Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

Circulatory Failure (Shock) Circulatory shock can be described as an acute failure of the circulatory system to supply the peripheral tissues and organs of the body with an adequate blood supply, resulting in cellular hypoxia. 48,49 Most often hypotension and hypoperfusion are present, but shock may occur in the presence of normal vital signs. Shock is not a specific disease but a syndrome that can occur in the course of many life-threatening traumatic conditions or disease states. Moreover, no single classification system exists; rather, shock can be classified by the cause, primary pathophysiological derangement, or clinical manifesta- tions. Generally each type of shock has certain distin- guishing features; nonetheless, all types of shock reflect an imbalance between oxygen supply and demand. As a result, all shock states share common derangements, such as inadequate peripheral tissue perfusion, altera- tions in cellular metabolism and function, and impaired organ perfusion, and all share common compensatory mechanisms in response to these derangements. Pathophysiology of Shock Circulatory failure results in hypoperfusion of organs and tissues, which in turn results in an insufficient sup- ply of oxygen and nutrients for cellular function and the accumulation of waste products. 6 The cellular injury cre- ated by an inadequate delivery of oxygen and substrates also induces the production and release of inflammatory mediators that further compromise perfusion through functional and structural changes within the microvas- cular circulation. This leads to a vicious cycle in which impaired perfusion is responsible for cellular injury, which causes maldistribution of blood flow, further compromising cellular perfusion, and can culminate in irreversible end-organ damage. Cellular Responses Shock ultimately exerts its effect at the cellular level, with failure of the circulation to supply body cells with the oxygen and nutrients needed for production of ATP. Cells require ATP for a number of functions, including opera- tion of the Na + /K + −ATPase membrane pump that moves sodium out of the cell and returns potassium to the inside of the cell. The cell uses two pathways to convert nutri- ents to ATP (see Chapter 1). The first is the anaerobic (non–oxygen-dependent) glycolytic pathway, located in the cytoplasm, which converts glucose to ATP and pyru- vate. The second is the aerobic (oxygen-dependent) path- way, which is located in the mitochondria. When oxygen is available, pyruvate from the anaerobic pathway moves into the mitochondria and enters the aerobic pathway, where it is transformed into ATP and the metabolic by- products carbon dioxide and water. When oxygen is lacking, pyruvate is converted to lactic acid. As a shock state progresses, cellular metabolism becomes anaerobic because of the decreased availability of oxygen. Excess amounts of lactic acid accumulate in

SUMMARY CONCEPTS (continued)

there is impaired ejection of blood from the heart during systole; with diastolic dysfunction, there is impaired filling of the heart during diastole. Left ventricular dysfunction is characterized by congestion in the pulmonary circulation and impaired blood flow in the peripheral circulation, and right ventricular dysfunction by congestion in the peripheral circulation. ■■ The manifestations of heart failure include fluid retention and edema, shortness of breath, fatigue and impaired exercise tolerance, impaired gastrointestinal function and malnutrition, and cyanosis. When performance of the right ventricle is impaired, there is dependent edema of the lower parts of the body, engorgement of the liver, and ascites. With failure of the left ventricle, pulmonary congestion with shortness of breath and chronic, nonproductive cough are common. ■■ The acute heart failure syndromes represent a gradual or rapid change in heart failure signs and symptoms, indicating the need for urgent therapy.These symptoms are primarily the result of pulmonary congestion due to elevated left ventricular filling pressures with or without a low cardiac output. ■■ The diagnostic methods in heart failure are directed toward establishing the cause and extent of the syndrome.Treatment is directed toward correcting the cause whenever possible, improving cardiac function, maintaining the fluid volume within a compensatory range, and developing an activity pattern consistent with individual limitations in cardiac reserve. Among the medications used in the treatment of heart failure are diuretics, digitalis, ACE inhibitors and angiotensin receptor blocking agents, β -adrenergic receptor blockers, vasodilators, and aldosterone blockers. ■■ Among the devices used to treat heart failure patients with a reduced ejection fraction are an implantable cardiac defibrillator and ventricular assist devices. Heart transplantation remains the treatment of choice for many persons with end- stage heart failure. ■■ The manifestations of heart failure in the elderly often are different and superimposed on other disease conditions; therefore, heart failure often is more difficult to diagnose in the elderly than in younger persons. Because the elderly are more susceptible to adverse and toxic medication reactions, medication doses need to be adapted and more closely monitored.