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INTRODUCTION

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Shock is a medical condition in which there is decreased perfusion to tissues resulting in injury to the cells. It is characterized by a mismatch between oxygen delivery and oxygen demand. Subsequently, inflammatory markers are released that further hinder tissue perfusion and result in organ failure or death if not immediately rectified. In some situations, correction of the underlying cause may lead to reversal of this state. However, at times it is irreversible and may progress to multiple organ dysfunction syndrome (MODS) and ultimately death.

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CLASSIFICATION

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There are four major types of shocks: hypovolemic, cardiogenic, obstructive, and distributive shock. Nonetheless, shock states can also be mixed wherein they are a combination of any of the shocks previously mentioned. Hypotension is the common factor in all types of shocks but the cause of the hypotension is what differentiates them.

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Hypovolemic shock is a result of insufficient circulating blood volume. Decrease in preload leads to decreased filling of the heart in diastole, resulting in inadequate cardiac output (CO). This in turn leads to hypotension and shock. Hemorrhagic shock is a type of hypovolemic shock.

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Cardiogenic shock is usually due to mechanical failure of the ventricles of the heart. Hence, if the ventricles are having difficulty pumping the blood forward, it backs up and increases the preload. The end-diastolic pressure of the ventricle increases secondary to the increased preload and over distends the heart. The failing ventricles can be the right, left, or both depending on the cause.

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Obstructive shock is usually due to hindrance of flow in the circulation leading to increase in afterload or decrease in diastolic filling, resulting in hypotension. It occurs despite normal circulating volume and myocardial function because the culprit is within the pulmonary or systemic circulation causing obstruction. Examples of obstructive shock include cardiac tamponade and pulmonary embolism.

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Distributive shock is defined by vasodilation leading to drop in preload resulting in hypotension with normal or increased CO. Included in this category are septic shock, anaphylactic shock, and neurogenic shock.

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Mixed shock states occur when two or more processes are present together, for example, cardiogenic shock in septic shock from Takotsubo cardiomyopathy.

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PATHOPHYSIOLOGY

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Although there are many different causes of shock, it ultimately results in hypotension and decreased perfusion. Mean arterial pressure (MAP) is a product of CO and systemic vascular resistance (SVR) (MAP = CO × SVR). Autoregulation of the cerebral and coronary circulation usually occurs over a wide range of blood pressures from MAP 50 to 150 mmHg. However, when the MAP falls below this range, there is extreme compromise of the CO. CO depends on the heart rate (HR) and stroke volume (SV) (CO = HR × SV). SV ultimately depends on preload, afterload, and contractility. When any of these factors are compromised, SV reduces, in turn decreasing ...

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