the patient's minute ventilation to prevent severe hypercapnia or hypocapnia.

The pressure, volume, and flow to time waveforms for airway pressure–release ventilation.

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The pressure, volume, and flow to time waveforms for airway pressure–release ventilation.

Conclusion

With computer feedback systems, many modern ventilators allow the operator to make fine adjustments in tidal volume, airway pressures, and the timing of the respiratory cycle. The desired result is improved ventilator-patient interaction and limitation of ventilator-induced lung injury. These newer methods of mechanical ventilation are often based on attractive physiologic hypotheses, and they are interesting to implement. Each method has its proponents, but objective evidence has failed to show that any of the alternative methods of ventilation is more successful than conventional mechanical ventilation with proper attention to tidal volume. Most clinicians use alternative methods of ventilation only in cases when conventional mechanical ventilation has failed.

Clinical Trials

• Weaning Children From Mechanical Ventilation:Computer-Driven System Versus Usual Care
• Pneumonectomy and Protective Mechanical Ventilation
• Music Therapy for Patients Being Weaned From Mechanical Ventilation

Multimedia

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Media file 1: An example of the Drinker and Shaw negative-pressure ventilator (iron lung).

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An example of the Drinker and Shaw negative-pressure ventilator (iron lung).

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Media file 2: The pressure, volume, and flow to time waveforms for assist-control ventilation.

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The pressure, volume, and flow to time waveforms for assist-control ventilation.

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Media file 3: The pressure, volume, and flow to time waveforms for controlled ventilation.

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The pressure, volume, and flow to time waveforms for controlled ventilation.

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The components of mechanical ventilation inflation pressures. Paw is airway pressure, PIP is peak airway pressure, Pplat is plateau pressure.

(	Media file 5: The effects of increased airway resistance (A) and decreased respiratory system compliance on the pressure-time waveform.

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The effects of increased airway resistance (A) and decreased respiratory system compliance on the pressure-time waveform.

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Media file 6: Determination of the lower inflection point to estimate the best (optimal) positive end-expiratory pressure (PEEP) from the pressure-volume hysteresis curve.

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Determination of the lower inflection point to estimate the best (optimal) positive end-expiratory pressure (PEEP) from the pressure-volume hysteresis curve.

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Media file 7: The effect of positive end-expiratory pressure (PEEP) on the pressure-time inflation curve.

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The effect of positive end-expiratory pressure (PEEP) on the pressure-time inflation curve.

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Media file 8: The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV).

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The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV).

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Media file 9: The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV) with pressure-support ventilation.

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The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV) with pressure-support ventilation.

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Media file 10: The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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Media file 11: The pressure, volume, and flow to time waveforms for pressure-regulated volume-controlled ventilation.

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The pressure, volume, and flow to time waveforms for pressure-regulated volume-controlled ventilation.

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Media file 12: The pressure, volume, and flow to time waveforms for proportional-assist ventilation.

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The pressure, volume, and flow to time waveforms for proportional-assist ventilation.

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Media file 13: The pressure, volume, and flow to time waveforms for airway pressure–release ventilation.

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The pressure, volume, and flow to time waveforms for airway pressure–release ventilation.

Keywords

mechanical ventilation, conventional respiratory support, positive pressure ventilation, positive-pressure ventilation, negative-pressure ventilation, negative pressure ventilation, iron lung, cuirass, shell unit, acute lung injury, chronic obstructive lung disease, apnea, respiratory arrest, neuromuscular disease, chronic obstructive pulmonary disease, COPD, acute respiratory distress syndrome, ARDS, permissive hypercapnia, iron lung

Today's most noteworthy pencils, styluses, and pen scanners

Hand Tools

"Mechanical pencils rule," my fifteen-year-old grandniece, Genevieve, declared when I invited her to be her generation's voice

on school supplies. "Nobody sharpens anymore." Then, continuing with a fashion maven's hyperbole and arbitrary imperatives, she gave a passionate disquisition on types of clickers, new grips, smaller lead sizes, and other niceties of pencil selection. As she consigned the yellow-painted wooden pencil to the wastebasket of history, I felt a rush of nostalgia for the perfumed sharpener shavings of my youth.

