What does a PetCO2 lower than the PaCO2 indicate?

When the PetCO2 (end-tidal carbon dioxide) is lower than the PaCO2 (arterial carbon dioxide), it typically indicates the presence of increased dead space ventilation. Dead space is the portion of the respiratory system where gas exchange does not occur, meaning that while air is being ventilated, it is not effectively participating in the exchange of gases—specifically, CO2. This can happen in conditions where ventilation exceeds perfusion, often seen in situations such as pulmonary embolism or certain types of lung disease.

Increased dead space ventilation leads to a scenario where the carbon dioxide that is being exhaled is less than what is present in the arterial blood, resulting in a PetCO2 that is lower than the PaCO2. This situation reflects an imbalance where the ventilation is not adequately matching perfusion, highlighting defective gas exchange.

The other options do not accurately describe this physiological outcome. Ventilatory failure typically would cause an increase in both PetCO2 and PaCO2, while a physical obstruction would usually lead to an inability to ventilate properly, impacting PetCO2 levels in a different manner. Normal respiratory function, on the other hand, would suggest that PetCO2 and PaCO2 are relatively equal, reflecting an