When considering metabolic changes in substitution cystoplasties using bowel, which of the following is CORRECT?A. A mild, subclinical hyperchloremic metabolic acidosis is encountered in all patients that undergo urinary diversion using ileal and/or colonic segments
In the bowel lumen, sodium ions (Na+) are secreted in exchange for hydrogen ions (H+) and bicarbonate ions (HCO3 − ) in exchange for chloride ions (Cl−). Urine has high concentrations of ammonia (NH3), ammonium (NH4 + ), hydrogen and chloride – these substances are reabsorbed in bowel segments exposed to urine resulting in chronic acid load. Whether this results in significant metabolic acidosis depends on the patient (comorbidities), bowel segment used and the duration of contact of the bowel with urine.
A mild, subclinical hyperchloremic metabolic acidosis is encountered in all patients that undergo urinary diversion using bowel. ≤20% of these will have episodes of severe acidosis. 10% of patients with an ileal conduit have a clinically important metabolic acidosis after 1 year.
Renal wasting and secretion of potassium from the bowel results in hypokalaemia. In general, this will not have important clinical consequences. However, when metabolic acidosis is treated, potassium is exchanged with the intracellular space causing further potassium depletion, manifesting clinically as muscle weakness. Several cases of muscle weakness mistaken for GuillainBarré syndrome after ureterosigmoidostomy are reported. Therefore, when correcting acidosis also supplement potassium (potassium citrate 15 mEq or 1.6 g BD-QDS). Lifelong follow-up of patients with bowel substitutions is recommended, comprising bloods (FBC, U&E, bicarbonate, chloride, B12, folate) and USS KUB at regular intervals.