In fact the classic wooden pencil is hardly extinct, but one need only take a quick look at the array of vibrantly colored, subtly textured, high-attitude, low-priced mechanical pencils widely available to see that this writing instrument has become a part of contemporary youth culture.

 

The emotional appeal of pencils is that they are the instruments of works in progress—the quick sketch of the artist, the lines drawn by the carpenter, the notes and speculations of the scientist. Often they are the tools of people who are themselves works in progress: those in school, trying to figure the world out. Many people never quite get over the allure of school supplies, those first tools of intellect. Throughout their lives they continue to seek out and acquire pencils and pens—and now newer items such as Palm Pilots and pen-shaped scanners

I had long thought of the mechanical pencil as the dandy of the desk set, an ostentatious substitute for the modest, perfect wooden pencil. In fact the mechanical pencil has changed, though so gradually that its progress has gone largely unheralded. Today you can spend just a few dollars and get a pencil that is easier and more comfortable to use than one that was top of the line, and expensive, two decades ago.

 

Most contemporary mechanical pencils have clickers or ratchet systems to advance the lead in very small increments, which reduces the likelihood that you will break the point. They make a fine yet dark line because they use slimmer leads that have been engineered to reduce breakage. The most recent innovations involve ergonomic hand grips that have been softened in some cases and reshaped in others to increase comfort and decrease the possibility of doing damage to one's hand. The mechanical pencil that finally won me over was a Sanford PhD, which has a fashionably large body; a tapered, textured, three-sided grip; and a sleeve into which the lead can retract. It costs about $8.00.

 

You can pay hundreds of dollars for a pencil, but its mechanism will be essentially the same as that in the pencils that cost much less. "There are a couple of factories in Japan that make the works for everybody," Marilyn Brown told me. She runs the "fine writing" department at the New York specialty writing store Art Brown (www.artbrown.com), named for her husband's late uncle. "But the guys who shop here aren't going to use a two-dollar pencil. They want to show off."

 

Brown took from her display case a mechanical pencil by Faber-Castell, a leading maker of traditional wooden pencils, and let me examine it. Its barrel was made of ribbed Pernambuco wood. "This is just like an old wood pencil," she said of the elegant object in my hand, "except that it costs $195." Then she handed me a $26.95 model from the same manufacturer, called the E-Motion. It had a wooden barrel, a brushed-metal clip, and an extra-thick 1.4mm lead, thus evoking both mechanical and wooden pencils of bygone days in a smoothly contemporary form. "This," she said, "is a real pencil pencil."

Researcher gives robotic surgery tools a sense of touch

Haptic technology will allow doctors to 'feel' the work of a mechanical helper

By substituting mechanical instruments for human fingers, robotic tools give surgeons a new way to perform medical procedures with great precision in small spaces. But as the surgeon directs these tools from a computer console, an important component is lost: the sense of touch.

Johns Hopkins researchers are trying to change that by adding such sensations, known as haptic feedback, to medical robotic systems. "Haptic" refers to the sense of touch.

"The surgeons have asked for this kind of feedback," says Allison Okamura, an associate professor of mechanical engineering at Johns Hopkins. "So we're using our understanding of haptic technology to try to give surgeons back the sense of touch that they lose when they use robotic medical tools."

Okamura is a leading researcher in human-machine interaction, particularly involving mechanical devices that convey touch-like sensations to a human operator. In recent years, she has focused on medical applications as a participant in the National Science Foundation Engineering Research Center for Computer-Integrated Surgical Systems and Technology, based at Johns Hopkins. With funding from the National Institutes of Health and the NSF, she has established a collaboration with Intuitive Surgical Inc., maker of the da Vinci robotic system used in many hospitals for heart and prostate operations.

