Carbon dioxide: Global warning for nephrologists

Abstract

The large prevalence of respiratory acid-base disorders overlapping metabolic acidosis in hemodialysis population should prompt nephrologists to deal with the partial pressure of carbon dioxide (pCO2) complying with the reduced bicarbonate concentration. What the most suitable formula to compute pCO2 is reviewed. Then, the neglected issue of CO2 content in the dialysis fluid is under the spotlight. In fact, a considerable amount of CO2 comes to patients' bloodstream every hemodialysis treatment and "acidosis by dialysate" may occur if lungs do not properly clear away this burden of CO2. Moreover, vascular access recirculation may be easy diagnosed by detecting CO2 in the arterial line of extracorporeal circuit if CO2-enriched blood from the filter reenters arterial needle.

Keywords: Acid-base assessment; Bicarbonate; Carbon dioxide; Expected pressure of carbon dioxide; Hemodialysis; Metabolic acidosis; Mixed disorders; Vascular access recirculation; Ventilatory response.

Figure 1

Risk of death and serum bicarbonate concentration in hemodialysis patients. Trend of risk inferred by data from Bommer et al[2] (A), Wu et al[3] (B), Tentori et al[4] (C), Yamamoto et al[5] (D). HCO₃: Bicarbonate concentration.

Figure 2

Relationship between pressure of carbon dioxide and bicarbonate concentration in chronic metabolic acidosis. The relationship between pCO₂ and HCO₃ during metabolic acidosis is graphically depicted as a curve. Two linear approximations (straight lines) equivalent to Winters’ formula (pCO₂ = 1.5 × HCO₃ + 8) and to the practical rule (pCO₂ = 1.2 × HCO₃ + 11.2) are also shown. HCO₃: Bicarbonate concentration; pCO₂: Pressure of carbon dioxide.

Figure 3

Example of gas analysis in on-line hemodiafiltration. Additional CO₂ load delivered via substitution fluid infusion during online hemodiafiltration. Reproduced with permission from Marano et al[30]. CO₂: Carbon dioxide.

Figure 4

Course of carbon dioxide in presence of vascular access recirculation. pCO₂ from dialyzer re-entering the extracorporeal circuit reveals vascular access recirculation. pCO₂: Pressure of carbon dioxide.

Figure 5

Performance of “RecirCO₂lation” test to detect vascular access recirculation. Receiver operating curves of pCO₂-increase (solid line) and pCO₂ at 5 min (dotted line) for diagnosis of vascular access recirculation using Glucose Infusion Test (> 0.3%) as reference. Reproduced with permission from Marano et al[32]. AUC: Area under curve; pCO₂: Pressure of carbon dioxide.