The effects of oral sodium bicarbonate on extracellular water in patients with chronic kidney disease

Background: Metabolic acidosis in chronic kidney disease (CKD) is often treated with oral sodium bicarbonate. There is limited evidence around the effects of sodium bicarbonate on extracellular fluid and blood pressure in CKD. Methods: In a double blind randomised comparison patients with stage 3-5 CKD were randomised to either oral sodium bicarbonate 1.5 g three times a day (n=18) or placebo (n=21) for 4 weeks. Assessments performed at 0 and 4 weeks included: body weight, office blood pressure and assessment for peripheral/pulmonary oedema; serum creatinine, electrolytes and venous bicarbonate; 24-hour urine for sodium excretion; extracellular fluid volume and total body water determined by sodium bromide and deuterium oxide dilution respectively; extracellular fluid volume and total body water by bioimpedance. Differences between the active and placebo groups at week 4 were analysed by ANCOVA. Results: At week 4, serum bicarbonate was higher (25.6±2.4 vs 23.3±3.1 mmol/l) and blood urea lower (14.2±5.6 vs 17.0±5.8 mmol/l) in the active treatment group. Urine sodium concentration was also higher (82.7±25.3 vs 59.0±21.9 mmol/l). Extracellular fluid volume (20.0±4.3 vs 18.0±2.9) and total body water (42.3±9.6 vs 39.0±6.8) measured by bioimpedance and total body water by deuterium dilution (41.7±8.3 vs 39.4±6.2) were significantly greater in the treatment arm at week 4. Differences in systolic and diastolic blood pressure did not reach statistical significance. Conclusions: Oral sodium bicarbonate has a biological effect and increases body water content, without evidence of a clinical consequence. This may reflect the fact that some of the ingested sodium is excreted in the urine