Dose-dependent plasma elimination of subcutaneously administered calcium heparin in horses

Pharmacokinetic parameters for subcutaneous low dose heparin in horses have been determined. Four groups of five and one group of eleven mature, healthy horses of various breeds were given single subcutaneous injections of 60, 80, 100, 125, and 150 units of calcium heparin/kg of body weight (U/kg) in the pectoral region. Jugular blood samples were collected prior to, and at hourly intervals for 12 h after injection. Heparin plasma concentrations were measured using a commercially available amidolytic assay. Peak concentrations 4 h after administration were 0.021 +/- 0.016 (mean +/- SD) units of heparin/ml of plasma (U/ml) after 60 U/kg, 0.035 +/- 0.025 U/ml after 80 U/kg, 0.023 +/- 0.004 U/ml after 100 U/kg, 0.034 +/- 0.019 U/ml after 125 U/kg, and 0.053 +/- 0.019 U/ml after 150 U/kg. Data from groups given 60 and 100 U/kg could not be used for kinetic calculations. Elimination constant (l/h), elimination half-life (h), and elimination time (h) calculated to reach base-line values after 80 U/kg were 0.182 +/- 0.041 l/h, 3.8 +/- 0.9 h, and 9.7 +/- 2.2 h. After 125 U/kg, corresponding values were 0.211 +/- 0.019 l/h, 3.3 +/- 0.3 h, 13.4 +/- 1.2 h, and after 150 U/kg 0.098 +/- 0.015 l/h, 7.1 +/- 1.1 h, and 20.6 +/- 3.2 h. Calculated heparin concentrations 12 h after administration of 80, 125, and 150 U/kg were 0.011 +/- 0.002, 0.010 +/- 0.001, and 0.027 +/- 0.004 U/ml.(ABSTRACT TRUNCATED AT 250 WORDS)