75 Pulmonary Hypertension Bryan Siegel, John Torrey Berger III Keywords Pulmonary Hypertension; Pediatric; Nitric Oxide; Sildenafil; Endothelium; Pulmonary vascular resistance Pulmonary Hypertension Pulmonary hypertension describes a number of different diseases involving the pulmonary vasculature, which have common hemodynamic, histologic, and therapeutic features The hallmark of these diseases is an abnormally high pulmonary vascular resistance The histologic changes such as medial hypertrophy and intimal fibrosis do not vary significantly between different etiologies, whether the cause is idiopathic, hemodynamic, inflammatory, or toxin mediated The hemodynamic and clinical consequences depend not only on the extent of vascular changes but also on the state of the subpulmonary ventricle, which is usually the morphologically right ventricle Currently, pulmonary arterial hypertension is defined as a mean pulmonary arterial pressure of greater than 20 mm Hg at rest, or greater than 30 mm Hg while exercising, with a normal pulmonary capillary wedge pressure less than 15 mm Hg and a pulmonary vascular resistance index of more than 3 Wood units multiplied by total square meters of body surface area The different diseases resulting in increased pulmonary vascular resistance show a spectrum of responses to the currently available specific pulmonary antihypertensive substances, with disease-specific profiles of response beginning to emerge With regard to the individual patient, nonetheless, a correct understanding of the precise pathophysiologic situation in which pulmonary vascular resistance is increased and causes pulmonary hypertension is of crucial importance for the correct commencement and evaluation of efficacy of treatment ...Pulmonary Hypertension Bryan Siegel, John Torrey Berger III Keywords Pulmonary Hypertension; Pediatric; Nitric Oxide; Sildenafil; Endothelium; Pulmonary vascular resistance Pulmonary Hypertension