In the da Vinci system, a surgeon sits at a computer console, looks through a three-dimensional video display of the surgery site and moves finger controls that direct the motion of robotic tools inside the patient. Currently, this system does not send haptic feedback to the surgeon to convey what the mechanical tool "feels" inside the body. Okamura's team seeks to add these sensations to the da Vinci and similar machines.

Through the arrangement with Intuitive Surgical, Okamura's lab has acquired da Vinci hardware and software that allow her to conduct experiments toward achieving that goal. For example, the da Vinci's tools can be directed to tie sutures, but if the operator causes the tools to pull too hard, the thread can break. The Johns Hopkins researchers want the human operator to be able to feel resistance when too much force is applied.

"The sense of touch is important to surgeons," Okamura says. "They like to feel what's happening when they're working inside the body. They feel a 'pop' when a needle pokes through tissue. They can feel for calcification. Their sense of touch helps tell them where they are within the body. In robotic procedures and other types of minimally invasive surgery, surgeons insert long tools between their hands and the patient. This approach has definite medical benefits, but for the surgeon, there's a loss of dexterity and haptic information. It's like operating with chopsticks that have grippers on the end."

To address this, Okamura's team is experimenting with several techniques that could give some of those sensations back to the surgeons. One option is to attach to the robotic tools force sensors capable of conveying to the human operator how much force the machine is applying during surgery. Another idea is to create mathematical computer models that represent the moves made by the robotic tools, and then use this data to send haptic feedback to the operator.

Both approaches have advantages and drawbacks. Force sensors may be highly accurate, but they are expensive and would have to be made of sterile, biocompatible materials in order to to be used in medical robots. Computer models could be less expensive but might not respond quickly enough. "I'm exploring both approaches to see which produces the best results," Okamura says. "The most important thing is that the haptic feedback sent to the human operator must feel right because the fingers aren't easily fooled."

While this research continues, Okamura's team has developed an interim system that instead sends "haptic" information to the eyes. When a surgeon is using a robotic tool to tie a suture, for example, a colored circle follows the image of the tool in the visual display, indicating how much force is being using. A red light may signal that too much force is being applied, and the thread is likely to break. Green and yellow lights may indicate that the right amount of force is being used or that the tool is edging toward excessive force.

Development and use of a new high-frequency, low mechanical impedance strain gauge

A low mechanical impedance strain gauge that imposed insignificant preload to the myocardial fibers was tested in vitro and in vivo. The dynamic response of the gauge to an abrupt change in length (step response) and to sinusoidal perturbation was determined. The electrical output reached 95% of maximum steady-state response within 3-5 ms after a step displacement. Frequency analysis indicated a flat response up to 80 oscillations/s. The in vivo testings of the gauges were performed on intact, working swine hearts during control and ischemic flows in a regionally perfused preparation. During control perfusion the gauges demonstrated epicardial shortening in systole and early-to-mid diastole. Relaxation was confined to late diastole. With ischemic perfusion there was a progressive loss of systolic shortening, but minimal disruption in global hemodynamics. Correlative measurements were also made with sonomicrometers positioned in subepicardial myocardium. Patterns of motion, shortening, and changes in strain were similar between the two types of gauges.

Small Mechanical Forces Have Big Impact On Embryonic Stem Cells

Applying a small mechanical force to embryonic stem cells could be a new way of coaxing them into a specific direction of differentiation, researchers at the University of Illinois report. Applications for force-directed cell differentiation include therapeutic cloning and regenerative medicine.

"Our results suggest that small forces may indeed play critical roles in inducing strong biological responses in embryonic stem cells, and in shaping embryos during their early development," said Ning Wang, a professor of mechanical science and engineering at the U. of I., and corresponding author of a paper accepted for publication in Nature Materials and posted on the journal's Web site.

Cell softness is an intrinsic property of embryonic stem cells and dictates how a cell responds to forces in its physical microenvironment. Those responses include how strongly the cell attaches to a surface, how far the cell spreads on a surface, and, most surprisingly, whether specific genes are expressed.

