Urine output in the first twelve hours after cardiopulmonary bypass does not predict the development of renal impairment

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Hourly urine output is one of the foremost indices which are monitored in patients who have undergone cardiac surgery. Traditionally, a urine output of less than 0.5 ml/kg/h triggers intervention [1]. However, renal failure may supervene in patients who have seemingly adequate renal function according to conventional monitoring. In this pilot study we analysed urine output and alterations in serum creatinine in 31 consecutive patients in the first 12 h following coronary artery bypass and valvular surgery. We found that an `adequate' urine output alone is an unreliable predictor of subsequent renal impairment (indicated by a rise in 24 h post-operative serum creatinine by more than 50%). Six patients demonstrated such a rise in serum creatinine despite their producing urine outputs of greater than or equal to an average of 0.5 ml/kg/h. Of these, five subsequently required renal replacement therapy. This finding may have implications for the monitoring of cardiac patients who return to low-dependency patient care areas within 12 h of surgery.

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