The hemodynamic effects of heparin and their relation to ionized calcium levels.

Heparin complexes calcium in vitro and possesses vasodilating properties when given as an intravenous bolus. To investigate the possibility that these hemodynamic effects could be related to the ability of heparin to induce hypocalcemia in vivo, we studied the response to a bolus of heparin, 300 IU/kg, in 20 patients undergoing cardiopulmonary bypass for cardiac operations. Ionized calcium decreased significantly after heparin administration, as did mean arterial pressure and systemic vascular resistance. In a further nine patients, 125 mg of calcium chloride was given immediately before the heparin bolus. It induced a small rise in calcium levels and maintained the mean arterial pressure at unchanged values. Our data confirm the vasodilating effect of an intravenous bolus of heparin and show that it is related to an acute lowering of ionized calcium levels. When calcium levels are not allowed to drop, the blood pressure is maintained at stable values. We advise slow injection of the heparin bolus and special attention to the ionized calcium levels before cardiopulmonary bypass.