To study cellular sensitivity to force, Wang and his collaborators first attached a magnetic bead, 4 microns in diameter, to the surface of a living embryonic stem cell. Then they applied a tiny oscillating magnetic field, which moved the bead up and down. By precisely measuring the magnetic field and the distance the bead traveled, the effect of the mechanical force and how soft the cells are could be determined.

The cyclic nature of the mechanical force is very important, Wang said, as it simulates natural forces within a living cell, such as the cyclic movement of the motor protein myosin.

The researchers found that mouse embryonic stem cells were softer and much more sensitive to localized cyclic forces than their more advanced, differentiated counterparts.

"As stem cells differentiate, they become stiffer," said Wang, who is affiliated with the university's Beckman Institute, Micro and Nanotechnology Laboratory, and department of bioengineering. "The stiffer the stem cell, the less it spreads under stress."

The researchers obtained the same results when they applied cyclic forces to stiff human muscle cells. They did not experiment with human embryonic stem cells.

To study some of the long-term effects of localized mechanical forces on the behavior of mouse embryonic stem cells, the researchers utilized the expression of an enhanced green fluorescent gene. Cells expressing this gene glow fluorescent green when exposed to blue light.

As the mechanical force was applied in the researchers' experiments, the green fluorescence in cells with magnetic beads faded, indicating reduced gene expression. Control cells (without beads) a few microns away continued to glow.

"The softness of mouse embryonic stem cells makes them very sensitive to localized cyclic forces," Wang said. "If our findings can be extended to early animal embryos, they could provide a new way of locally differentiating a single cell of early lineage, while leaving nearby cells alone."

With Wang, co-authors of the paper are graduate student and lead author Farhan Chowdhury, postdoctoral research associates Sungsoo Na (now an assistant professor at Indiana University) and Dong Li, graduate student Yeh-Chuin Poh, animal sciences professor Tetsuya S. Tanaka, and cell and developmental biology professor Fei Wang.

The work was funded by the National Institutes of Health, the U.S. Department of Agriculture, and the University of Illinois

Automated mechanical monitoring system from InnerSense prevents wafer handling issues

Product Briefing Outline: InnerSense LTD (a Ricor company) now offers a new wafer handling analysis product, the SMW2, for automated mechanical monitoring of the 300mm semiconductor process tools. The new product offering is based on over 6 years of experience in preventing excursions related to wafers’ micro-cracks and other mechanical defects, as well as providing capabilities to detect worn-out process tool mechanical components and to plan more effective periodic maintenance.

Problem: Wafer handling issues are responsible for about 30% of the overall yield loss in the fab. Collision, skidding, rubbing and abrupt lifting and chucking cause wafer micro cracks, breakage, and backside/edge defects. Any mechanical contact with the wafer is suspected to generate particles. The more aggressive such contact is, the higher the particle size and count are likely to be. In addition, troubleshooting and reactive maintenance of those handling issues impacts tool availability and disrupts production. Periodic monitoring of the entire wafer handling system by means of an instrumented wafer (Smart Wafer) has been shown to pinpoint the root cause of such issues and confirm the effectiveness of a corrective action taken. This method has been adopted and implemented by leading IC manufacturers and equipment suppliers. However, in the existing method the smart wafer is manually loaded from the docking station into a FOUP. This operation requires a special tool for opening the FOUP. It has to be done in a specific area in the clean room, and the wafer needs to be cleaned before the run. In addition the raw data recorded in the old method required expert analysis to return a meaningful diagnosis. This has limited its usage in high-volume manufacturing environment.

Solution: The new product provides customers with the capability of monitoring the process tools mechanical health, using the new “Smart FOUP” which interfaces with the process tools like any standard FOUP. New GUI software includes additional user friendly analysis capabilities, such as automatic SPC tool monitoring, tool-to-tool performance comparisons, and more. The new product employs simplified routines that can be easily implemented in a high volume manufacturing environment with minimal interference with the normal production flow.

