If minute ventilation is constant and dead space ventilation increases, then PaCO2 will ...

If minute ventilation is constant and dead space ventilation increases, then PaCO2 will ...

• A. PaCO2 increases
• B. PaCO2 decreases
• C. No change
• D. Cannot be determined

Answer: (A)

This question tests how dead space affects alveolar ventilation and PaCO2 when minute ventilation is kept constant. Minute ventilation equals tidal volume times rate, but only the portion of each breath that reaches the alveoli—alveolar ventilation—removes CO2. Increasing dead space means a larger fraction of each breath fills the conducting airways without participating in gas exchange, so alveolar ventilation declines while CO2 production stays the same. Since PaCO2 is governed by the balance between CO2 production and alveolar ventilation (PaCO2 rises as VA falls with constant production), the result is an increase in PaCO2.

This question tests how dead space affects alveolar ventilation and PaCO2 when minute ventilation is kept constant. Minute ventilation equals tidal volume times rate, but only the portion of each breath that reaches the alveoli—alveolar ventilation—removes CO2. Increasing dead space means a larger fraction of each breath fills the conducting airways without participating in gas exchange, so alveolar ventilation declines while CO2 production stays the same. Since PaCO2 is governed by the balance between CO2 production and alveolar ventilation (PaCO2 rises as VA falls with constant production), the result is an increase in PaCO2.