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 O₂ 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, β standardized coefficient pH = 0.45, model R² = 0.25, P < .001). Alkalemia, hypokalemia, decreased calcium and increased hemoglobin-O₂ affinity (leftward shift in the oxyhemoglobin dissociation curve) were evident at the end of hemodialysis. pH and hemoglobin-O₂ 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 O₂ 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.