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C H A P T E R 15 MANAGEMENT OF SHOCK A 42-year-old woman has presented toyour hospital with a severe subarachnoid hemorrhage. She is intubated and unconscious. Her blood pressure is 164(92 with a heart rate 0/62. She tes a very poor urine output, is peripherally mottled and has poor peripheral perfusi&n, What would you do? DAVE NEILIPOVITZ, PIERRE CARDINAL, AND PETER BRINDLEY The patient above is clearly not doing well. Aside from her significantneurologic problems, she appears to have impaired circulation. Is she in shock? John Collins Warren described shock as "... a momentary pause in the act of death/'1 Although this pragmatic definition aptly describes a potential outcome despite therapy, it fails to assist the physician. Indeed, despite the common use of the term shock, it is often poorly understood. Since shock has a high associated morbidity and mortality,it is essential that it be promptly recognized and aggressively treated. The goal of this review is to aid rapid diagnosis and treatment primarilyby understanding the physiology of shock. • Overview of Shock The definition of shock is controversial. Once synonymous with hypotension, it is now clear that this definition is overly simplistic. In addition, in those in whom hypotension is present correctionof the blood pressure does not always reverse shock. Further, while some hypotensive patients may not be in shock, others may be hypertensive, yet be in shock.2 Shock exists whenever oxygen delivery is inadequate to meet tissue oxygen requirements. This is not to say that other physiologic parameters are not important. Blood pressure is important insofar as pressure differences are the forces that drive blood through the vessels. Severe hypoxemia or anemia may also cause tissue ischemia regardless ofpressure or blood flow.3 Failure to promptly correct any of 145 these parameters can result in organ dysfunction and possibly death.4 "7 The commonly accepted classification of shock is from Hinshaw and Cox,8 who in 1972 subdivided shock into four major categories (Table 15.1). Their classification, a modification of what Blalock proposed in the 1930s,9 is sensible, but in practice has limitations especially early when diagnoses are not yet confirmed. Often it is difficult to classify a patient in a specific category and multiple categories ofshock coexist. For example, a septic patient may have an element of myocardial impairment10 '12 and relative hypovolemia, as well as a distributive problem. Early management is often not guided by the different categories of shock. Thus, although it is important to identify the type of shock as it helps us to refine our therapy, it should not delay the initiation of therapy. An alternative to the categorical approach to shock is to use a physiologic approach to simplify early management. Essentially,the approach isto optimize blood flow, blood pressure and oxygen content to ultimately improve tissue oxygen delivery. of various cellular metabolic by-products and other vasoactive agents produced by the endothelium. B. Blood Pressure Regulation Just as blood flow is precisely regulated to meet the metabolic needs of the various tissues, cardiac output is in large part regulated by the sum of all local tissue flows. When localvasodilatation increases tissue perfusion,, this immediately increases venous return, which the heart in turn is able to eject. However, the nervous response also assists the heart and circulation to make changes in blood pressure that are independent of blood flow and to better respond to changing metabolic needs especially in times of stress. For example, a small reduction in blood pressure activates the sympathetic nervous system, which increases heart rate and contractility and causes vasoconstriction of arterioles and, more importantly, venoconstriction. The increased sympathetic discharge thus restores blood pressure and flow but also redirects blood flow to more vital organs. • Basic Physiology A. Blood Flow Blood flow is tightly regulated to meet the metabolic needs ofeach tissue and organ.13 This occurs through local vasodilatation or vasoconstriction in response to increased or decreased local needs. Whenever the metabolic needs of a specific organ increase, blood flowincreases through localvasodilatation; whenever metabolic needs decrease, blood flow decreases through local vasoconstriction. This tight regulation of local blood flow is mediated by the local release • Management The therapy of shock, however crude, attempts to mimic the finely tuned physiological response of the heart and circulation to meet the body's metabolic needs. The goals of therapy are therefore twofold: first,to restore the balancebetween metabolic supply and demand; arid second, to avoid hypotension. Restoring the balancebetween metabolic supply and demand...

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