Chapter 046. Sodium and Water (Part 3)

HypovolemiaEtiology True volume depletion, or hypovolemia, generally refers to a state of combined salt and water loss exceeding intake, leading to ECF volume contraction. The loss of Na+ may be renal or extrarenal (Table 46-1).Table 46-1 Causes of HypovolemiaI. ECF volume contractedA. Extrarenal Na+ loss1.Gastrointestinal(vomiting,nasogastricsuction,drainage, fistula, diarrhea)2. Skin/respiratory (insensible losses, sweat, burns)3. HemorrhageB. Renal Na+ and water loss1. Diuretics2. Osmotic diuresis3. Hypoaldosteronism4. Salt-wasting nephropathiesC. Renal water loss1. Diabetes insipidus (central or nephrogenic) ...
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Chapter 046. Sodium and Water (Part 3) Chapter 046. Sodium and Water (Part 3) Hypovolemia Etiology True volume depletion, or hypovolemia, generally refers to a state ofcombined salt and water loss exceeding intake, leading to ECF volumecontraction. The loss of Na+ may be renal or extrarenal (Table 46-1). Table 46-1 Causes of Hypovolemia I. ECF volume contractedA. Extrarenal Na+ loss 1. Gastrointestinal (vomiting, nasogastric suction, drainage, fistula, diarrhea) 2. Skin/respiratory (insensible losses, sweat, burns) 3. HemorrhageB. Renal Na+ and water loss 1. Diuretics 2. Osmotic diuresis 3. Hypoaldosteronism 4. Salt-wasting nephropathiesC. Renal water loss 1. Diabetes insipidus (central or nephrogenic) II. ECF volume normal or expanded A. Decreased cardiac output 1. Myocardial, valvular, or pericardial disease B. Redistribution 1. Hypoalbuminemia (hepatic cirrhosis, nephrotic syndrome) 2. Capillary leak (acute pancreatitis, ischemic bowel, rhabdomyolysis) C. Increased venous capacitance 1. SepsisNote: ECF, extracellular fluid. Renal Many conditions are associated with excessive urinary NaCl and waterlosses, including diuretics. Pharmacologic diuretics inhibit specific pathways ofNa+ reabsorption along the nephron with a consequent increase in urinary Na+excretion. Enhanced filtration of non-reabsorbed solutes, such as glucose or urea,can also impair tubular reabsorption of Na+ and water, leading to an osmotic orsolute diuresis. This often occurs in poorly controlled diabetes mellitus and inpatients receiving high-protein hyperalimentation. Mannitol is a diuretic thatproduces an osmotic diuresis because the renal tubule is impermeable to mannitol.Many tubule and interstitial renal disorders are associated with Na+ wasting.Excessive renal losses of Na+ and water may also occur during the diuretic phaseof acute tubular necrosis (Chap. 273) and following the relief of bilateral urinarytract obstruction. Finally, mineralocorticoid deficiency (hypoaldosteronism)causes salt wasting in the presence of normal intrinsic renal function. Massive renal water excretion can also lead to hypovolemia. The ECFvolume contraction is usually less severe since two-thirds of the volume lost isintracellular. Conditions associated with excessive urinary water loss includecentral diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI). Thesetwo disorders are due to impaired secretion of and renal unresponsiveness to AVP,respectively, and are discussed below.