Urinary osmolal changes in renal dysfunction following open-heart operations.

Forty consecutive patients who underwent open-heart procedures using a hyperosmolar perfusion prime were studied to determine the significance of free-water clearance and urinary osmolality early after bypass in predicting the likelihood of postoperative renal dysfunction, defined as a blood urea nitrogen (BUN) level over 50 mg/100 ml. Urinary osmolality increased in all patients during the first 18 hours after bypass, but the increase was substantially less for those who subsequently developed renal dysfunction. Free-water clearance, which was significantly less negative in the patients with renal dysfunction by 2 hours after bypass and remained so throughout the 18 hours of this study, served as an early postoperative indicator of impaired renal function in the patients who eventually developed BUN elevation. Moreover, it was more sensitive as an index of renal dysfunction than was osmolality alone. Early recognition of renal impairment is important, as it may prevent dangerous fluid overloading and allow for corrective measures to be undertaken before frank renal failure develops.