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 (S_a-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 S_a-vO₂ from the body surface, pump flow, blood haemoglobin and patient temperature; a table is also presented which supplies the S_a-vO₂ ranges corresponding to VO₂ mean ±1 and ±2SD. The nomogram was tested on 10 subjects undergoing CPB for myocardial revascularization. S_a-vO₂ was determined by arterial and mixed venous blood oximetry 5, 20, and 35 min after the start of CPB. The measured S_a-vO₂ was 27.1 ± 7.2% while S_a-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 S_a-vO₂ value corresponding to VO₂ mean - 2SD and was associated with the lowest value of blood haemoglobin. Two values were higher than the S_a-vO₂ value corresponding to VO₂ mean + 2SD and were associated with inadequate muscle relaxation. By comparing measured S_a-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.