The activated clotting time loading dose response ratio (ACTLORR) as an indicator of heparin demand during cardiopulmonary bypass.

Data relating to the activated clotting time response to a 4 mg/kg heparin loading dose were collected prospectively in 358 patients having cardiopulmonary bypass. After excluding patients with factors known to cause relative heparin resistance or sensitivity, the activated clotting time (ACT) loading dose response ratio (ACTLORR) was calculated retrospectively in 263 patients and found to correlate significantly (p = 0.0001) with the need for extra heparin administration during bypass. Where the ACTLORR was above 5.5, 92% of patients required no additional heparin during the first 90 minutes of bypass (n = 98). Where the ACTLORR was between 4.0 and 5.0, it was far less predictive, with approximately 35% of patients requiring additional heparin. This study indicates that a large ACT response to the initial heparin loading dose (a high ACTLORR) is predictive of stable, adequate anticoagulation during the first 90 minutes of bypass, but that a low initial response is not necessarily associated with declining ACTs and the need for additional heparin administration.