During a left hepatectomy under general isoflurane anesthesia, arterial blood gases are: O2 138, CO2 39, pH 7.38, saturation 99%. At the same time, CO2 on mass spectrometer is 26 mm Hg. The most plausible explanation for the difference between CO2 measured with mass spectrometer versus arterial blood gas is which of the following?

During a left hepatectomy under general isoflurane anesthesia, arterial blood gases are: O2 138, CO2 39, pH 7.38, saturation 99%. At the same time, CO2 on mass spectrometer is 26 mm Hg. The most plausible explanation for the difference between CO2 measured with mass spectrometer versus arterial blood gas is which of the following?

• A. Mainstem intubation
• B. Atelectasis
• C. Shunting through thebesian veins
• D. Hypovolemia

Answer: (D)

The key idea is that end-tidal CO2 (the CO2 reported by the mass spectrometer from exhaled gas) reflects CO2 in ventilated, well-perfused alveoli, and it depends on pulmonary blood flow. Arterial CO2 (PaCO2) reflects overall CO2 in arterial blood from tissue production and gas exchange. Normally, end-tidal CO2 sits a bit lower than PaCO2, but a large and abrupt difference suggests a problem with pulmonary perfusion rather than ventilation. In this case, hypovolemia reduces venous return and cardiac output, which decreases blood flow through the lungs. Less blood reaching the pulmonary capillaries means less CO2 is transferred to the alveolar air to be exhaled. This creates more alveolar dead space and drops end-tidal CO2 markedly, even though arterial CO2 remains relatively high from ongoing CO2 production and systemic gas exchange. So the CO2 reading from the mass spectrometer (end-tidal CO2) is much lower than the arterial CO2. Other options would tend to produce different patterns: mainstem intubation or significant atelectasis would more consistently alter oxygenation or produce different signs on examination and ABGs; a thebesian vein shunt would mix deoxygenated blood into the arterial system with a different, less direct effect on the ETCO2–PaCO2 gradient; and hypovolemia best accounts for the observed large ETCO2 drop with relatively preserved oxygenation and a near-normal pH.

The key idea is that end-tidal CO2 (the CO2 reported by the mass spectrometer from exhaled gas) reflects CO2 in ventilated, well-perfused alveoli, and it depends on pulmonary blood flow. Arterial CO2 (PaCO2) reflects overall CO2 in arterial blood from tissue production and gas exchange. Normally, end-tidal CO2 sits a bit lower than PaCO2, but a large and abrupt difference suggests a problem with pulmonary perfusion rather than ventilation.

In this case, hypovolemia reduces venous return and cardiac output, which decreases blood flow through the lungs. Less blood reaching the pulmonary capillaries means less CO2 is transferred to the alveolar air to be exhaled. This creates more alveolar dead space and drops end-tidal CO2 markedly, even though arterial CO2 remains relatively high from ongoing CO2 production and systemic gas exchange. So the CO2 reading from the mass spectrometer (end-tidal CO2) is much lower than the arterial CO2.

Other options would tend to produce different patterns: mainstem intubation or significant atelectasis would more consistently alter oxygenation or produce different signs on examination and ABGs; a thebesian vein shunt would mix deoxygenated blood into the arterial system with a different, less direct effect on the ETCO2–PaCO2 gradient; and hypovolemia best accounts for the observed large ETCO2 drop with relatively preserved oxygenation and a near-normal pH.