Aspirin unresponsiveness predicts thrombosis in high-risk pediatric patients after cardiac surgery.

OBJECTIVE: Thrombosis occurs in up to 26% of patients with congenital heart disease after cardiac surgery and is associated with increased morbidity and mortality. Aspirin is commonly administered to reduce the risk of thrombosis, yet aspirin responsiveness is rarely assessed. In this study, we hypothesize that inadequate response to aspirin is associated with increased risk of thrombosis after selected congenital cardiac procedures considered to be high risk for thrombosis.

METHODS: Patients undergoing high-risk congenital cardiac surgery who received postoperative aspirin (N = 95) were studied. Response to aspirin was determined using the VerifyNow system several days after administration. Patients were monitored prospectively for 30 days for the development of a thrombosis event and the relationship between aspirin unresponsiveness and a thrombosis event was determined by the Fisher exact test.

RESULTS: Rate of aspirin unresponsiveness (≥550 aspirin reaction units [ARU]) was 10 of 95 (10.5%) and was highest in patients weighing less than 5 kg given 20.25 mg/d of aspirin. Thrombosis events occurred in 7 patients (7.4%). Thrombosis was observed in 6 of 10 (60%) patients who were unresponsive to aspirin, compared with 1 of 85 (1.2%) patients who were responsive to aspirin (P < .001). In 2 patients who were unresponsive to the initial aspirin dose, an increase in dose resulted in an adequate therapeutic aspirin response (ARU < 550), suggesting insufficiency rather than true resistance in a subset of patients.

CONCLUSIONS: Postoperative thrombosis is associated with aspirin unresponsiveness in this patient population. In high-risk patients, monitoring of aspirin therapy and consideration of dose adjustment or alternative agents for unresponsive patients may be justified and warrants further investigation in a prospective trial.