Applications: Troubleshooting, monitoring and predicting mechanical failures in any robotic wafer handling system. Could be also used by OEMs to select moving components and improve mechanical design.

Platform: The new Smart Wafer- SMW2- incorporates contact less communication and charging to allow regular robotic handling in and out of the “Smart FOUP”.. In the new product. the docking station is integrated into a dedicated FOUP. The data is downloaded via optical communication and the battery is recharged via induction, so that no mechanical contact is made with the wafer. Hence, the smart wafer can be handled like any other production or test wafer. Gathering periodic readings of every tool on the floor supports historical tracking and comparison of similar tools, enabling closer tool matching for tighter process and equipment control. It also allows harnessing the power of AEC (Automated Equipment Control) for managing the entire line through the central factory data system. Predictive maintenance can be more effective by addressing only the trending up or "out of control" mechanical parts Collaboration between the tool manufacturer and the user in sharing typical vibration signatures and indicative signals may facilitate reliable diagnosis and remote assistance. In the long run such collaboration will inevitably contribute to improving handling system designs and enhancing yields

Small Mechanical Forces Have Big Impact On Embryonic Stem Cells

Applying a small mechanical force to embryonic stem cells could be a new way of coaxing them into a specific direction of differentiation, researchers at the University of Illinois report. Applications for force-directed cell differentiation include therapeutic cloning and regenerative medicine.

"Our results suggest that small forces may indeed play critical roles in inducing strong biological responses in embryonic stem cells, and in shaping embryos during their early development," said Ning Wang, a professor of mechanical science and engineering at the U. of I., and corresponding author of a paper accepted for publication in Nature Materials and posted on the journal's Web site.

Cell softness is an intrinsic property of embryonic stem cells and dictates how a cell responds to forces in its physical microenvironment. Those responses include how strongly the cell attaches to a surface, how far the cell spreads on a surface, and, most surprisingly, whether specific genes are expressed.

To study cellular sensitivity to force, Wang and his collaborators first attached a magnetic bead, 4 microns in diameter, to the surface of a living embryonic stem cell. Then they applied a tiny oscillating magnetic field, which moved the bead up and down. By precisely measuring the magnetic field and the distance the bead traveled, the effect of the mechanical force and how soft the cells are could be determined.

The cyclic nature of the mechanical force is very important, Wang said, as it simulates natural forces within a living cell, such as the cyclic movement of the motor protein myosin.

The researchers found that mouse embryonic stem cells were softer and much more sensitive to localized cyclic forces than their more advanced, differentiated counterparts.

"As stem cells differentiate, they become stiffer," said Wang, who is affiliated with the university's Beckman Institute, Micro and Nanotechnology Laboratory, and department of bioengineering. "The stiffer the stem cell, the less it spreads under stress."

The researchers obtained the same results when they applied cyclic forces to stiff human muscle cells. They did not experiment with human embryonic stem cells.

To study some of the long-term effects of localized mechanical forces on the behavior of mouse embryonic stem cells, the researchers utilized the expression of an enhanced green fluorescent gene. Cells expressing this gene glow fluorescent green when exposed to blue light.

As the mechanical force was applied in the researchers' experiments, the green fluorescence in cells with magnetic beads faded, indicating reduced gene expression. Control cells (without beads) a few microns away continued to glow.

"The softness of mouse embryonic stem cells makes them very sensitive to localized cyclic forces," Wang said. "If our findings can be extended to early animal embryos, they could provide a new way of locally differentiating a single cell of early lineage, while leaving nearby cells alone."

With Wang, co-authors of the paper are graduate student and lead author Farhan Chowdhury, postdoctoral research associates Sungsoo Na (now an assistant professor at Indiana University) and Dong Li, graduate student Yeh-Chuin Poh, animal sciences professor Tetsuya S. Tanaka, and cell and developmental biology professor Fei Wang.

The work was funded by the National Institutes of Health, the U.S. Department of Agriculture, and the University of Illinois.