Changes of apparent bicarbonate space (ABS) in uremic patients undergoing hemodialysis

Purpose. The aim of this study was to estimate the apparent bicarbonate space percentage (ABS%) of total body weight pre- and post-hemodialysis (HD), and to determine the relationship between the ABS% and the interdialytic body weight gain percentage (IBWG%) in uremic patients. Patients & Methods. To this end, data were collected on 12 uremic patients undergoing regular HD with similar HD-session characteristics, but who had widely varying amounts of IBWG. Blood samples were drawn from the arterial line pre- and post-HD (anaerobically, in heparinized syringes) for the detennination of HCO 3 -, pH, and PaCO 2 during two HD sessions per patient, for a total of 24 HD sessions. The IBWG% was estimated using the formula by Fernandez et al., and the ABS% was calculated pre- and post-HD. Results. The mean values (±SD) pre-HD (HCO 3 - = 18.25 ± 1.88 mmol/L, pH = 7.31 ± 0.03, PaCO = 36.16 ± 2.5 mmHg) and post-HD (HCO 3 - = 26.43 ± 1.8, pH = 7.43 ± 0.04, PaCO 2 = 38.45 ± 2.48) show that patients have metabolic acidosis pre-HD and slight alkalosis post-HD. A comparison of the ABS% of total body weight pre-HD (54.2 ± 1.47%) and post-HD (49.86 ± 0.70%) shows significantly higher levels pre-HD than post-HD (p< 0.0001). The correlation between ABS% and IBWG% pre-HD (r = 0.626, p < 0.001) and post-HD (r = 0.809, p < 0.0001) shows a significant and positive relationship. Conclusion. According to our findings, the metabolic acidosis pre-HD increases the ABS%, while the correction of acidosis by HD reverts the ABS% post-HD to about normal levels. The positive relationship between ABS% and IBWG% pre- and post-HD shows indirectly that the IBWG negatively influences the plasma HCO 3 - concentration pre- and post-HD, as it is known that the ABS is inversely related to the plasma HCO 3 - concentration.