Pulmonary Hypertension

Knowledge concerning pulmonary hypertension is relatively new. In contrast to the systemic arterial pressure, which can be measured by a simple apparatus, the sphygmomanometer, pressures within the pulmonary circulation can only be measured during cardiac catheterization or indirectly by Doppler echocardiography. Consequently, a major procedure must be performed each time pressure in the pulmonary artery has to be obtained. Elevated pressure in the pulmonary artery can be caused by one of three principal mechanisms: (1) increased resistance within the pulmonary vascular tree, (2) increased blood flow through the lungs, and (3) increased pressure in the left atrium transmitted back through the pulmonary veins into the pulmonary arterial system.

Pulmonary hypertension owing to increased resistance within the pulmonary circulation may be caused by several factors. Spasm and constriction of the pulmonary arterioles—the only mechanism within the pulmonary circulation analogous to mechanisms of systemic hypertension—occur in a variety of diseases. Great efforts are being made to reveal the circumstances under which pulmonary arterial spasm may be successfully eliminated, restoring more-normal pressure in the pulmonary circulation. Other factors causing increased resistance include organic changes within the arterial tree, closing off some channels; occlusion of smaller branches of the pulmonary artery by emboli; and the sudden blockage of the main pulmonary artery or its principal branches by a large thrombus.

Pulmonary arteriolar spasm leading to pulmonary hypertension may be caused by specific stimuli. Two such stimuli have been discovered, and their role in pulmonary hypertension is considerable: (1) reduced oxygen content in the blood (hypoxemia ), such as occurs at high altitude or in some diseases of the lungs, and (2) increased pressure within the left atrium, characteristic of mitral stenosis and left ventricular failure. Pulmonary arteriolar spasm caused by these stimuli is reversible, and their elimination produces a fall in pulmonary arterial pressure.

Pulmonary hypertension caused by increased blood flow to the

lungs is almost entirely restricted to congenital heart disease. An increase in blood flow within the pulmonary circulation does not automatically raise pressure within this circuit. The lungs have a large vascular reserve capacity that can accommodate excess blood without increasing resistance to flow. Thus in healthy persons during exercise cardiac output increases without raising pressure in the pulmonary circulation. Large shunts due to the various congenital communications between the two sides of the circulation may persist without elevating pressure in the pulmonary circuit. However, for various reasons the reserve capacity of the pulmonary vascular tree may not be available in some cases of congenital heart disease, so that pulmonary arterial pressure rises in direct proportion to increases of pulmonary blood flow. This type of pulmonary hypertension associated with congenital heart disease is called hyperkinetic pulmonary hypertension .

Pressure transmitted backward from the left atrium is termed passive pulmonary hypertension . High left atrial pressure in mitral stenosis and in left ventricular failure dams up the blood flow and forces the right ventricle to eject blood under high pressure. This passive pulmonary hypertension is not to be confused with pulmonary arteriolar spasm, which is also related to rise in pressure in the left atrium and can occur in conjunction with passive elevation of pressure.

Pulmonary hypertension presents problems to the physician different from those related to systemic arterial hypertension. The latter, in addition to causing overload of the left ventricle, may have such secondary serious effects as accelerating atherosclerotic disease of the coronary circulation and other regions and damaging the kidneys and brain. Such secondary effects do not develop in pulmonary hypertension, and its problems concern primarily its effects on the right ventricle. However, the challenge of pulmonary hypertension is that it occurs frequently in children and young adults and produces significant disability earlier than does systemic hypertension. The crucial question involving pulmonary hypertension is its reversibility. Some forms respond well to treatment of their causes. Surgical closure of a left-to-right shunt associated with hyperkinetic pulmonary hypertension brings about an immediate fall in pulmonary arterial pressure, which can be measured during surgery. Similarly, in passive pulmonary hypertension relief of mitral stenosis

producing a fall in left atrial pressure immediately lowers pulmonary pressure. Other forms of reversible pulmonary hypertension are less dramatic and may require more time for a fall in pressure. These include pulmonary arteriolar spasm related to high left atrial pressure and hypoxemia.

Perhaps the most striking point about pulmonary hypertension is that certain reversible varieties turn into permanent and irreversible forms. This transition occurs most frequently in congenital heart disease associated with high pulmonary blood flow and is related to the development of organic diseases of the small pulmonary blood vessels. The importance of this self-perpetuating form of pulmonary hypertension has been discussed in connection with ventricular septal defect and other forms of congenital disease (chapter 11). Since in congenital heart disease pulmonary hypertension is the leading factor in reducing the lifespan of children and young adults, the physician has to be acutely aware that a patient may at one point have a reversible form of pulmonary hypertension and be a candidate for complete surgical cure but a year or two later, if surgical treatment is postponed, may develop an inoperable form because of pulmonary vascular disease.