Hydrogen ion concentration and coronary artery bypass graft surgery with and without cardiopulmonary bypass

Acidosis during cardiopulmonary bypass (CPB) has been related to the strong ion difference (SID) and the composition of intravascular fluids that are administered. Less intravascular fluids tend to be administered during off- than on-pump CABG and should influence the degree of acidosis that develops. This study aimed to explore the role of CPB in the development of acidosis by comparing changes in hydrogen ion concentration ([H+]) and electrolytes in patients undergoing on- and off-pump coronary artery bypass graft (CABG) surgery.

Eighty two patients had arterial blood gas measurements pre-operatively, following CABG and at approximately 0600 h the morning after surgery. Carbon dioxide tension (PaCO2) and concentrations of sodium, potassium, chloride, [H+], bicarbonate and haemoglobin were measured and strong ion difference calculated. Data was analysed using mixed repeated-measures analysis of variance.

Intra-operatively, mean SID decreased more in the on- compared to the off-pump group (4.0 mmol/L, 95% confidence interval 2.8-5.3 mmol/L, p < 0.001). Neither [H+] or PaCO2 changed significantly and there were no significant difference between the groups. By the morning following surgery, [H+] and PaCO2 had both increased (p < 0.001) and difference in SID had disappeared (p = 0.17).

Despite significant differences in changes in SID, there were no differences in [H+] between patients during or after CABG surgery whether performed on- or off-pump. This may be have been the result of greater haemodilution in the on- compared to the off-pump group, compensating for change in SID by reducing the concentration of weak acids. Although it was associated with significantly greater decrease in SID, CPB was not associated with any significant increased risk of acidosis.