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Chapter Seven Cardiac Emergencies
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Shock

When the blood supply to vital organs is severely reduced, a person goes into shock . The most important manifestation of shock is abnormally low arterial pressure, which is usually caused by one or more of the following mechanisms: depressed function of the cardiac pump, faulty regulation of blood pressure, and sudden loss of blood in the body.

Shock, a variety of cardiocirculatory failure, differs in many respects from heart failure, described in chapter 5. In heart failure the arterial pressure is usually maintained at a normal or near-normal level, and blood supply to vital organs is only slightly impaired; its principal deleterious consequences are related to backing up of the blood in the lungs and systemic veins, sometimes called backward heart failure. In shock, or forward failure, the blood pressure is so low that the pumping force is inadequate to maintain the circulation in various organs. There is an obvious similarity between shock and syncope: the former is partial failure


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of circulatory adjustment, the latter total. Furthermore, shock represents a prolonged state, whereas syncope is of very short duration. Reduced blood supply to the brain in shock often causes extreme fatigue, restlessness, confusion, and even a semicomatose state. Reduced blood flow to the skin affects temperature control: the skin becomes cool, and a cold perspiration is common. Pallor with bluish tinge (cyanosis) may also be observed.

Shock is a persistent state with a low probability of spontaneous improvement. The prognosis of shock varies widely and depends on its cause and the response to treatment. The most common causes of shock are noncardiac; they include severe infection, serious allergic reaction (anaphylactic shock ), serious hemorrhage, intense pain, and catastrophic events in the gastrointestinal tract, such as perforation of an organ.

The least reversible, and hence most serious, form of shock is cardiogenic shock , produced by a sudden reduction in the pumping efficiency of a cardiac ventricle. The commonest cause of cardiogenic shock is myocardial infarction (due either to the initial injury to the heart muscle or to a later major complication). Cardiogenic shock may also develop as a result of sudden severe overload of the heart, such as rupture of a cardiac valve or massive pulmonary embolism. A state similar to shock but not directly related to malfunction of the cardiac muscle is pericardial tamponade , accumulation of fluid in the sac around the heart that compresses the heart and interferes with its pumping (see chap. 10). Shock may also result from rapid ectopic tachycardia.

The prognosis of cardiogenic shock is always guarded. The mortality rate under drug therapy or insertion of an intraaortic balloon pump remains high. Elimination or amelioration of the cause of cardiogenic shock is occasionally possible by surgical means.


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Chapter Seven Cardiac Emergencies
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