Is a standard regime for anticoagulation with heparin in unstable angina adequate?

To establish the proportion of patients with unstable angina in whom adequate anticoagulation is achieved using a standard regime of intravenous heparin.

A prospective series of 108 Emergency Department attendees over a six-month period with a clinical diagnosis of unstable angina for whom anticoagulation with heparin was prescribed were included in the study. The standard regime was a 5000 unit bolus followed by an intravenous infusion of 1000 units per hour (1200 units if the patient's weight was greater than 80 kg), with subsequent adjustments being made by reference to a nomogram. The activated partial thromboplastin time (APTT) was measured at six and 12 hours after treatment began. Two commonly used criteria for adequate heparinisation were compared: 1. APTT greater than 1.5 times control and 2. APTT in the range of 60-85 seconds.

There were valid data for 90 patients at six hours and 79 at 12 hours. Compared to the criterion for adequate anticoagulation of APTT greater than 1.5 times the control, 25% of patients were subtherapeutic at six hours and 12% at 12 hours. Compared to the criterion APTT greater than 60 seconds, 53% of patients were subtherapeutic at 6 hours and 47% at 12 hours. At 6 hours, 26% of patients were over-anticoagulated as defined as APTT greater than 85 seconds. This had reduced to 13% by 12 hours.

In the context of recent research suggesting that an APTT of greater than 1.5 times the control is sufficient to reduce complications in unstable angina, our results demonstrate that a standard regime of heparinisation will achieve this goal in the majority of patients within 6 hours of starting heparin therapy. However, if an APTT of 60-85 seconds is the goal, this standard regime is inadequate.