Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

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ventricular diastolic dysfunction. In each of these altera- tions, the elevated left atrial pressure is transmitted to the pulmonary circulation. Continued increases in left atrial pressure can lead to medial hypertrophy and intimal thickening of the small pulmonary arteries, causing sustained hypertension. Increased pulmonary blood flow results from increased flow through left-to-right shunts in congenital heart diseases such as atrial or ventricular septal defects and patent ductus arteriosus. If the high-flow state is allowed to continue, morphologic changes occur in the pulmonary vessels, leading to sustained pulmonary hypertension. The pulmonary vascular changes that occur with congenital heart disorders are discussed in Chapter 19. Obstruction of pulmonary blood flow is commonly due to pulmonary thromboemboli. Persons who are promptly treated for acute pulmonary thromboembolism with anticoagulants rarely develop pulmonary hyperten- sion. However, in some persons chronic obstruction of the pulmonary vascular bed develops because of impaired resolution of the thromboemboli. Continued exposure of the pulmonary vessels to hypoxemia is a common cause of pulmonary hyperten- sion. Unlike blood vessels in the systemic circulation, most of which dilate in response to hypoxemia and hypercapnia, the pulmonary vessels constrict. The stim- uli for constriction are thought to originate in the air spaces near the smaller branches of the pulmonary arter- ies. In regions of the lung that are poorly ventilated, the response is adaptive in that it diverts blood flow away from the poorly ventilated areas to those areas that are more adequately ventilated. This effect, however, becomes less beneficial as more and more areas of the lung become poorly ventilated. Pulmonary hypertension is a common problem resulting from hypoxemia that develops in persons with advanced COPD or interstitial lung disease. 62 It also may develop at high altitudes in persons with normal lung function. Persons who expe- rience marked hypoxemia during sleep (such as those with sleep apnea) often experience marked elevations in pulmonary arterial pressure. Secondary pulmonary hypertension is difficult to rec- ognize in its early stages, when the signs and symptoms are primarily those of the underlying disease. Pulmonary hypertension may cause, or contribute to, dyspnea, present initially with exertion and later at rest. Fatigue and syncope on exertion also occur, presumably the result of reduced cardiac output and elevated pulmonary artery pressures. Diagnosis is based on radiographic findings and echocardiography. Doppler ultrasonography is a reli- able noninvasive method for estimating pulmonary artery systolic pressure. Treatment measures are directed toward the underlying disorder. Vasodilator therapy may be indicated. Primary Pulmonary Arterial Hypertension Primary pulmonary arterial hypertension (PAH) is per- sistent elevation in pulmonary artery pressure that occurs in the absence of identified cardiopulmonary or other secondary causes of pulmonary hypertension. 10,15,16,63–66

insertion of a filter to prevent emboli from traveling to the lung may be indicated in life-threatening situations or in cases where thrombolytic therapy or anticoagulation is contraindicated. Venous thromboembolism is often clinically silent until it presents with significant morbidity and mortal- ity. Thus, recognition of risk factors and appropriate pre- ventative treatment are essential. Prophylactic measures include early ambulation for postoperative and postpar- tum patients and the use of graded compression elastic stockings and intermittent pneumatic compression (IPC) boots for bedridden patients. Intermittent pneumatic compression boots provide intermittent inflation of air- filled sleeves that prevent venous stasis. Some devices pro- duce sequential gradient compression that moves blood upward in the leg. Anticoagulant therapy may be used to decrease the likelihood of deep vein thrombosis, throm- boembolism, and fatal pulmonary embolism after major surgical procedures (see Chapter 12). Low–molecular- weight heparin, which can be administered subcutane- ously, often is used. Warfarin, an oral anticoagulation drug, may be used for persons with a long-term risk for development of thromboemboli. Newer factor Xa inhibi- tors are gaining popularity as an alternative to warfarin due to their efficacy and fewer food and drug interactions. Pulmonary Hypertension The pulmonary circulation is normally a low-pressure system designed to accommodate varying amounts of blood delivered from the right heart. The main pulmonary artery and major branches are relatively thin-walled, compliant vessels. The distal pulmonary arterioles also are thin walled and have the capacity to dilate, collapse, or constrict depending on the presence of vasoactive substances released from the endothelial cells of the vessel, neurohumoral influences, flow veloc- ity, oxygen tension, and alveolar ventilation. Pulmonary hypertension is a disorder characterized by an abnormal elevation of pressure within the pulmonary circulation—namely, the pulmonary arterial system. 62,63 Once present, pulmonary hypertension is self-perpetu- ating. It introduces secondary structural abnormalities of pulmonary vessels including smooth muscle hypertro- phy and proliferation of the vessel intima. Although pul- monary hypertension can develop as a primary disorder, most cases develop secondary to other conditions. Secondary Pulmonary Hypertension Secondary pulmonary hypertension refers to an increase in pulmonary pressures associated with other disease conditions, usually cardiac or pulmonary. 10,15,16 Often more than one disorder, such as COPD, heart failure, and sleep apnea, contributes. Secondary pulmonary hypertension may develop at any age. Mechanisms include: (1) elevation of pulmonary venous pressure, (2) increased pulmonary blood flow, (3) pulmonary vascu- lar obstruction, and (4) hypoxemia. Elevation of pulmonary venous pressure is com- mon in conditions such as mitral valve disorders or left