Changes in arterial-mixed venous oxygen content difference (CaO2 - CvO2) and the effect on shunt calculations in...

Many commonly used indices of pulmonary oxygen transfer assume that CaO2 - CvO2 is constant. We studied changes in CaO2 - CvO2 in critically ill patients to determine the effect on calculated shunt of assuming a fixed CaO2 - CvO2. Two hundred pairs of arterial and mixed venous blood gas measurements were obtained retrospectively from 43 patients, each providing four or five pairs from a period of 48 h. CaO2 - CvO2 range from 13 to 74 ml l-1. The mean within-patient sd was 6.8 ml l-1 and the overall between-patient sd was 10.9 ml l-1. Our results show that assuming a fixed CaO2 - CvO2 leads to errors in quantifying pulmonary oxygen transfer.