What is the desired equation for tidal volume?

• A. actual Vt x actual CO2/desired CO2
• B. desired Vt x actual CO2/desired CO2
• C. actual CO2/desired CO2
• D. actual Vt + CO2

Answer: (A)

Tidal volume is adjusted based on how CO2 in the blood responds to changes in ventilation. CO2 removal rises with greater alveolar ventilation, which depends on tidal volume (assuming the respiratory rate and dead space stay constant). Because CO2 is inversely related to alveolar ventilation, the correct way to scale the current tidal volume to reach the desired CO2 is to multiply the current tidal volume by the ratio of the current CO2 to the desired CO2: new Vt = current Vt × (current CO2 / desired CO2). If the CO2 is higher than desired, this ratio is greater than one, increasing tidal volume to blow off more CO2; if CO2 is lower than desired, the ratio is less than one, reducing tidal volume. This approach relies on stable CO2 production and a constant rate, and it aligns with how alveolar ventilation governs CO2 levels. The other options either start from the wrong baseline, yield a dimensionless value, or combine values in a way that doesn’t produce a valid tidal volume.