Which statement is true about the constants in the tidal volume calculation?

• A. The Known PaCO2 is fixed at 40
• B. The Desired PaCO2 is fixed at 40; all else is given
• C. The Known Vt is fixed at 40
• D. The Desired PaCO2 is variable

Answer: (B)

The main idea is that you aim for a specific PaCO2 and then work out the tidal volume needed to reach that target. In this calculation, CO2 production and anatomical dead space, along with the respiratory rate, are treated as given constants. PaCO2 is determined by how much ventilation you provide: higher alveolar ventilation lowers PaCO2, while lower ventilation raises it. By fixing the desired PaCO2 (commonly 40 mmHg) and keeping the other factors constant, you can solve for the tidal volume that yields that target. That’s why stating that the Desired PaCO2 is fixed at 40, with all other variables given, is the correct approach. The actual measured PaCO2 isn’t fixed here; you’re choosing a target.