Activated coagulation time in monitoring heparinized dogs.

Heparin was administered to normal dogs either IV or subcutaneously and the anticoagulant effects were monitored by determining prolongation of activated partial thromboplastin time (APTT) and activated coagulation time (ACT). Significant correlation was found between ACT and log APTT for all heparin doses. Intravenously administered heparin was rapidly cleared, with the anticoagulant effects lasting only 2 to 3 hours. One hour after IV heparin at dose levels of 100 and 200 U/kg was given, the APTT were prolonged to 1.8 and 2.8 X base-line values, respectively, whereas the ACT were prolonged to 1.4 and 1.6 X base-line values, respectively. Subcutaneously administered heparin was rapidly absorbed, approaching maximal anticoagulation by 2 hours. Thereafter, a fairly stable plateau of anticoagulation was maintained for 4 to 14 hours depending on dose. Two hours after administration of heparin dosages of 250, 500, 750, and 1,000 U/kg, the dog's APTT values were prolonged by 1.6, 2.4, 3.1, and 3.2 X base line, respectively. Corresponding values for ACT revealed prolongations of 1.3, 1.4, 1.6, and 1.7 X base line. Considerable variation in APTT response to a given dose of heparin was noted, particularly in dogs given the higher heparin doses. Subcutaneous heparin administration at a dosage of 500 U/kg maintained an APTT target value of 1.5 to 2.5 X base line for 8 hours. On the basis of the regression line equation, ACT = -7.2 + 73.5 log APTT, ACT values corresponding to the APTT target value ranged from 92.2 to 108.5 s (1.23- to 1.45-fold base line). Monitoring anticoagulation in dogs given subcutaneous heparin is recommended at 2 hours to evaluate maximal anticoagulation effects. The inexpensiveness, technical simplicity, and linear response to heparin support the use of ACT in monitoring heparinized dogs.