The pulmonary vascular bed is normally a low-pressure, low-resistance system. When it is involved in disease, either through obliteration of cross-sectional area or less commonly through an increase in tone, the resulting pulmonary hypertension and redistribution of pulmonary blood flow lead to profound changes in cardiac function and pulmonary gas exchange.
Physiological Effects of Pulmonary Hypertension
The right and left sides of the heart are functionally integrated by their anatomical contiguity. There is continuity between their free walls, they share a common wall (the intraventricular septum), and they are covered by the pericardium. When pulmonary vascular resistance is normal, the right ventricle serves as a capacitance chamber, performing only minimal contractile work. It compensates ineffectually for acute rises in pulmonary artery pressure and acutely can only generate a mean pressure of 40 mm Hg. Acute elevations of right ventricular pressure also interfere with left ventricular performance, presumably owing to a shift in the intraventricular septum to the left which decreases left ventricular compliance. Chronic elevations of pulmonary artery pressure cause gradual hypertrophy of the right ventricle, which eventually allows it to generate pressures equal to those in the left ventricle.
Abnormalities of pulmonary function in patients with pulmonary vascular disease are usually a consequence of the underlying lung disease rather than an intrinsic effect of the pulmonary vascular disease. An exception is the decreased diffusing capacity due to capillary obliteration. In addition, pulmonary vascular occlusion and obliteration cause shunt and ventilation-perfusion inequality by undefined mechanisms. The resulting hypoxemia is further exaggerated by the associated reduction of cardiac output and low mixed venous Po2.
CAUSES OF PULMONARY
Table 1 categorizes the causes of pulmonary hypertension by the underlying pathophysiological mechanism. Hyperkinetic and post capillary disorders cause increased pulmonary pressures indirectly when cardiac abnormalities produce an increase in pulmonary blood flow and outﬂow pressure, respectively. Reactive pulmonary hypertension is due predominantly to hypoxic vasoconstriction. This chapter will focus on two disorders that typify primary involvement of the pulmonary vessels one acute, pulmonary embolism, and one chronic, primary pulmonary hypertension.
Table 1. Pulmonary Hypertension
A great variety of substances may embolize to the pulmonary vascular bed with the resulting clinical presentation dependent on the composition of the embolic material. Talc granules and cotton fibers injected along with illicit drugs, sickled red blood cells, and blood-borne parasites like schistosomes lead to slowly progressive disease clinically similar to primary pulmonary hypertension. Embolization of fat, air, or amniotic ﬂuid, however, alters alveolar capillary membrane integrity and presents as the adult respiratory distress syndrome. The consequences of the most common embolic material, thromboemboli, depend on the amount of clot reaching the lung and the cardiopulmonary status of the patient. It may vary from a persistent tachycardia or mild dyspnea to cardiopulmonary arrest. Thromboemboli directly or indirectly cause 200,000 deaths per year.
Most thromboemboli originate in the iliofemoral deep veins. Less common sources include the vessels below the knee, pelvic veins, upper extremities, and mural thrombi in the right side of the heart. Stasis and intimal injury are important factors in the development of thrombosis. The clinical diagnosis of deep venous thrombosis is highly inaccurate, as physical examination is often misleading, and specialized diagnostic techniques should be utilized whenever it is suspected (Table 2).
Table 2. Methods of diagnosing deep venous thrombosis
Dr. Afsaneh Jeddi