Lack of in vitro interaction between heparin and nitroglycerin

Nitroglycerin has been reported to reduce activated partial thromboplastin time (aPTT) values in patients treated with concurrent heparin and nitroglycerin. However, in vivo studies have yielded conflicting results. In this in vitro evaluation, nitroglycerin was added to samples of pooled plasma from normal volunteers in concentrations of 0, 1, 10, 50, 100, 150, and 200 ng/mL. Preservative-free heparin was then added to the samples to produce final concentrations of 0, 0.3, and 0.6 U/mL. Activated partial thromboplastin time (aPTT) was determined for each sample using a single reagent. There were no significant differences in aPTT values among increasing nitroglycerin concentrations for any of the three levels of heparinization. No direct effect of nitroglycerin on the anticoagulant effect of heparin was observed, as measured by aPTT.