How are PaCO2 and minute ventilation related?
The arterial carbon dioxide tension (partial pressure of carbon dioxide in the arteries, PaCO2) is determined by the rate of carbon dioxide production (VCO2) and the level of minute alveolar ventilation (VA).
Hence, at a constant rate of carbon dioxide production, the arterial carbon dioxide tension remains constant as long as alveolar ventilation remains constant.
Clinically, however, we don’t measure minute alveolar ventilation; rather, we measure the overall level of a patient’s ventilation, or minute ventilation (VE), which is the alveolar ventilation plus the dead space ventilation (VD). So, we can use this to calculate the partial pressure of carbon dioxide in the arteries by reworking the above equation as follows.
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Pathological changes to PaCO2
At a constant rate of carbon dioxide production, the arterial partial pressure of carbon dioxide falls with increasing minute ventilation and rises with declines in minute ventilation.
Furthermore, PaCO2 rises with increasing dead space, as seen in various disease states like chronic obstructive pulmonary disease (COPD) and acute respiratory distress syndrome (ARDS), and falls with declines in the proportion of dead space—as seen with large volume breaths during which VD stays constant but the difference between VD and VT increases.
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