Pulmonary hypertension is a feature common to many acute and chronic lung diseases. Acute pulmonary hypertension may be reversible if the provoking stimulus is removed, whereas chronic pulmonary hypertension is often irreversible owing to fixed structural changes in the pulmonary arterial wall ( Fig 1). Mechanical factors, including vasoconstriction due to hypoxemia or mediators, passive pulmonary venous hypertension, and luminal thrombosis, are primarily involved in acute pulmonary hypertension. In chronic pulmonary hypertension, remodeling of pulmonary vessels occurs with medial hypertrophy, intimal proliferation, fibrosis, and more complex changes such as plexiform lesions resulting in narrowing of the vascular lumen and decreased distensibility of the vascular wall. Altered hemodynamics due to increased blood flow or pressure, and vascular injury, as a response to inflammation or direct toxic effects of drugs, are two important factors involved in vascular remodeling. This remodeling process is reversible if the hemodynamic stress is reduced in the presence of still intact endothelial cells. Studies in the 1960s demonstrated that correction of mitral valve stenosis or surgical banding of pulmonary arteries in congenital pulmonary hypertension was associated with a regression of medial hypertrophy.
Fig. 1 CT scan of a patient with chronic pulmonary hypertension. Note the abrupt and extensive reduction in the diameters of the pulmonary arteries as a result of the remodeling of pulmonary vessels.
Therapeutic considerations for congenital or acquired cardiac malformations associated with high pulmonary flow or pressure should include early surgical correction of these malformations. However, heart-lung or lung transplantation often represents the only therapeutic option in some of these patients, as well as in others suffering from severe idiopathic pulmonary hypertension. In the interim period, these patients have to be treated with vasodilators since right heart failure is the predominant cause of death in this pathophysiological constellation. Pharmacological interventions are also necessary in diseases associated with acute pulmonary hypertension, since right ventricular dysfunction due to increased afterload may complicate acute respiratory distress syndrome, cardiac surgery, and heart transplantation.
Under normal physiological conditions, the functional significance of the right ventricle appears minimal; it acts more or less as a passive conduit responsible for accepting venous blood and propelling it through the pulmonary system to the left ventricle. However, development of sudden pulmonary hypertension induces a decrease in right ventricular ejection fraction and increased right ventricular volumes. Provided that right ventricular contractile function is not depressed, the right ventricle may be able to pump sufficiently to maintain a normal cardiac output; in this situation a severely reduced right ventricular ejection fraction indicates right ventricular dysfunction rather than failure. However, if pulmonary hypertension increases further and/or right ventricular contractile function is depressed, the right ventricle may not be able to generate a sufficient output, resulting in right ventricular failure. Moreover, an increased right ventricular end-diastolic volume may cause a left shift of the ventricular septum which, in turn, may alter left ventricular performance. Thus right ventricular failure may influence the systemic circulation and may even limit survival. In addition to these secondary alterations in left ventricular performance, the leftward septal shift may result in an increased wedge pressure, possibly enhancing the development of interstitial pulmonary edema.
Intravenously administered vasodilators used to be the only pharmacological option to treat acute and chronic pulmonary hypertension. However, in the last few years, inhaled nitric oxide (NO) and aerosolized epoprostenol (prostaglandin I 2) have been demonstrated to be suitable alternatives, particularly in the treatment of acute pulmonary hypertension.
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