Which ventilation setting affects which ABG value?

At this point in your learning, the terms ventilation (the exchange of pCO2) and oxygenation (pO2) are probably very familiar to you. Remember, pCO2 and pO2 are values in an ABG reading. Let’s relate these concepts to settings on the ventilator, and learn which specific setting affects ventilation or oxygenation.

We have repeatedly discussed how ventilation refers specifically to the exchange of air in and out of the lungs. In terms of blood gas effects, ventilation directly refers to the removal of CO2. So, what ventilator settings would affect this? What settings directly impact the amount of air going in and out of the lungs? If you guessed the tidal volume, you are correct! The size of breath will directly impact the amount of air going in and CO2 coming out of the lungs.

There is another setting that will impact the amount of CO2 clearance. What do the chemoreceptors in the brain trigger if the CO2 levels start to rise? If you answered the respiratory rate (RR), then you are remembering correctly! The RR also has direct impact on the amount of CO2 leaving the lungs over time. If you breathe faster, you are getting rid of CO2 more often and this will help drive CO2 levels down.

Increased CO2 clearance is commonly referred to as blowing off CO2. Try out this term when talking to ventilation clinicians to look like a real expert.

Key Concept

To affect ventilation, the RR and the tidal volume are the two settings with direct impact on the amount of air into the lungs and CO2 out of the lungs.

What about oxygenation? Which ventilator settings directly impact the patient’s oxygenation status? This concept should not be new to you. We have talked extensively about FiO2 and PEEP as working together to deliver oxygen into the body (see Chapter 2). The FiO2 can be increased to deliver higher amounts of oxygen to the lungs, while PEEP can push the oxygen across the alveolar-capillary membrane.

Please note that this is a slight simplification of these concepts. There is some crossover between the ventilator settings that effect oxygenation and ventilation. But for beginner learners, it is better to keep oxygenation and ventilation and the settings that affect them separate in your minds without crossover.

Take a look at the following table, which summarizes the mechanical ventilation settings that affect oxygen versus ventilation:

Settings that affect Oxygenation: Settings that affect Ventilation:
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If you ever get the chance to work in flight medicine or medevacs, you may be the sole or lead care provider for a ventilated patient and need to know how to make appropriate decisions.

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