Acid-base assessment of patients receiving hemodialysis

In regulating blood bicarbonate concentration the kidneys play a central role in acid-base homeostasis, and severe renal failure causes metabolic acidosis.

This is a disturbance of acid-base characterized by primary reduction in blood bicarbonate and pH, and secondary (compensatory) decrease in pCO₂.

One of the aims of the regular, intermittent (usually three times a week) hemodialysis therapy prescribed for patients with end-stage chronic kidney disease, is correction of this metabolic acidosis by addition of bicarbonate to dialysate fluid.

The aim of this bicarbonate replacement is to maintain blood bicarbonate at, or close to physiological concentration, so for patients receiving hemodialysis the only acid-base parameter that would normally be monitored is bicarbonate, measured as total CO₂ by routine biochemistry analyzer, it is one element of the urea and electrolyte (U&E) profile. 

The authors of a recently published clinical study sought to examine the acid-base status of patients receiving hemodialysis more completely by submitting them for blood gas analysis, which allows measurement of, not only bicarbonate, but also pH and pCO₂.

The studied population comprised 53 patients who had been receiving hemodialysis three times a week at a dialysis unit in Pompeii, Italy for several years.

All were receiving the same concentration of bicarbonate in dialysis fluid.

Arterial blood was sampled for gas analysis from each of the studied patients before dialysis sessions; the number of samples per patient ranged from 2 to 14 (average 7). A total of 362 blood gas results (pH, pCO₂ and calculated bicarbonate) from the 53 patients accumulated for analysis during a 5-year (2008-2012) period. 

The pH, pCO₂ and bicarbonate results of each sample were together used to assess acid-base status, and in the case of abnormality, diagnose the type of acid-base disturbance.

Normal or near-normal acid-base status was found in only 17.4 % of the 362 samples and 15 % of the 53 patients.

As expected for a cohort of patients with chronic kidney disease, metabolic acidosis was the most common acid-base disturbance, evident in 140 (38.7 %) samples and 22 (41.5 %) patients.

Respiratory disturbances (predominantly respiratory alkalosis), were also evident. The second most common acid-base abnormality was mixed disturbance (respiratory alkalosis and metabolic acidosis).

This was found in 64 (17.7 %) samples and 13 (24.5 %) patients. Respiratory alkalosis was evident in five (9.4 %) patients and respiratory acidosis in two (3.8 %).

This study has demonstrated that for patients receiving hemodialysis, acid-base disturbance is not confined to metabolic acidosis, and that measurement of bicarbonate (as total CO₂) alone, provides an inadequate account of patient acid-base status.

Current recommendation for hemodialysis is that predialysis total CO₂ be maintained within 20-24 mmol/L; 90 % of patients in this study had levels within this target range.

But this clearly does not mean that 90 % of patients maintained normal acid-bases status.

Without full blood gas analysis the respiratory acid-base disturbances revealed by this study go unrecognized.

Furthermore, the adequacy of patient’s ventilatory response to metabolic acidosis, and therefore the patient’s pH is unclear if only total CO₂ is measured.

The authors of this study cite recent evidence that blood pH may be “key for assessing mortality risk in hemodialysis patients.”

If that is the case then blood gas analysis will presumably be viewed as the more appropriate tool for assessing the effectiveness of hemodialysis to restore normal acid-base status.