Which statement is true about the CO2-based formulas for RR and Vt?

• A. Both RR and Vt equations incorporate actual CO2 and desired CO2
• B. Only RR uses CO2 terms
• C. Only Vt uses CO2 terms
• D. Neither uses CO2 terms

Answer: (A)

The key idea is that CO2-based control uses both the actual CO2 level and the target CO2 level to drive ventilation adjustments. In these schemes, the difference between what the CO2 is now and what you want it to be (the CO2 error) is fed into the equations that set ventilation. Since alveolar ventilation—and thus CO2 elimination—depends on both how fast you breathe (RR) and how much air you move with each breath (tidal volume), both parts of ventilation are adjusted in response to the CO2 error. When the measured CO2 is higher than the target, the controller increases ventilation to blow off more CO2; when it’s lower than the target, it decreases ventilation. So both the respiratory rate and the tidal volume formulas incorporate the actual CO2 and the desired CO2, making this statement the best one.