Comparison of pH and PCO2 between arterial and venous blood gas samples in critically ill patients

Abstract BACKGROUND The reliability of peripheral venous blood sampling as a substitute for arterial blood sampling in determining the partial pressure of carbon dioxide (pCO₂) in critically ill patients remains controversial. We aimed to determine the utility of venous blood in a medical intensive care unit population. METHODS Seventy-six critically ill patients requiring arterial blood sampling as part of their standard clinical management were prospectively enrolled in this study. A venous blood sample was drawn from the same upper extremity within 5 min of the arterial blood sampling. Venous pH, pCO₂, and pO₂ were measured in venous samples and compared with those in their arterial counterparts. RESULTS The venous pCO₂ was, on average, higher by 5.8 mmHg (standard deviation = 4.6 mmHg) than the arterial pCO₂ with Bland-Altman 95% limits of agreement of −3.4, 15.1 mmHg. CONCLUSIONS Obtaining venous blood from an intact upper extremity in critically ill individuals can provide a reliable estimate of arterial pCO₂ and eliminate the need for invasive arterial blood sampling.