A patient in the intensive care unit is receiving mechanical ventilation, has a pulmonary artery catheter in place, and is being monitored continuously with a capnometer

The patient's arterial partial pressure of carbon dioxide (PaCO2) is 41 mm Hg and the partial pressure of end-tidal carbon dioxide (PetCO2) is 36 mm Hg. There is a sudden decrease in the PetCO2 to 18 mm Hg causing an alarm to sound. The most likely cause of this development is which of the following?
a. Hypovolemia
b. Apneic episode
c. Pulmonary embolism
d. Increased cardiac output

ANS: C
Pulmonary embolism will cause a decrease in blood flow to the lungs. This increases alveolar dead space and leads to a decrease in the partial pressure of end-tidal carbon dioxide (PetCO2). Hypovolemia would also cause a decrease in the PetCO2, but it would not occur as suddenly as it did in this situation. The fact that the patient has an indwelling pulmonary artery catheter in-creases the risk of developing a pulmonary embolism, which often will have a quick onset. An apneic episode would have increased the PetCO2. An increased cardiac output would increase the PetCO2 because increases in cardiac output result in better perfusion of the alveoli and a rise in PetCO2.