Beyond bicarbonate: complete acid–base assessment in patients receiving intermittent hemodialysis

Abstract Background: Acid–base assessments in hemodialysis patients have been limited almost entirely to measurements of total CO2 concentration, and assumptions have been made about the presence of acid–base disorders. To gain a fuller understanding of the acid–base status of stable hemodialysis patients, we analyzed measurements of pCO2, pH and HCO3− obtained in a cohort of chronic stable hemodialysis patients over a 5-year period. Methods: We reviewed acid–base measurements taken pre-dialysis from fistula blood in 53 outpatients receiving hemodialysis thrice weekly between 2008 and 2012. In these patients, pH and pCO2 were measured using an onsite blood gas analyzer, and HCO3− was computed. Relevant clinical and laboratory data were obtained from medical records. Factors affecting serum HCO3− were identified. Simple and mixed acid–base disorders were diagnosed using accepted rules. Results: Serum HCO3− was affected by age, normalized protein catabolic rate, interdialytic weight gain and length of interval between treatments. As expected, metabolic acidosis was the most common acid–base disorder, but respiratory acid–base disturbances, as simple or complex disorders, were found in 41% of the measurements. Respiratory alkalosis was seen more frequently than respiratory acidosis, but the latter disorder was more commonly associated with serious comorbidities. Conclusions: Respiratory acid–base disorders are an important component of the acid–base abnormalities seen in hemodialysis patients and are not identified by measuring total CO2 concentration; hence, complete acid–base measurements are needed to determine the components of hemodialysis patients' acid–base status that are contributing to mortality risk.