Pathways to complement activation during cardiopulmonary bypass.

Complement activation was assessed in 34 patients undergoing cardiopulmonary bypass. Arterial concentrations of complement fragments Ba and C3d rose in all patients, the increase in Ba preceding that of C3d. At the same time as complement fragments were being generated the arterial neutrophil count fell. These findings suggest (a) that complement activation is initiated by the alternative pathway during cardiopulmonary bypass and (b) that complement activation mediates loss of neutrophils during bypass. Complement mediated loss of neutrophils during the analogous setting of haemodialysis is the result of leucosequestration in the pulmonary vasculature. During cardiopulmonary bypass the lungs are out of circuit, so that activated leucocytes may sequester in other target organs. This may be an aetiological factor in the multi-organ failure occasionally seen after uneventful cardiopulmonary bypass.