Renal · Hypovolemia
The facts most likely to be tested
Orthostatic hypotension and tachycardia are the most sensitive physical exam findings for significant intravascular volume depletion.
Fractional excretion of sodium (FeNa) less than 1% indicates prerenal azotemia due to intact tubular reabsorptive capacity.
BUN-to-creatinine ratio greater than 20:1 is a classic laboratory marker of prerenal physiology caused by increased urea reabsorption.
Urine sodium concentration less than 20 mEq/L confirms the kidneys are avidly retaining sodium in response to hypoperfusion.
Isotonic saline (0.9% NaCl) or lactated Ringer's are the preferred initial resuscitation fluids to restore effective circulating volume.
Skin turgor and mucous membrane moisture are unreliable clinical indicators of volume status in the elderly population.
Urine osmolality greater than 500 mOsm/kg reflects the appropriate release of antidiuretic hormone (ADH) to conserve free water during volume depletion.
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A 72-year-old male is brought to the emergency department by his daughter due to lethargy and decreased oral intake over the past 3 days. Physical examination reveals dry mucous membranes, delayed capillary refill, and a blood pressure of 98/60 mmHg that drops to 82/50 mmHg upon standing. Laboratory studies show a serum creatinine of 1.8 mg/dL (baseline 1.0 mg/dL) and a BUN of 45 mg/dL. Urinalysis reveals a FeNa of 0.6% and a urine sodium of 12 mEq/L.
What is the most likely diagnosis and the underlying pathophysiology?
Prerenal azotemia due to hypovolemia.
The patient's clinical presentation of orthostasis combined with a FeNa <1% and a BUN:Cr ratio >20:1 is diagnostic of prerenal azotemia, confirming the kidneys are responding appropriately to volume depletion.
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Etiology / Epidemiology
Common in elderly and diabetics due to GI losses, renal wasting, or insensible losses.
Clinical Manifestations
Look for tenting skin, orthostatic hypotension, and dry mucous membranes.
Diagnosis
Assess BUN/Creatinine ratio > 20:1 and FENa < 1% to confirm prerenal state.
Treatment
Initiate Isotonic Saline (0.9% NaCl) resuscitation; avoid rapid correction in chronic hyponatremia.
Prognosis
Monitor urine output > 0.5 mL/kg/hr to prevent progression to Acute Tubular Necrosis.
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Epidemiology & Etiology
Volume depletion results from absolute fluid loss (hemorrhage, vomiting, diarrhea) or relative loss (third-spacing). Elderly patients are at highest risk due to blunted thirst mechanisms. Diabetic ketoacidosis and diuretic overuse are frequent clinical triggers.
Pertinent Anatomy
The juxtaglomerular apparatus senses decreased perfusion pressure, triggering the Renin-Angiotensin-Aldosterone System. This anatomy dictates the renal response of sodium retention to preserve intravascular volume.
Pathophysiology
Decreased effective arterial blood volume leads to hypoperfusion of the kidneys. The body compensates via vasoconstriction and sodium/water reabsorption. If uncorrected, prolonged ischemia leads to ischemic acute tubular necrosis, shifting from prerenal to intrinsic renal failure.
Clinical Manifestations
Patients present with tachycardia, orthostatic hypotension, and decreased skin turgor. Altered mental status and oliguria are signs of severe end-organ hypoperfusion. Look for sunken eyes in pediatric patients and dry axillae in the elderly.
Diagnosis
The Fractional Excretion of Sodium (FENa) is the gold standard for distinguishing prerenal from intrinsic causes. A FENa < 1% indicates intact tubular function responding to hypovolemia. A BUN/Cr ratio > 20:1 is a classic marker of prerenal azotemia.
Treatment
Administer Isotonic Saline (0.9% NaCl) or Lactated Ringer's for rapid volume expansion. Avoid hypotonic fluids in acute resuscitation as they do not remain in the intravascular space. Monitor for pulmonary edema in patients with underlying heart failure.
Prognosis
Most cases resolve rapidly with fluid replacement. Failure to respond suggests progression to Acute Tubular Necrosis (ATN). Monitor serum creatinine daily to ensure return to baseline.
Differential Diagnosis
Congestive Heart Failure: elevated JVP and peripheral edema
Sepsis: warm extremities and fever
Adrenal Insufficiency: hyperkalemia and hyponatremia
Diabetes Insipidus: polyuria and hypernatremia
Acute Tubular Necrosis: FENa > 2% and muddy brown casts