OBJECTIVE: To evaluate bicarbonate fluxes across the peritoneal membrane and bicarbonate gain in patients treated with continuous ambulatory peritoneal dialysis (CAPD) using dialysis solutions with different bicarbonate concentrations. PATIENTS AND DESIGN: Ninety-seven exchanges, using different dwell times and glucose and bicarbonate concentrations were performed in 43 stable CAPD patients. Dialysate effluent bicarbonate concentration and volumes were measured at different dwell times. Net dialytic bicarbonate gain was calculated. Patients' acid-base status was determined at the middle of the dwell. RESULTS: In prolonged dwells (6-12 hours) the dialysate effluent bicarbonate concentration correlated with arterial plasma bicarbonate concentration (F = 129, p < 0.0001), but not with ultrafiltration rate or dialysis solution bicarbonate concentration. In 4-hour dwells, effluent bicarbonate concentration correlated with both plasma bicarbonate concentration and ultrafiltration rate (F = 32.52, p < 0.0001 and F = 4.4, p < 0.05, respectively). The effluent bicarbonate concentration may be predicted from the patient's plasma bicarbonate concentration and net ultrafiltration rate for either a 4-hour or prolonged (6-12 hours) dwell time. Net bicarbonate gain by the patient correlated with ultrafiltration rate, plasma bicarbonate, and dialysis solution bicarbonate concentration (F = 100.56, p < 0.0001 at 4 hours and F = 108.08, p < 0.0001 at 6-12 hours), with the ultrafiltration rate being the predominant parameter. CONCLUSIONS: The effluent bicarbonate concentration is related to plasma bicarbonate concentration, with ultrafiltration playing a marginal role only during short dwells. However, the ultrafiltration rate has a profound effect on net patient bicarbonate gain. Multiple linear regression analysis allows the prediction of the effect of acid-base status, ultrafiltration, dwell time, and dialysis solution bicarbonate content on net patient bicarbonate gain. It seems that bicarbonate content in the CAPD dialysis solution should be progressively increased with increasing solution osmolality.

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