Acute renal failure in the postoperative period significantly increases the morbidity and mortality of the surgical patient. The physiology, diagnosis, and treatment of renal abnormalities are described in Chapter 5: Specific Considerations with Renal Disease. Three common conditions encountered in the PACU that can indicate renal malfunction include oliguria, polyuria, and electrolyte disturbances. For more information on the management of these conditions, see Chapter 5.
Oliguria is defined as a urine output of less than 0.5 mL/kg/h over 6 or more hours; however, reduced urine output in the PACU often precedes the official diagnosis. Hypovolemia is the most frequent cause of postoperative oliguria. Administration of a fluid bolus (250 to 500 mL of crystalloid or colloid), even when other etiologies are not yet excluded, is acceptable, as well as placement of a urinary catheter. Further diagnostic tests (eg, plasma and urine electrolytes) and invasive monitoring should be considered when oliguria persists. Diuretics (see Chapter 5: Specific Considerations with Renal Disease) should be used only when deemed necessary, such as in congestive heart failure and chronic renal insufficiency. Modern enhanced recovery after surgery (ERAS) protocols emphasize conservative intraoperative fluid management, and thus hypovolemia should be considered in the oliguric ERAS patient in the PACU.
Polyuria, defined as a urine output disproportionately high for a given fluid intake, occurs less frequently. Symptomatic treatment is based on volume replacement to maintain hemodynamic stability and adequate fluid balance. Electrolyte and acid-base equilibrium may be perturbed secondary to the etiology or to large volume loss. The differential diagnosis includes excessive volume, pharmacologic diuresis, osmotic diuresis, postobstructive diuresis, acute tubular necrosis, and diabetes insipidus.
Electrolyte disturbances such as hyperkalemia and acidemia can develop within hours and must be corrected emergently to avoid ventricular dysrhythmias and death (see Chapter 5). Polyuria may cause profound dehydration, with massive potassium losses and resulting alkalemia. Hypokalemia, often associated with hypomagnesemia, may also trigger atrial and ventricular dysrhythmias, although not as severe as those associated with hyperkalemia. Potassium should be replaced cautiously to avoid overdose. Magnesium replacement may effectively treat atrial and ventricular dysrhythmias, especially if the latter are in the form of torsade de pointes.