If a patient’s total respiratory rate exceeds the ventilator’s set rate, what effect does changing the set rate have on the arterial blood gas?

• A. Increases PaCO2
• B. No Effect
• C. Decreases PaO2
• D. Increases pH

Answer: (B)

When the patient is driving most of the breaths, the ventilator’s rate becomes less influential on overall ventilation. Total alveolar ventilation is what determines the arterial gas values, and if the patient’s spontaneous breaths (which exceed the ventilator’s set rate) are providing the bulk of that ventilation, changing the ventilator’s rate doesn’t significantly alter gas exchange. In this scenario, the patient’s own tidal volume and breathing pattern largely dictate PaCO2, PaO2, and pH, so adjusting the set rate has little to no effect on the arterial blood gas.