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Abstract: PO1495

Re-Evaluation of Renal Bicarbonate Compensation in the Setting of Extreme Chronic Respiratory Acidosis

Session Information

Category: Fluid, Electrolyte, and Acid-Base Disorders

  • 902 Fluid, Electrolyte, and Acid-Base Disorders: Clinical

Authors

  • Khan, Asad H., University of Massachusetts Medical School-Baystate, Springfield, Massachusetts, United States
  • Nathanson, Brian Harris, University of Massachusetts Medical School-Baystate, Springfield, Massachusetts, United States
  • Landry, Daniel L., University of Massachusetts Medical School-Baystate, Springfield, Massachusetts, United States
  • Mcgee, Bill, University of Massachusetts Medical School-Baystate, Springfield, Massachusetts, United States
  • Braden, Gregory Lee, University of Massachusetts Medical School-Baystate, Springfield, Massachusetts, United States
Background

In chronic respiratory acidosis (CRA), the expected renal compensatory increase in serum bicarbonate (HCO3) is 0.35-0.4 mEq/L for every 1 mmHg PaCO2 over 40 mmHg. However, at extreme hypercarbia, at PaCO2 >80 mmHg, limited research dating from the 1960s suggests the HCO3/PaCO2 relationship flattens at a PaCO2 >70 mmHg.

Methods

We analyzed 2840 arterial blood gas (ABG) observations from 761 patients with extreme chronic hypercarbia with PaCO2 > 80 mmHg from 7/2015-7/2017 from a single, US teaching hospital. We included all patients with a pH ranging from 7.20 to 7.35 who had serum bicarbonate levels measured by the main lab autoanalyzer, within 12 hours of the ABG. To reduce the bias of acute respiratory acidosis (RA), we performed a sensitivity analysis on 101 patients who had 603 ABGs 4-10 days apart to assure chronicity of RA. Generalized linear regression models were developed to predict HCO3.

Results

For the 2840 ABGs, the mean (SD) was: pH: 7.27 (0.05), PaCO2: 92.51 (10.95) mmHg (range 80-177), and HCO3 : 37.9 (4.5) mEq/L. In Figure 1. the increase in serum HCO3 was 0.26; 95% CI (0.25, 0.28) for each value of PaCO2 above 80 mmHg after adjusting for pH. The regression equation yielded: HCO3= 88.64*pH +0.34 *PaCO2 - 637.2. The sensitivity analysis produced very similar findings.

Conclusion

At extreme hypercarbia, defined as PaCO2 >80 mmHg, the kidneys have limited ability to retain bicarbonate compared to established formulae for lesser increases in PaCO2. Our new equation predicts this response is a 0.26 mEq/L increase in HCO3 for each 1 mmHg of PaCO2 increase over 80 mmHg.

Measured bicarbonate (Y axis) plotted against measured PaCO2 (X axis) showing an increase of 0.26 mmol/L of bicarbonate for each value of PaCO2 above 80 mmHg, with adjustments for different values of pH (colored lines).