An experimental study of urodynamic effects of epidural morphine and of naloxone reversal

Urinary retention after epidural morphine is a distressing and troublesome complication. This study was undertaken to determine the effects of morphine (epidural, intramuscular, and intravenous) on urinary bladder function and possible reversal by naloxone. Thirty male volunteers (aged 20-28 years) were studied; the doses studied were 2, 4, and 10 mg of epidural morphine, 10 mg intramuscular morphine, and 10 mg intravenous morphine. The urodynamic study consisted of measurements of urine flow rate, strength of detrusor contraction, bladder capacity, and urethral pressure. Irrespective of dose, all subjects receiving epidural morphine showed marked relaxation of the detrusor shortly after injection. There was a corresponding increase in the maximal bladder capacity leading to urinary retention. These effects lasted an average of 14-16 h. Urethral pressures remained unchanged. Urodynamic changes after intramuscular and intravenous morphine were minimal. Treatment using a single intravenous injection of 0.8 mg naloxone promptly reversed the effects of epidural morphine on the bladder. Prevention of urinary retention was achieved with an intravenous infusion of naloxone started before administration of epidural morphine. The possible mechanisms of urinary retention following epidural opiates are discussed.