Bioequivalence and Pharmacokinetics of Intravenous Calcium during Cesarean Delivery

Background: Few studies have assessed the dose ratio of calcium gluconate to calcium chloride or defined the time course of change in serum ionized calcium concentration after intravenous injection. Methods: In a bioequivalence (dose ratio) trial, parturients undergoing cesarean delivery were randomly assigned to receive calcium chloride (0.5 g) or calcium gluconate (1.5 or 2 g) by 10-min intravenous infusion. Venous serum ionized calcium concentration was measured before calcium infusion and approximately 5, 10, 15, 30, and 60 min after infusion start. These data were combined with serum ionized calcium concentration measurements in parturients who received 1 g calcium chloride or saline placebo in two recent clinical trials to define the pharmacokinetics of intravenous calcium over the first hour during and after drug administration. Results: The bioequivalence study enrolled 34 participants, from whom were collected 181 serum ionized calcium concentration measurements. The dose ratio of calcium gluconate to calcium chloride was 3.11 (95% CI, 2.77 to 3.48). Population pharmacokinetics of calcium were determined using 311 serum ionized calcium concentration measurements from 70 parturients. The pharmacokinetics of intravenous calcium were described by a two-compartment model with systemic clearance of 0.18 (95% CI, 0.07 to 0.27) l/min, distributional clearance of 1.25 (95% CI, 1.03 to 1.56) l/min, central volume of 10.9 (95% CI, 9.3 to 12.6) l, and peripheral volume of 16.5 (95% CI, 12.5 to 24.7) l. After adjusting for the dose ratio, calcium gluconate and calcium chloride had identical time courses. A 1-g infusion of calcium chloride resulted in a peak increase in serum ionized calcium concentration of 0.39 (0.38 to 0.42 mM), which decreased by half (29 [23 to 40] min) after initiation of the 10-min infusion. Conclusions: This study confirmed a 3:1 dose ratio of calcium gluconate to calcium chloride and estimated the pharmacokinetics over the first hour after intravenous delivery. These data inform clinical care and may guide future trials assessing calcium efficacy to reduce bleeding in obstetric patients.