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Difficulty: HardUrologic Surgery

A 6565-year-old man comes to the clinic for a follow-up examination 66 weeks after undergoing a radical cystectomy and urinary diversion via an ileal conduit for muscle-invasive urothelial carcinoma. Over the past two weeks, he has experienced progressive fatigue and generalized muscle weakness. His temperature is 37.6∘C37.6^\circ\text{C} (99.7∘F99.7^\circ\text{F}), blood pressure is 118/76 mm Hg118/76\text{ mm Hg}, pulse is 88/min88/\text{min}, and respirations are 22/min22/\text{min}. Physical examination shows a well-healed abdominal incision. The stoma is pink and viable, outputting clear yellow urine into the ostomy bag. The lungs are clear to auscultation bilaterally.

Laboratory studies are obtained:

Laboratory TestResult
Sodium139 mEq/L139\text{ mEq/L}
Potassium3.2 mEq/L3.2\text{ mEq/L}
Chloride115 mEq/L115\text{ mEq/L}
Bicarbonate14 mEq/L14\text{ mEq/L}
Blood urea nitrogen20 mg/dL20\text{ mg/dL}
Creatinine1.2 mg/dL1.2\text{ mg/dL}

Arterial blood gas analysis on room air shows:
- pH: 7.287.28
- pCO 2_2: 28 mm Hg28\text{ mm Hg}
- pO 2_2: 96 mm Hg96\text{ mm Hg}

Which of the following is the most appropriate next step in the management of this patient?

  1. A
    Placement of a nasogastric tube and initiation of bowel rest
  2. B
    Admission for blood cultures and empirical intravenous piperacillin-tazobactam
  3. Initiation of oral potassium citrate therapyAnswer
  4. D
    Initiation of noninvasive positive pressure ventilation
  5. E
    Computed tomography of the abdomen and pelvis with intravenous contrast

Answer

Initiation of oral potassium citrate therapy
The correct answer is the administration of an oral alkalinizing agent that also replenishes potassium, such as potassium citrate. This patient has a normal anion gap metabolic acidosis (anion gap = 139−[115+14]=10139 - [115 + 14] = 10) with marked hyperchloremia and hypokalemia. This is a classic complication of urinary diversion utilizing an ileal conduit (or ureterosigmoidostomy). The ileal mucosa actively absorbs ammonium and chloride from the urine while secreting bicarbonate. The chronic metabolic acidosis and increased sodium delivery to the distal tubule promote renal potassium wasting, leading to hypokalemia. The patient's fatigue and muscle weakness are direct consequences of the hypokalemia and acidosis. Arterial blood gas analysis reveals a pCO 2_2 of 28 mm Hg28\text{ mm Hg}, which exactly matches the expected respiratory compensation calculated by Winter's formula [Expected pCO 2_2 = (1.5×14)+8=29±2(1.5 \times 14) + 8 = 29 \pm 2]. Therefore, there is no primary respiratory disorder. Treatment consists of reversing the acidosis and replenishing potassium, making potassium citrate the ideal pharmacotherapy.

Step-by-Step Solution

1
Analyze the patient's acid-base status and electrolyte panel.
The anion gap is normal (Na+−[Cl−+HCO3−]=139−[115+14]=10\text{Na}^+ - [\text{Cl}^- + \text{HCO}_3^-] = 139 - [115 + 14] = 10). The patient has a hyperchloremic (115 mEq/L115\text{ mEq/L}) normal anion gap metabolic acidosis with hypokalemia (3.2 mEq/L3.2\text{ mEq/L}).
To identify the primary physiologic derangement causing the patient's fatigue and weakness.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO 2_2 = (1.5×14)+8=29±2 mm Hg(1.5 \times 14) + 8 = 29 \pm 2\text{ mm Hg}. The measured pCO 2_2 of 28 mm Hg28\text{ mm Hg} represents appropriate and complete respiratory compensation.
To determine if the tachypnea and low pCO2 indicate a secondary respiratory failure or an appropriate compensatory mechanism.
3
Correlate the metabolic findings with the patient's surgical history of an ileal conduit.
The ileal segment used for the urinary diversion actively reabsorbs chloride and ammonium from the urine while secreting bicarbonate. This causes a hyperchloremic metabolic acidosis, which in turn leads to renal potassium wasting.
To establish the underlying etiology of the normal anion gap metabolic acidosis in a post-cystectomy patient.
4
Select the most appropriate pharmacotherapy to correct the underlying derangements.
Oral potassium citrate is selected because it provides base (citrate is metabolized to bicarbonate in the liver) and replenishes the potassium deficit.
To safely and effectively reverse the acidosis and hypokalemia without causing further complications.

Key Concept

Hyperchloremic normal anion gap metabolic acidosis and hypokalemia are classic complications of urinary diversion via an ileal conduit. Treatment involves oral alkalinization with potassium citrate.
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Topics

Urologic Surgery

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