“Knowing when to adjust mechanical ventilator settings” is the third article in a series of articles written to help medical students understand mechanical ventilation.
If you already get the basics of mechanical ventilation (FiO2, PEEP, tidal volume, RR), please read on. If you do not understand the basics of mechanical ventilation, please start at the beginning of this short series.
As a brief review, we initially set mechanical ventilators with attention to the oxygen and carbon dioxide levels. FiO2 is set at 100% with the intention to decrease this to below 60% once the PEEP is at or close to 5 cmH2O. The tidal volume is set at 8 to 10 ml/kg of the patient’s ideal body weight and is usually not adjusted thereafter. The patient’s respiratory rate is determined on a best guess kind of scenario. Finally, about 30 minutes after the initial settings we check an arterial blood gas and adjust the settings as needed.
Keep in mind that this explanation of adjusting mechanical ventilation settings is simplified for the medical student. To really understand ventilation you should become familiar with the various ventilation settings that are used. For example:-Assist Control Ventilation: A minimum number of supported breaths plus patient-triggered fully assisted breaths.
-Synchronized Intermittent Mandatory Ventilation: A minimum number of supported breaths plus patient-triggered breaths with the tidal volume determined by the patient’s efforts.
-Pressure Support Ventilation: No set respiratory rate, instead the vent supports patient-initiated breaths with a set inspiratory pressure.
Until you understand the complexities of mechanical ventilation (which you probably don’t need to do as a medical student), you should understand how you know when to adjust the ventilator settings.
You need to adjust the ventilator settings when your arterial blood gas is not at goal. If oxygenation is off, adjust the FiO2 first and the PEEP second, keeping within the circumscribed limits. Simple enough. If the pH is off goal then you need to adjust the pCO2. This is a little more complicated than adjusting oxygen.
For every 10 mmHg decrease in pCO2, the pH will increase by 0.08. So if your pH is too low (too acidic), you should decrease your CO2 (an acid) levels and vice versa. Remember, you change your pCO2 by changing the respiratory rate (because tidal volume is usually set and never touched again).
How do we know how much to change the respiratory rate in order to reach our goal pH? An easy way to do this is to work in percentages. Figure out what percentage you want to decrease the pCO2 by. This determined percentage is the percentage that you will increase the respiratory rate in order to make your patient blow off more CO2.
For example, your patient’s arterial blood gas reads 7.3 pH/ 50 pCO2/ 80 O2. You want to increase the pH a little bit and get that CO2 down to at least 40 mmHg. Decreasing the pH from 50 to 40 is a 20% drop (50-40= 10, 10/50= 20%) so you need to increase the respiratory rate by 20%. If the patient’s respiratory rate is 10 breaths/min, 20% of 10 is 2 breaths per minute. So you would now set the respiratory rate on the ventilator to 12 breaths per minute (10 +2).
So in review, adjust the ventilator settings based on the arterial blood gas readings. Change FiO2, then PEEP when needed for optimum oxygenation. Adjust the pH and pCO2 using percentages and respiratory rate as described above.
