What should a therapist evaluate when a patient's end-tidal CO2 levels differ significantly from arterial CO2 levels?

When evaluating discrepancies between end-tidal CO2 (ETCO2) levels and arterial CO2 (PaCO2) levels, it is essential to consider the possibility of a pulmonary embolism. A pulmonary embolism can lead to ventilation-perfusion (V/Q) mismatch, where areas of the lung are ventilated but not perfused, which can result in lower ETCO2 levels compared to arterial CO2 levels. This significant difference indicates that while the patient's lungs are exchanging gases, blood flow to those areas is impaired, leading to poor oxygenation and CO2 elimination.

In the context of the other options, metabolic acidosis and impending respiratory failure are important clinical considerations, but they do not specifically account for the discrepancy observed between ETCO2 and arterial CO2 levels. Underlying cardiovascular complications can also impact CO2 transport and levels, but they are less directly related to the differential observed in ETCO2 versus arterial CO2. The key focus here is on the acute nature of a pulmonary embolism and its direct impact on gas exchange dynamics; hence, assessing for this condition is crucial when significant differences in these measurements arise.