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Introduction

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Cardiovascular failure, or shock, can be caused by (1) depletion of the vascular volume, (2) compression of the heart or great veins, (3) intrinsic failure of the heart itself or failure arising from excessive hindrance to ventricular ejection, (4) loss of autonomic control of the vasculature, (5) severe untreated systemic inflammation, and (6) severe but partially compensated systemic inflammation. If the shock is decompensated, the blood pressure or the cardiac output will be inadequate for peripheral perfusion; in compensated shock, the perfusion will be adequate but only at the expense of excessive demands on the heart. Depending on the type and severity of cardiovascular failure and on response to treatment, shock can go on to compromise other organ systems. This chapter discusses the cardiovascular and pulmonary disorders associated with shock.

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Hypovolemic Shock

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Diagnosis

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Hypovolemic shock (shock caused by inadequate circulating blood volume) is most often caused by bleeding but may also be a consequence of protracted vomiting or diarrhea, sequestration of fluid in the gut lumen (eg, bowel obstruction), or loss of plasma into injured or burned tissues. Regardless of the etiology, the compensatory responses, mediated primarily by the adrenergic nervous system, are the same: (1) constriction of the venules and small veins in the skin, fat, skeletal muscle, and viscera with displacement of blood from the peripheral capacitance vessels to the heart; (2) constriction of arterioles in the skin, skeletal muscle, gut, pancreas, spleen, and liver (but not the brain or heart); (3) improved cardiac performance through an increase in heart rate and contractility; and (4) increased sodium and water reabsorption through renin-angiotensin-aldosterone as well as vasopressin release. The result is improved cardiac filling, increased cardiac output (both directly by the increase in contractility and indirectly through increased end-diastolic volumes), and increased blood flow to organs with no or limited tolerance for ischemia (brain and heart).

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The symptoms and signs of hypovolemic shock are many and can be caused either by the inadequate blood volume or by the compensatory responses. Some manifest themselves early, some late. One of the earliest signs is that of postural hypotension—a fall in the systolic blood pressure of more than 10 mm Hg that persists for more than 1 minute when the patient sits up. It can be very useful in patients who are suspected of being hypovolemic from either dehydration or occult internal blood loss (eg, in a patient who might have gastrointestinal bleeding). Other signs will have to be used, however, in very ill patients and in injured patients, who might not tolerate changes in position.

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Another early sign can make itself known when the physician has difficulty in establishing intravenous access. In addition, the skin might be cold and pale. The pallor, which can be detected in all patients, including those with deeply pigmented skin, is best detected by compressing a toe to produce blanching on ...

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