Objective: To assess renal function in patients with obstructive sleep apnea syndrome (OSAS) during two nights when they were untreated and treated with continuous positive airway pressure (CPAP).
Methods: Tubular function was estimated on the basis of the renal lithium clearance and its derived formulae, urinary osmolality, osmolal clearance and free water clearance; while glomerular function was evaluated by endogenous creatinine clearance. Twenty patients with OSAS and sixteen normal controls were studied.
Results: The untreated patients with OSAS had significantly lower fractional proximal tubular sodium reabsorption (59.1% +/- 7.1%, P < 0.001 vs controls), lower fractional distal tubular sodium reabsorption (93.8% +/- 1.7%, P < 0.05 vs controls) and urinary osmolality (573.0 +/- 107.9 mosm.kg-1, P < 0.05 vs controls). The absolute distal tubular reabsorption rate of sodium and osmolal clearance in the untreated patients were higher 47.0 +/- 26.0 mmol.L-1 and 25.0 +/- 4.1 ml.L-1; P < 0.01 and P < 0.05 respectively vs controls). During CPAP-treated nights all abnormal renal function indexes in the OSAS patients restored to normal control values (P > 0.05).
Conclusions: The natriuresis and diuresis of OSAS patients were due to the decrease in proximal and distal tubular sodium reabsorption and in tubular concentration ability during their nocturnal sleep and were returned to normal by CPAP therapy.