Gas exchange during dialysis. Contrasting mechanisms contributing to comparable alterations with acetate and bicarbonate buffers

Although arterial hypoxemia during hemodialysis is common and may contribute to dialysis morbidity, the mechanisms responsible remain uncertain. Additionally, controversy exists as to whether bicarbonate dialysate produces less hypoxemia than acetate dialysate. The short- and long-term effects of acetate dialysate and bicarbonate dialysate on gas exchange were compared in eight stable patients undergoing dialysis using a closed, proportioning system and a double-blind, crossover study design. Dialysate was sampled immediately proximal and distal to the dialyzer to determine its contribution to total carbon dioxide elimination. Ventilatory parameters and blood gas values were measured before dialysis, at one hour, and after dialysis. Arterial oxygen tension fell significantly and comparably at one hour with both dialysates, whereas the alveolar-arterial oxygen gradient increased only slightly. Despite hypoxemia, minute ventilation decreased by 4 to 18 percent, and arterial carbon dioxide tension was unchanged. Although total carbon dioxide elimination was unchanged in all groups, there was a significant decrease in lung total carbon dioxide elimination with acetate dialysate of 9.23 ± 2.69 to 7.74 ± 1.57 mmol per minute on Day 1 (mean ± SD, p < 0.025) concomitant with a loss of total carbon dioxide into the bath of 2.04 ± 0.20 mmol per minute, resulting in a significant reduction in respiratory quotient (0.92 ± 0.07 to 0.75 ± 0.05, p < 0.01). In contrast, there was a gain of total carbon dioxide into the blood of 1.64 ± 0.45 mmol per minute with bicarbonate dialysate, which resulted in an increased pH at one hour compared with acetate dialysate (7.39 ± 0.04 versus 7.35 ± 0.03, p < 0.05). Hypoxemia persisted after dialysis in all groups and was associated with an increased alveolar-arterial oxygen gradient in three of the four groups. It is concluded that transitory hypoventilation contributes to comparable hypoxemia with both acetate and bicarbonate dialysates by different mechanisms. With acetate dialysate, there is a decrease in carbon dioxide load to the lungs, whereas with bicarbonate dialysate, the mechanism responsible appears to be a suppression of respiratory drive resulting from a gain of bicarbonate from the dialysate. Additionally, neither dialysate prevents post-dialysis hypoxemia, which is associated with an increased alveolar-arterial oxygen gradient resulting from a mechanism that remains to be elucidated.