In screening a patient suspected of having pulmonary embolism, which of the following determinations is most helpful?

In the context of screening for pulmonary embolism (PE), measuring the arterial carbon dioxide tension (PaCO2) is particularly informative. Pulmonary embolism can lead to a mismatch in ventilation and perfusion due to obstruction of blood flow to areas of the lung. This effect often results in decreased levels of carbon dioxide in the blood because the ventilated portions of the lung are not being adequately perfused. Therefore, a low PaCO2 can indicate hyperventilation as a response to the hypoxemia caused by perfusion problems.

In managing PE, clinicians look for changes in the partial pressure of carbon dioxide to assess the patient's ventilatory status. A significant drop in PaCO2 can suggest that the patient is experiencing rapid and shallow breaths due to hypoxemia, a common physiological response when a pulmonary embolism is present.

While other measurements, like the P/F ratio (the ratio of arterial oxygen partial pressure to inspired oxygen fraction), can provide insights into the severity of hypoxemia and the degree of respiratory failure, PaCO2 directly reflects the patient's ventilatory efficacy and can be more immediate in indicating the acute effects of an embolism on gas exchange. Therefore, PaCO2 measurement is a critical component when screening for pulmonary embolism.