The clinical onset of heparin is rapid.

This study used the activated clotting time (ACT) to determine the clinical onset of four different doses of heparin after bolus injection into the central circulation. Ten consenting adults (Group A) undergoing coronary artery bypass grafting were given 350 U/kg of bovine lung heparin and had simultaneous duplicate arterial and venous ACT determinations at baseline and at 30, 60, 90, 120, 180, and 600 s after heparin injection. Twenty additional coronary artery bypass grafting patients were alternately assigned to one of two 10-patient groups (B and C), which were given 200 and 300 U/kg of bovine lung heparin, respectively. Group D consisted of 10 abdominal aortic aneurysmectomy patients who received 70 U/kg of bovine lung heparin. In Groups B, C, and D, duplicate ACT measurements were taken from an indwelling arterial catheter at baseline and at 30, 60, 90, 120, 180, and 300 s after completion of a bolus injection of heparin into the central circulation. After a 70 U/kg heparin dose, all patients had significant ACT prolongation within 30 s, and 8 of 10 had effectively achieved their peak anticoagulation response by that time. In all patients receiving 200, 300, and 350 U/kg of heparin, arterial anticoagulation (ACT > 300 s) occurred and in most patients peaked within 30 s after heparin administration (P < 0.05). Arterial and venous ACTs did not differ significantly from each other at any measurement period, but venous ACTs peaked slightly later than arterial ACTs (within 60 s in 9 of 10 patients). When 200 U/kg or more of heparin is administered into the central venous circulation in hemodynamically stable anesthetized patients, peak arterial ACT prolongation occurs within 30 s and peak venous ACT prolongation within 60 s.