Impaired carbon dioxide transport during and after cardiopulmonary bypass

Changes in the CO2 carrying power of blood were evaluated during and after cardiopulmonary bypass (CPB) by calculating the equation of the whole blood CO2 dissociation curve and the ratio between the arterial-venous differences of CO2 content and CO2 tension (Ra-v). Sixteen patients undergoing normothermic CPB for coronary revascularization were studied; arterial and mixed venous blood gas analyses were performed prior to CPB, at the end of first cardioplegia infusion, 25 and 45 min after CPB commencement and 10 min after the termination of CPB. After CPB commencement, the whole blood CO2 dissociation curve became flatter and did not further change during or after CPB. Ra-v decreased from 1.06 +/- 0.16 to 0.72 +/- 0.12 ml/mmHg after the start of CPB and did not change significantly during CPB; it was still 0.73 +/- 0.13 ml/mmHg after CPB. The data indicate that during CPB the amount of CO2 removed from tissues by 1 litre of blood decreases by about 30% and that impairment in CO2 transport persists after the restoration of physiological circulation. Impairment in CO2 transport is mainly caused by haemodilution, but it could be worsened by acidosis.