A man arrives with a distended abdomen and a bladder holding 1,800 ml. The catheter goes in, everyone is relieved, and the risk starts. Post-obstructive diuresis follows decompression of chronic high-pressure retention in a substantial minority of patients, and it can run to litres over hours — driven by accumulated urea acting as an osmotic load, by the tubular damage that chronic back-pressure causes, and by the sodium-losing defect that comes with it.
The practical thresholds are worth having to hand: more than about 200 ml per hour for two consecutive hours, or more than 3 litres in 24 hours, is a diuresis that needs managing rather than admiring. Untreated it produces hypovolaemia, hypotension, hypokalaemia, hyponatraemia or hypernatraemia depending on the pattern of loss, and occasionally a collapse on the ward overnight. The management is to measure hourly output, replace roughly two-thirds to three-quarters of the loss with an appropriate crystalloid rather than matching it millilitre for millilitre, and check electrolytes and creatinine at least twice in the first 24 hours.
Two other things follow the same catheter. Haematuria ex vacuo — bleeding from decompressed bladder mucosa — is usually self-limiting and is not a reason to clamp the catheter; the old practice of intermittent clamping to decompress slowly has no evidence behind it and delays relief. And the creatinine that was raised on admission needs a repeat before anyone concludes the patient has chronic kidney disease: much of it resolves.
The patients to watch are the ones with a high residual volume, a raised creatinine, bilateral hydronephrosis on ultrasound, or a palpable bladder they had stopped noticing. Those are the four things to record in the notes when the catheter goes in.
- Record the drained volume, the creatinine and whether there is hydronephrosis when the catheter goes in
- Chart hourly urine output for 24 hours in anyone who drained more than about a litre
- Treat more than 200 ml/hour for two hours, or 3 litres a day, as a diuresis needing fluid replacement
- Replace about two-thirds to three-quarters of the loss rather than matching it, and recheck electrolytes twice in 24 hours
- Do not clamp the catheter to decompress slowly, and repeat the creatinine before diagnosing chronic kidney disease
Why it matters
The dangerous part of chronic retention is the hours after it is relieved, and that is when attention usually moves on.
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