9.4 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 [latex](RR)[/latex], then you are remembering correctly! The [latex]RR[/latex] 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.

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. 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 affect 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. With practice you will be able to see the big picture and even make predictions on what order to use ventilation settings.

Take a look at the following table, which summarizes the mechanical ventilation settings that affect oxygen versus ventilation (keep in mind that [latex]V_T[/latex] can also be affected by other settings on a ventilator):

“Which ventilation setting affects which ABG value?” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.