Effects of sildenafil on platelet activation of patients with SCD and PAH. The lines connect the percentages of activated platelets of 12 patients before and after (off-on) a dose of sildenafil. The percentage of activated platelets decreased after sildenafil from 30.6% (± 4.4%) to 17.3% (± 3.6%) for activated GPIIb/IIIa (P = .01, paired t test), and from 6.3% (± 1.6%) to 3.2% (± 0.6%) for P-selectin (P = .08).