Changes in ionized calcium concentrations and acid-base status during abdominal aortic vascular surgery

Abdominal aortic surgery may produce significant haemodynamic instability (from a combination of factors: hypovolaemia, acid-base disturbances, vasoactive metabolite release from ischaemic tissues and hypocalcaemia). Calcium is often given after aortic unclamping to attenuate this instability. We studied 20 patients undergoing elective abdominal aortic surgery and observed a triphasic change in ionized calcium concentrations and acid-base status. Initially, during the cross-clamp period (when patients were cardiovascularly stable), ionized calcium concentrations decreased significantly (mean 1.06 (SD 0.08) to 0.91 (0.13) mmol litre-1; P < 0.01), while a significant metabolic acidosis developed (pH 7.38 (0.05) to 7.30 (0.05); P < 0.05). Second, release of the aortic cross-clamp resulted in further acidosis (pH 7.27 (0.05) (P < 0.05) mixed respiratory and metabolic) with a decrease in mean arterial pressure, with no change in ionized calcium concentrations. The third phase was associated with spontaneous restoration of acid-base status and ionized calcium concentrations to normal over 2 h. There was no correlation between units of blood given, volume of blood lost, fluid volume given or duration of aortic cross-clamping and degree of ionized hypocalcaemia. We conclude that ionized hypocalcaemia occurred during the cross-clamp period of aortic surgery, was unrelated to the volume of blood given and did not appear to be responsible for the changes in arterial pressure during surgery.