Using Vt = ( Known PaCO2 × Known Vt in ml ) ÷ ( Desired PaCO2 ), if Known PaCO2 = 60, Known Vt = 600 ml, and Desired PaCO2 = 40, what is the resulting Vt?

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Multiple Choice

Using Vt = ( Known PaCO2 × Known Vt in ml ) ÷ ( Desired PaCO2 ), if Known PaCO2 = 60, Known Vt = 600 ml, and Desired PaCO2 = 40, what is the resulting Vt?

Adjusting tidal volume to reach a target PaCO2 uses a proportional relationship: PaCO2 is inversely related to alveolar ventilation, so to lower PaCO2 you increase ventilation (here by increasing Vt). The formula scales the known tidal volume by the ratio of the current PaCO2 to the desired PaCO2. Compute: 60 × 600 = 36,000; 36,000 ÷ 40 = 900. Alternatively, 600 ml × (60/40) = 900 ml. So the resulting tidal volume is 900 ml.

Adjusting tidal volume to reach a target PaCO2 uses a proportional relationship: PaCO2 is inversely related to alveolar ventilation, so to lower PaCO2 you increase ventilation (here by increasing Vt). The formula scales the known tidal volume by the ratio of the current PaCO2 to the desired PaCO2.

Compute: 60 × 600 = 36,000; 36,000 ÷ 40 = 900. Alternatively, 600 ml × (60/40) = 900 ml.

So the resulting tidal volume is 900 ml.

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