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

A nomogram providing the arterial mixed venous haemoglobin saturation difference (S(a-v)O2) 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 degrees C) was obtained from the literature. The nomogram computes the S(a-v)O2 from the body surface, pump flow, blood haemoglobin and patient temperature; a table is also presented which supplies the S(a-v)O2 ranges corresponding to VO2 mean +/-1 and +/-2SD. The nomogram was tested on 10 subjects undergoing CPB for myocardial revascularization. S(a-v)O2 was determined by arterial and mixed venous blood oximetry 5, 20, and 35 min after the start of CPB. The measured S(a-v)O2 was 27.1 +/- 7.2% while S(a-v)O2 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-v)O2 value corresponding to VO2 mean - 2SD and was associated with the lowest value of blood haemoglobin. Two values were higher than the S(a-v)O2 value corresponding to VO2 mean + 2SD and were associated with inadequate muscle relaxation. By comparing measured S(a-v)O2 values with those obtained by the nomogram and the table, anaesthesiologists and perfusionists can easily detect patients presenting abnormally low or high VO2 values.