Pathophysiology

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The total amount of water in a typical 70 kg (154 lb) adult is approximately 42 L (Fig. 13-2). About 28 of the 42 L are inside the cells of the body (intracellular fluid) while the remaining 14 L are in the extracellular fluid space (fluid outside of cells: interstitial fluid and plasma). Circulating blood volume for a normal adult is roughly 5 L (70 mL/kg) and is comprised of 2 L of red blood cell fluid (intracellular) and 3 L of plasma

(extracellular). By definition, hypovolemic shock occurs as a consequence of in adequate intravascular volume to meet the oxygen and metabolic needs of the body. Diminished intravascular volume can result from severe external or internal bleeding, profound fluid losses from GI sources such as diarrhea or vomiting, or urinary losses such as diuretic use, diabetic ketoacidosis, or diabetes insipidus (Table 13-1). Other sources of intravascular fluid loss can occur through damaged skin, as seen with burns, or via "capillary leak" into the interstitial space or peritoneal cavity, as seen with edema or ascites. This latter phenomenon is often referred to as "third spacing" since fluid accumulates in the interstitial space disproportionately to the intracellular and extracellular fluid spaces. Regional ischemia may also develop as blood flow is naturally shunted from organs such as the GI tract or the kidneys to more immediately vital organs such as the heart and brain.

FIGURE 13-2. Distribution of body fluids showing the extracellular fluid volume, intracellular body fluid volume, and total body fluids in a 70 kg (154 lb) adult. Extracellular volume (ECV) comprises 14 L of total body fluid (42 L). Plasma volume makes up approximately 3 L of the 14 L of ECV. Intracellular volume accounts for the remaining 28 L of total body fluids with roughly 2 L being located within the red blood cells. Blood volume (approximately 5 L) is also depicted and is made up of primarily red blood cells and plasma. (From Guyton AC, Hall JE. Textbook of Medical Physiology. 8th ed. Philadelphia: Saunders, 1991: 275, with permission.)

FIGURE 13-2. Distribution of body fluids showing the extracellular fluid volume, intracellular body fluid volume, and total body fluids in a 70 kg (154 lb) adult. Extracellular volume (ECV) comprises 14 L of total body fluid (42 L). Plasma volume makes up approximately 3 L of the 14 L of ECV. Intracellular volume accounts for the remaining 28 L of total body fluids with roughly 2 L being located within the red blood cells. Blood volume (approximately 5 L) is also depicted and is made up of primarily red blood cells and plasma. (From Guyton AC, Hall JE. Textbook of Medical Physiology. 8th ed. Philadelphia: Saunders, 1991: 275, with permission.)

Hypovolemic shock symptoms begin to occur with decreases in intravascular volume in excess of 750 mL or 15% of the circulating blood volume (20 mL/kg in pediatric patients).6 As previously stated, decreases in preload or left ventricular end-diastolic volumes result in decreases in SV. Initially, CO may be partially maintained by compensatory tachycardia. Similarly, reflex increases in SVR and myocardial contractility may diminish arterial hypotension. This neurohumoral response to hypo-volemia is mediated by the sympathetic nervous system in an attempt to preserve perfusion to vital organs such as the heart and brain (Fig. 13-3). Two major endpoints of this response are to conserve water to maximize intravascular volume and to improve tissue perfusion by increasing BP and CO (oxygen delivery). The body attempts to maximize its fluid status by decreasing water and sodium excretion through release of ADH, aldosterone, and cortisol. BP is maintained by peripheral vasoconstriction mediated by catecholamine release and the renin-angiotensin system.5 CO is aug-

mented by catecholamine release and fluid retention. ' However, when intravascular volume losses exceed 1,500 mL (about 3 pints), the compensatory mechanisms are inadequate, typically resulting in a fall in CO and arterial BP, while acute losses greater than 2,000 mL (about 4 pints) are life-threatening (35 mL/kg in pediatric patients). The decrease in CO results in a diminished delivery of oxygen to tissues within the body and activation of an acute inflammatory response.5 Oxygen delivery can be further compromised by inadequate blood hemoglobin levels due to hemorrhage and/or diminished hemoglobin saturation due to impaired ventilation. Decreased delivery of oxygen and other vital nutrients results in diminished production of the energy substrate, adenosine triphosphate (ATP). Lactic acid is then produced as a by-product of anaerobic metabolism within tissues throughout the body. Hyperglycemia produced during the stress response from cortisol release is also a contributing factor in the development of lactic acidosis. Lactic acidosis indicates that inadequate tissue perfusion has occurred.3 Protracted tissue hypoxia sets in motion a downward spiral of events leading to organ dysfunction and eventual failure if untreated.6 Table 13-2 describes the effects of shock on the body's major organs. Relative failure of more than one organ, regardless of etiology, is referred to as the multiple organ dysfunction syndrome (MODS). Involvement of the heart is particularly devastating considering the central role it plays in oxygen delivery and the potential for myocardial dysfunction to perpetuate the shock state. Pre-existing organ dysfunction and build up of inflammatory mediators can also exacerbate the effects of hypovolemic shock to the point of irreversibility.5 For example, acute or chronic heart failure can lead to pulmonary edema, further aggravating gas exchange in the lungs and, ultimately, tissue hypoxia. MODS develops in approximately 20% of trauma patients who require fluid resuscitation. Only about one-third of early-onset MODS is quickly reversible (within 48 hours) with proper fluid resuscitation. Thus, it is imperative that hypovolemic shock be treated quickly to avoid MODS.8

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