Is the pCO2 gap obtained from the superior vena cava in agreement with that from the pulmonary artery?

Background: The partial pressure of carbon dioxide (pCO2) gap is the difference between pCO2 values in the arterial and mixed venous blood; values higher than six mmHg may be predictive of tissue hypoperfusion. It is still doubtful if central venous blood can be used to assess the gap. This study was aimed to compare the values obtained with blood collected from the superior vena cava and the pulmonary artery.
Methods: Data were obtained from a previous study. In 30 patients who underwent on-pump myocardial revascularization, blood samples from a radial artery, the pulmonary artery, and the superior vena cava were collected eight times in the perioperative period. Two-hundred determinations were utilized to calculate the pCO2 gap from central and mixed venous pCO2.
Results: The pCO2 gap was 8.7±2.6 mmHg in the superior vena cava, and 6.0±2.7 in the pulmonary artery. The difference between the two values fell within a wide interval between -4.1 mmHg and +9.5 mmHg in 95% of cases. pCO2 values were significantly higher in the superior vena cava than in the pulmonary artery (mean difference 2.7±2.4 mmHg; P<0.0001), causing a systematic bias between the two estimates; such bias increased during spontaneous breathing (P=0.0249).
Conclusions: pCO2 values are higher in the superior vena cava than in the pulmonary artery. As a consequence, the pCO2 gap calculated with the blood taken from the superior vena cava is overestimated, probably because it reflects the pCO2 arteriovenous difference of the upper part of the body, in particular of the brain.