- is a functional measurement.
- is defined as the volume of the lungs that does not participate in gas exchange.
- is approximately equal to the anatomic dead space in normal lungs.
- may be greater than the anatomic dead space in lung disease in which there are ventilation/perfusion (V/Q) inequalities.
- is calculated by the following equation:
Vd = physiologic dead space (ml) Vt = tidal volume (ml) Paco2 = Pco2 of arterial blood (mm Hg) Peco2 = Pco2 of expired air (mm Hg)
- In words, the equation states that physiologic dead space is tidal volume multiplied by a fraction. The fraction represents the dilution of alveolar Pco2 by dead-space air, which does not participate in gas exchange and does not therefore contribute C02 to expired air.
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