We investigated the plasma concentration of thromboxane B2 (TXB2), a stable metabolite of thromboxane A2 (TXA2), to assess platelet activation in 78 patients who had pulmonary hypertension associated with congenital heart disease (PH group) and 16 patients with almost normal hemodynamics (control group).

The PH group was divided into two subgroups: pulmonary vascular resistance (Rp) < or = 10 U/m2 (Rp < or = 10 group) and > 10 U/m2 (Rp > 10 group). In addition, the Rp < or = 10 group was divided on the basis of clinical symptoms into groups with dyspnea (dyspnea[+] group) and without dyspnea (dyspnea[-] group). Plasma TXB2 levels were measured by radioimmunoassay. Plasma TXB2 levels in the three groups (control, Rp < or = 10, and Rp > 10) were significantly different (P < .005); the TXB2 levels in the Rp < or = 10 group were significantly higher than the others. Among the Rp < or = 10 patients, the plasma TXB2 levels were significantly higher in the dyspnea(+) group than in the dyspnea(-) group (P < .0001). In addition, the pulmonary-to-systemic flow ratio and pulmonary blood flow divided by body surface area were significantly higher in the dyspnea(+) group than in the dyspnea(-) group (P < .02 and P < .002, respectively).

These findings suggest that platelet activation led to increased TXA2 release in patients with pulmonary hypertension, especially those with dyspnea and Rp < or = 10. TXA2 release from platelets probably caused constriction of the pulmonary arterioles and the bronchi, thus worsening pulmonary hypertension and dyspnea in these patients. In the patients with high Rp values, it was considered that the number of pulmonary arterioles where platelets could be activated had been reduced.