Electrolyte Abnormalities Following Urinary Diversion
A 70-year-old man with carcinoma of the bladder presents for radical cystectomy and ileal loop urinary diversion. He weighs 70 kg and has a 20-year history of hypertension. Preoperative laboratory measurements revealed normal plasma electrolyte concentrations and a blood urea nitrogen (BUN) of 20 mg/dL with a serum creatinine of 1.5 mg/dL. The operation lasts 4 h and is performed under uncomplicated general anesthesia. The estimated blood loss is 900 mL. Fluid replacement consists of 3500 mL of lactated Ringer’s injection and 750 mL of 5% albumin.
One hour after admission to the postanesthesia care unit, the patient is awake, his blood pressure is 130/70 mm Hg, and he appears to be breathing well (18 breaths/min, Fio2 = 0.4). Urinary output has been only 20 mL in the last hour. Laboratory measurements are as follows: Hb, 10.4 g/dL; plasma Na+, 133 mEq/L; K+, 3.8 mEq/L; Cl−, 104 mEq/L; total CO2, 20 mmol/L; Pao2, 156 mm Hg; arterial blood pH, 7.29; Paco2, 38 mm Hg; and calculated HCO3−, 18 mEq/L.
What is the most likely explanation for the hyponatremia?
Multiple factors tend to promote hyponatremia postoperatively, including nonosmotic antidiuretic hormone ( ADH) secretion (surgical stress, hypovolemia, and pain), large evaporative and functional fluid losses (tissue sequestration), and the administration of hypotonic intravenous fluids. Hyponatremia is particularly common postoperatively in patients who have received relatively large amounts of lactated Ringer’s injection ([Na+] 130 mEq/L); the postoperative plasma [Na+] generally approaches 130 mEq/L in such patients. (Fluid replacement in this patient was appropriate considering basic maintenance requirements, blood loss, and the additional fluid losses usually associated with this type of surgery.)
Why is the patient hyperchloremic and acidotic (normal arterial blood pH is 7.35-7.45)?
Operations for supravesical urinary diversion utilize a segment of bowel (ileum, ileocecal segment, jejunum, or sigmoid colon) that is made to function as a conduit or reservoir. The simplest and most common procedure utilizes an isolated loop of ileum as a conduit: the proximal end is anastomosed to the ureters, and the distal end is brought through the skin, forming a stoma.
Whenever urine comes in contact with bowel mucosa, the potential for significant fluid and electrolyte exchange exists. The ileum actively absorbs chloride in exchange for bicarbonate, and sodium in exchange for potassium or hydrogen ions. When chloride absorption exceeds sodium absorption, plasma chloride concentration increases, whereas plasma bicarbonate concentration decreases—a hyperchloremic metabolic acidosis is established. In addition, the colon absorbs NH4+ directly from urine; the latter may also be produced by urea-splitting bacteria. Hypokalemia results if significant amounts of Na+ are exchanged for K+. Potassium losses through the conduit are increased by high urinary sodium concentrations. Moreover, a potassium deficit may be present—even in the absence of hypokalemia—because movement of K+ out of cells (secondary to the acidosis) can prevent an appreciable decrease in extracellular plasma [K+].