Pathological basis of failure of concurrent glyceryl trinitrate therapy to improve efficacy of tissue type plasminogen activator in coronary thrombosis.

Study Objective: The aim was to examine the effect of glyceryl trinitrate, a potent coronary vasodilator, on the thrombolytic effects of tissue type plasminogen inhibitor (t-PA) in dogs with coronary thrombosis.
Design: The thrombus was formed by delivery of anodal direct current into the left anterior descending coronary artery. Fourteen dogs were randomly given t-PA alone (0.75 mg.kg-1 over 20 min) or t-PA with glyceryl trinitrate (125 micrograms.min-1 for 40 min). In four other dogs, glyceryl trinitrate was given after t-PA induced thrombolysis. Its effect on t-PA induced thrombolysis, in terms of reperfusion rate, time to thrombolysis, peak coronary blood flow, and reocclusion rate, was quantitated. Peripheral blood platelet counts and whole blood platelet aggregation were measured in all dogs. The thrombosed left anterior descending artery and normal circumflex coronary artery segments were examined by scanning electron microscopy at the end of the experiment.
Measurements and Main Results: The reperfusion rate in the t-PA plus glyceryl trinitrate (t-PA + GTN) group was 57% (4/7 dogs) and with t-PA alone, 71% (5/7 dogs). The time to thrombolysis in the t-PA + GTN group was greater than with t-PA alone, at 30(SD 10) v 18(7) min, p less than 0.02, and the duration of reperfusion much shorter, at 11(17) v 48(15) min, p less than 0.02. Peak coronary blood flow in the t-PA + GTN group following reperfusion was less compared with t-PA alone, at 36(52) v 53(18) ml.min-1, p less than 0.02. Reocclusion rates in the two groups were similar. Peripheral blood platelet counts decreased during thrombus formation in all dogs; this decrease stabilised when t-PA was given alone but not when it was given with glyceryl trinitrate [mean platelet count at the end of t-PA infusion 7.23(1.68) and 4.78(3.00) X 10(8).ml-1 respectively, p less than 0.02], suggesting continued sequestration of platelets in the intracoronary thrombus in the latter group. Whole blood platelet aggregation decreased significantly with t-PA alone, but less so with t-PA + GTN [magnitude of platelet aggregation 0.23(0.57) and 5.67(6.23) ohms, respectively, p less than 0.02], suggesting lower plasma concentrations of t-PA when given with glyceryl trinitrate. Glyceryl trinitrate given after thrombolysis induced by t-PA failed to sustain reperfusion. Scanning electron microscopy of occluded left anterior descending artery showed extensive endothelial injury and a thrombus composed of platelet--red blood cells--fibrin mesh. The reperfused left anterior descending artery showed extensive endothelial injury and residual thrombus consisting mostly of fibrin and red blood cells with some platelets.
Conclusion: Glyceryl trinitrate given concurrently with t-PA or after t-PA induced thrombolysis does not modify the thrombolytic potential of t-PA. The potentially "detrimental" effects of glyceryl trinitrate may be due to increase in hepatic blood flow and subsequent enhanced catabolism of t-PA. Lack of any significant effect of therapeutic dosage of glyceryl trinitrate on platelet--fibrin rich thrombus may explain the absence of a salutary action of this agent. Dynamic coronary vasoconstriction does not play an important role in coronary reocclusion after initial thrombolysis.