the molecular mechanism of pulmonary arterial hypertension (pah) is not known yet, but it is believed that the endothelial dysfunction results in a decrease in the synthesis of endothelium-derived vasodilators such as nitric oxide and prostacyclin. moreover, there is a stimulation of the synthesis of vasoconstrictors such as thromboxane and vascular endothelial growth factor (vegf). these results in a severe vasoconstriction and vascular smooth muscle and adventitial hypertrophy characteristic of patients with pah.