Your patient is a 43-year-old male who is experiencing post-operative voiding difficulty after an elective...

The patient is experiencing a common complication of low abdominal surgery. Post-operative urinary retention occurs in almost 25% of patients after low abdominal surgical procedures. A normal post-void residual volume is less than 50 cc or urine. The effects of anesthesia and analgesia both contribute to bladder distension, decreased micturition reflex, reduction of contractility of the detrusor muscle of the bladder, and incomplete voiding. The detrusor muscle of the bladder is stimulated to contract by muscarinic cholinergic agonists. Bethanechol is a muscarinic agonist and is frequently used in this setting to improve bladder emptying. Finasteride is a drug that is a 5 alpha reductase inhibitor indicated for use in patients with bladder outlet obstruction as a result of prostatic hypertrophy. The inhibition of 5 alpha reductase decreases local conversion of testosterone to dihydrotestosterone in the prostate gland, which results in gradual shrinkage over a period of six to twelve months. Phenylephrine is an alpha-adrenergic agonist that is selective for alpha-1 receptors. Activation of the alpha 1 receptors in the bladder results in contraction of the trigone muscle and sphincter.

This promotes urinary retention. Oxybutynin is an antimuscarinic agent that is useful for treatment of urge incontinence, and would have a detrimental effect on this patient’s bladder disorder. Imipramine is a medication with anticholinergic properties that would also cause worsening of the patient’s condition. Take home message: Post-operative urinary retention with concomitant incomplete voiding is a complication that results from a decreased micturition reflex, increased vesical sphincter tone, or decreased contractility of the detrusor muscle of the bladder. It can be successfully treated with a muscarinic agonist, such as bethanechol, or with an alpha-1 adrenergic antagonist.