A nomogram to evaluate the arterial mixed venous oxygen saturation difference during cardiopulmonary bypass

Abstract

A nomogram providing the arterial mixed venous haemoglobin saturation difference (Sa-vO2) corresponding to normal oxygen consumption (VO2) during cardiopulmonary bypass (CPB) was produced. Normal VO2 during CPB (95.8 ± 20.1 ml/min/m2 at 37°C) was obtained from the literature. The nomogram computes the Sa-vO2 from the body surface, pump flow, blood haemoglobin and patient temperature; a table is also presented which supplies the Sa-vO2 ranges corresponding to VO2 mean ±1 and ±2SD. The nomogram was tested on 10 subjects undergoing CPB for myocardial revascularization. Sa-vO2 was determined by arterial and mixed venous blood oximetry 5, 20, and 35 min after the start of CPB. The measured Sa-vO2 was 27.1 ± 7.2% while Sa-vO2 obtained from the nomogram was 24.9 ± 4.0%, the difference was not statistically significant. Eighteen values (60%) were within the range corresponding to VO2 mean ±1SD. One value was lower than the Sa-vO2 value corresponding to VO2 mean - 2SD and was associated with the lowest value of blood haemoglobin. Two values were higher than the Sa-vO2 value corresponding to VO2 mean + 2SD and were associated with inadequate muscle relaxation. By comparing measured Sa-vO2 values with those obtained by the nomogram and the table, anaesthesiologists and perfusionists can easily detect patients presenting abnormally low or high VO2 values.