A mechanism of hypoxemia during hemodialysis. Consumption of CO2 in metabolism of acetate.

The present study is an investigation of the role of acetate metabolism in dialysis-induced hypoxemia and of the relative roles of acetate metabolism, bicarbonate loss, and CO2 gas (g) loss in causation of hypoxemia. The loss of CO2 (g) measured in 23 patients during acetate dialysis was found to be negligible (0.21 +/- 0.01 mmol/min). The HCO-3 loss was substantial (3.4 +/- 0.5 mmol/min), but its predicted effect on dialysis hypoxemia was modest. The infusion of acetate at 4 mmol/min into 6 normal volunteers decreased the respiratory exchange ratio (R) from 0.83 +/- 0.06 to 0.71 +/- 0.06 with constant O2 consumption (VO2) and reduced net CO2 production (VCO2). In another experiment, the infusion of sodium acetate into 9 normal volunteers resulted in a similar reduction in R (from 0.82 +/- 0.04 to 0.71 +/- 0.04) and arterial pO2 (from 92.3 +/- 1.1 to 78.3 +/- 1.7 mm Hg). The results indicate that acetate metabolism can lead to reduction in R and hypoxemia and suggest that the same mechanism is responsible for hypoxemia during hemodialysis using acetate dialysate.

Bartter's syndrome due to a defect in salt reabsorption in the distal convoluted tubule.

Bartter's syndrome is generally attributed to a primary defect in salt reabsorption either in the ascending limb of Henle's loop or in the proximal tubule. 2 siblings presented here have all the clinical and biochemical features of Bartter's syndrome but seem to have defective salt reabsorption in the distal convoluted tubule. A surreptitious use of diuretics was ruled out. Free water clearance was reduced in both patients and also was low after the addition of furosemide when compared with controls. Urine osmolalities following overnight dehydration were 883 and 1,000 mosm/l. The reduced maximal free water clearance argues against a proximal defect, and the normal urine concentration against a Henle's loop defect. Low free water clearance after furosemide suggests a defect in the distal convoluted tubule.

Hemodynamic changes during hemodialysis: role of dialyzate.

To determine the contribution of the dialyzate to the hemodynamic changes that occur during hemodialysis, echocardiographic measurements obtained during identical conditions of hemodialysis except for the use of different dialyzates were compared. With an acetate-buffer systolic blood pressure and arterial oxygen tension declined but mean rate of left ventricular fiber shortening increased. These changes occurred after only 15 min of hemodialysis. By contrast, with a bicarbonate buffer these alterations did not ensue. Thus, during hemodialysis with limited ultrafiltration, the fall in arterial pressure observed is caused by the acetate-buffer. In hemodynamically unstable patients, bicarbonate may be the preferable dialyzate.