[Inhibition of the heparin effect by nitroglycerin]

The interaction of intravenous (i.v.) nitroglycerin (glyceryl trinitrate) on the anticoagulant effect of heparin was studied in 27 patients. The heparin dose was adjusted (800-1400 IU/h) to achieve partial thromboplastin time (PTT) of more than 100 s (130 +/- 28 s). While the heparin infusion was continued at the same dosage, the patients received 2-5 mg/h nitroglycerin i.v. (Perlinganit without propylene glycol [15 patients] or Gilustenon with propylene glycol [12 patients]). During combined heparin and nitroglycerin (NG) administration PTT decreased significantly (60 +/- 23 s; P less than 0.01). After withdrawal of NG, PTT increased to the initial levels (126 +/- 30 s). The drug interaction was seen in both groups--with or without propylene glycol. In nine of the 27 patients plasma heparin levels were measured; they remained unchanged during NG administration. The results indicate that i.v. NG induces heparin resistance. After NG is withdrawn, a rebound increase in PTT may occur. Adequate monitoring of PTT and heparin dosage adjustment are thus required during combined heparin and NG administration.