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 (see image below).
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 (see image below).
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 (see Complications of Mechanical Ventilation) 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 (see Complications of Mechanical Ventilation). 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 (see image below).
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 (see image above). Monitoring the peak and plateau pressures allows physicians to make clinical judgments on the progress of their patient (see image below).
The effects of increased airway resistance (A) and decreased respiratory system compliance on the pressure-time waveform.
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.
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 (see Complications of Mechanical Ventilation).
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 trials comparing higher versus lower levels of PEEP in adults with acute lung injury or acute respiratory distress syndrome (ARDS) have been underpowered to detect small but potentially important effects on mortality or to explore subgroup differences, Briel et al performed a systematic review and meta-analysis of data from 2299 patients in 3 trials. Treatment with higher versus lower levels of PEEP was not associated with improved hospital survival: 374 hospital deaths occurred in 1136 patients (32.9%) assigned to treatment with higher PEEP, and 409 hospital deaths occurred in 1163 patients (35.2%) assigned to lower PEEP (adjusted relative risk [RR], 0.94; 95% confidence interval [CI], 0.86-1.04; P = .25). However, in the subgroup of patients with ARDS, higher levels were associated with improved survival: 324 hospital deaths (34.1%) occurred in the higher PEEP group and 368 (39.1%) occurred in the lower PEEP group (adjusted RR, 0.90; 95% CI, 0.81-1.00; P = .049).
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 (see image below).
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:
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%