Hypoxia during maintenance hemodialysis-the critical role of pH

Background The impact and management of subclinical hypoxia during hemodialysis is a significant medical challenge. As key determinants of O-2 availability and delivery, proposed mechanisms contributing to hypoxia include ischemia, alkalemia and pulmonary leukocyte sequestration. However, no study has comprehensively investigated and compared these interrelated mechanisms throughout a typical hemodialysis treatment week. This study aimed to comprehensively assess the physiological mechanisms that contribute to hypoxia during hemodialysis. Methods In 76 patients, we measured arterial blood gases and pH at four time-points during hemodialysis (start, 15 min, 60 min, end) over the course of a standard treatment week. For the mid-week hemodialysis session, we additionally measured central hemodynamics (non-invasive cardiac output monitoring) and white blood cell count. Results Linear regression modelling identified changes in pH, but not central hemodynamics or white blood cell count, to be predictive of changes in PaO2 throughout hemodialysis (e.g. at 60 min, beta standardized coefficient pH = 0.45, model R-2 = 0.25, P < .001). Alkalemia, hypokalemia, decreased calcium and increased hemoglobin-O-2 affinity (leftward shift in the oxyhemoglobin dissociation curve) were evident at the end of hemodialysis. pH and hemoglobin-O-2 affinity at the start of hemodialysis increased incrementally over the course of a standard treatment week. Conclusion These data highlight the important role of pH in regulating O-2 availability and delivery during hemodialysis. Findings support routine pH monitoring and personalized dialysate bicarbonate prescription to mitigate the significant risk of alkalemia and subclinical hypoxia. Lay Summary Low blood oxygen levels are common during hemodialysis and can cause unpleasant symptoms. There are many things that can contribute to low blood oxygen levels, such as low blood pressure, but this area has not been fully investigated. We measured oxygen levels during hemodialysis over the course of a normal treatment week. We also measured blood flow, blood pressure, blood acidity (pH) and white blood cell count. Changes in pH were found to be the best predictor of blood oxygen levels. pH changes mostly due to the use of bicarbonate to reduce acidity during hemodialysis. The findings suggest that people undergoing hemodialysis may benefit from regular pH measurement during treatment, and that bicarbonate should be individually prescribed rather than a standard dose being given to everyone.