Sunday, December 5, 2010

Mechanical Ventilation for Medical Students

Patients may require mechanical ventilation for chronic or acute respiratory failure.  When a patient is intubated and placed on ventilation, there are several settings a physician may have to manage on the mechanical ventilator.  These ventilator settings include the positive end expiratory pressure (PEEP), the fraction of inspired oxygen (FiO2), the respiratory rate and the tidal volume.

As a medical student in the ICU, you will surely get asked several basic questions on mechanical ventilation.  Wrong answers to these questions will bore your attending, amuse your residents, and feed that desperate fear you have of accidentally killing your future patients through your own ineptness.
With that being said, correctly answering questions on the mechanical ventilation of your ICU patients could make your team nod proudly, yes, proudly.  The simplified basics of mechanical ventilation are described below.  Once you get the basics, move on to the second article in this series, “How to determine the initial settings for mechanical ventilation”.

There are two aspects medical students must understand about ventilation control settings:
1. Oxygenation and pH. 
2. Volume and Pressure
The reason we want to control inspiratory volume and pressure is because we are trying to walk a fine line between assisting the patient with breathing without allowing their respiratory muscles to atrophy or drastically decreasing the cardiac preload.
We want to control the oxygen levels and pH of our patient’s bodies to sustain healthy tissues and metabolic function.  The pH is reflected in our carbon dioxide and bicarbonate balance.  So we adjust oxygen and carbon dioxide levels as needed.

In the ICU, an oxygen concentration on the arterial blood gas (pO2) can be as low as 70 and still be considered acceptable.  How do we keep the pO2 at or above 70 mmHg?  Adjust the FiO2 and PEEP on the ventilator settings. Altering the patient’s pO2 can be just that simple- FiO2 and PEEP
The FiO2 is the fraction of inspired oxygen, while PEEP is the positive end expiratory pressure.
Carbon dioxide and pH:

The blood pH in your patient should be 7.4 plus or minus 0.5 and the carbon dioxide levels on the arterial blood gas (pCO2) should read 40 mmHg plus or minus 5.  How do we keep the pH and CO2 at safe levels? Adjust the respiratory rate and tidal volume.  Again, it’s just that simple.  You can alter the pH and pCO2 for your patient by adjusting the respiratory rate and the tidal volume on the ventilator. 

[Remember: The rate multiplied by the tidal volume will give you the minute ventilation (in units of L/min).  You may or may not get pimped on minute ventilation but it’s easy enough to remember: tidal volume x respiratory rate.]

So in short, the two basic goals of mechanical ventilation are to get the oxygen and carbon dioxide on the arterial blood gas close to perfect, and to do this with a pressure and volume that is appropriate for the medical situation the patient is in.  Alter the oxygen with the FiO2 and PEEP.  Alter the carbon dioxide (and thus pH) with the respiratory rate and tidal volume

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