Inhaled Nitric Oxide Inhibits Platelet Aggregation after Pulmonary Embolism in Pigs

BACKGROUND: Inhaled nitric oxide (NO) is reported to prolong bleeding time in animals and humans and to inhibit platelet aggregation in persons with acute respiratory distress syndrome. In pulmonary embolism (PE), inhibition of platelet aggregation appears useful because further thrombus formation may lead to right ventricular dysfunction that results in circulatory failure. In the present study, the effect of inhaled NO on platelet aggregation after acute massive PE was investigated. METHODS: After acute massive PE was induced in 25 anesthetized pigs by injecting microspheres, 5, 20, 40, and 80 parts per million inhaled NO were administered stepwise for 10 min each in 11 animals (NO group). In the control group (n = 14). NO was not administered. Adenosine diphosphate-induced initial and maximal platelet aggregation were measured before PE (10), immediately after induction of PE (PE), at the end of each 10-min NO inhalation interval (t10-t40), and 15 min after cessation of NO inhalation (t55) in the NO group, and at corresponding times in the control group, respectively. RESULTS: Two animals in the control group and one in the NO group died within 10 min after PE induction and were excluded from analysis. Peaking at t40 and t55, respectively, initial (-13 +/- 6%; P < 0.05) and maximal (+44 +/- 17%; P < 0.05) platelet aggregation increased significantly after PE in the control group. In contrast, NO administration after PE led to a significant decrease in initial (maximum decrease, -9 +/- 3% at t40; P < 0.05) and maximal (maximum decrease, -15 +/- 7% at t30; P < 0.05) platelet aggregation. In the NO group, platelet aggregation had returned to baseline levels again at t55. In addition, NO administration significantly decreased mean pulmonary artery pressure and significantly increased end-tidal carbon dioxide concentration and mean systemic blood pressure. CONCLUSIONS: Inhaled NO has a systemic and rapidly reversible inhibitory effect on platelet aggregation after acute massive PE in pigs. This may be beneficial in treating acute massive PE.