Chapter Thirteen
Diseases of the
Pulmonary Circulation

The lesser, or pulmonary, circulation differs in many respects from the systemic circulation. The volume of blood directed into a single organ, the lungs, equals that ejected into the aorta for the entire remainder of the body. Pressure in the pulmonary circulation is influenced by many factors, in contrast to pressure in the systemic circulation, which is controlled solely by constriction and relaxation of the arterioles. The arterial pressure in the pulmonary circulation is roughly one-fifth that in the systemic circulation. With these differences it is not surprising that abnormalities affecting the pulmonary circulation are unlike those affecting other regions of the circulation.

The principal significance of disturbances of the pulmonary circulation is that they impose an increased workload on the right ventricle of the heart, which may produce its hypertrophy and failure. (Similar effects on the left side of the heart result from problems of the systemic circulation, as we have seen.) Such overload within this circuit is caused primarily by two mechanisms: increased blood flow to the lungs and increased pressure within the pulmonary circulation. Increased blood flow is a characteristic feature of left-to-right shunts, occurring almost exclusively in congenital heart disease (see chapter 11). Pulmonary hypertension is the primary problem in disorders of the pulmonary circulation. It is of such magnitude and complexity that it will be presented here in its

entirety at the risk of repeating some points already made in previous chapters.

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.

Diseases Affecting the
Pulmonary Circulation

Pulmonary Embolism

Perhaps the most serious and unpredictable condition affecting the pulmonary circulation is pulmonary embolism. It consists of occlusion of sections of the pulmonary arterial tree by thrombi carried in the bloodstream from the venous part of the circulation or from the right side of the heart. The sites of formation of thrombi vary. Whereas inflammatory diseases of the veins are usually associated with thrombosis (thrombophlebitis), pulmonary embolism occurs much more frequently in veins not affected by inflammation (phlebothrombosis). It is known that in susceptible persons thrombi can form in veins, particularly during periods of inactivity. The veins most commonly affected by "silent" thrombosis are deep veins of the legs (not varicose veins) and veins in the pelvic organs (reproductive organs in women and the prostate in men). Because inactivity is an important cause of such thrombi,

venous thrombosis and pulmonary emboli are particularly likely to develop in people who are bedridden.

Several types of pulmonary embolism can be distinguished. In pulmonary infarct a small single embolus affects a small segment of a lung, which becomes clogged with blood. This relatively benign condition may cause chest pain, spitting up of blood, and some fever; it appears on the chest X ray as a shadow similar to that of pneumonia. Prompt recovery usually occurs in a single infarct without sequelae. The significance of pulmonary infarct is as a predictor of larger, more serious emboli developing from the source of the small embolus.

A large pulmonary embolism or multiple emboli can cause overloading of the pulmonary circulation. When a certain area of the pulmonary arterial tree (usually more than half) is occluded by clots, pressure in the pulmonary artery rises. The sudden onset of pulmonary hypertension often does not give the heart time to adapt and rapidly leads to right ventricular failure. Such an event, occasionally referred to as acute cor pulmonale, can be fatal, but it may reverse itself when pulmonary emboli are no longer forthcoming, either because of cessation of thrombosis or in response to surgical intervention. Patients with acute cor pulmonale are short of breath, may be in shock, and show signs of right ventricular failure.

Massive pulmonary embolism occluding the main pulmonary artery or the two principal branches causes death, usually instantly and without warning. It is the prevention of this dreaded complication that has placed so much emphasis on signs of any disturbance of the venous circulation.

Small, repeated pulmonary emboli may feed the pulmonary circulation over a period of weeks, months, or years, causing gradual elevation of pressure in the pulmonary artery and leading to chronic, irreversible pulmonary hypertension. Fortunately uncommon, this severe, usually fatal form of pulmonary hypertension is almost indistinguishable from primary pulmonary hypertension.

Treatment of pulmonary embolism mainly involves anticoagulant therapy to prevent recurrences of emboli. Acute cases may call for thrombolytic therapy. Interventional therapy includes surgical removal of clots, located by angiography, from the pulmonary artery or its principal branches. Various methods of occluding the inferior vena cava in cases where emboli are thought to arise in the

lower part of the body are occasionally used. Occlusion blocks the pathway for emboli and forces the blood to return to the heart through small collateral veins.

Cor Pulmonale

The term "cor pulmonale" literally means heart disease related to the lungs or pulmonary circulation; it should logically include all disturbances of the pulmonary circulation. However, in current medical terminology the term "chronic cor pulmonale" is used exclusively to indicate the cardiac effect of chronic diseases of the lungs. "Acute cor pulmonale" has already been mentioned as referring to large pulmonary embolism, but there are some who question any use of the term "cor pulmonale" not connected with diseases of the lungs.

The essential feature of chronic cor pulmonale is pulmonary hypertension caused most frequently by hypoxemia. Thus cor pulmonale occurs when some parts of the lungs remain unventilated, though their blood supply is unimpaired, as in the late stages of pulmonary emphysema, pulmonary fibrosis, and some varieties of chronic bronchitis. Other, rarer conditions producing chronic hypoxemia include chronic mountain sickness (an exaggerated response in some persons to low oxygen content at high altitude) and extreme obesity. These two conditions are of course reversible— chronic mountain sickness by transferring the patient to a location at sea level, obesity by dieting. As a rule, however, chronic cor pulmonale is a serious condition indicating the late stages of lung disease. Treatment is limited to controlling heart failure and is usually ineffective.

Primary Pulmonary Hypertension

A rare, severe disease, primary pulmonary hypertension usually affects young adults and occasionally children; its cause is unknown. The onset of the disease is inconspicuous and its progress very slow, causing excessive fatigue and shortness of breath. Patients may live for several years partly incapacitated, but they eventually develop intractable heart failure. Treatment has little to offer: various drugs used for essential hypertension have been tried, but success, if any,

is only of short duration. In cases where there is even a suspicion that pulmonary hypertension may have been caused by repeated small emboli, anticoagulant therapy may arrest the progress of the disease, though significant improvement is uncommon.

Other Diseases Associated with
Pulmonary Hypertension

The conditions discussed above cause pulmonary hypertension, which is their primary link with heart disease. There are a number of diseases of the heart in which pulmonary hypertension develops secondarily and is only one of several factors affecting the heart. They include congenital heart disease, mitral stenosis and mitral insufficiency, and chronic failure of the left ventricle of the heart. These conditions may produce all three types of pulmonary hypertension—passive, hyperkinetic, and that related to high pulmonary vascular resistance. However, the first two types are less likely to cause severe pulmonary hypertension than the third. Consequently, in ordinary clinical terms the phrase "pulmonary hypertension" is most often applied to elevated pulmonary vascular resistance. Thus pulmonary hypertension in connection with mitral stenosis connotes that in addition to passive elevation of pressure in the pulmonary artery, arteriolar constriction has taken place. In congenital heart disease, separating operable cases with high flow (hyperkinetic) from inoperable cases with high resistance constitutes the basic problem (see chapter 11).