Kidney Week

Abstract: TH-PO0092

Short of the Mark: Renal Adaptation in Extreme Chronic Hypercapnia Revisited

Session Information

• Fluid, Electrolyte, and Acid-Base Disorders: Clinical Research
  October 22, 2026 | Location: Exhibit Hall A, Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Fluid, Electrolytes, and Acid-Base Disorders

• 1102 Fluid, Electrolyte, and Acid-Base Disorders: Clinical

Authors

• Hashmi, Razi Haider, Baystate Medical Center, Springfield, Massachusetts, United States

Razi Haider Hashmi, MD

• Khan, Asad H., Baystate Medical Center, Springfield, Massachusetts, United States

Asad H. Khan, MD

• McGee, Bill, Baystate Medical Center, Springfield, Massachusetts, United States

Bill McGee, MD

• Nathanson, Brian Harris, OptiStatim, LLC, LongMeadow, Massachusetts, United States

Brian Harris Nathanson, PhD

• Landry, Daniel L., Baystate Medical Center, Springfield, Massachusetts, United States

Daniel L. Landry, DO, FASN

• Braden, Gregory Lee, Baystate Medical Center, Springfield, Massachusetts, United States

Gregory Lee Braden, MD

Group or Team Name

• Baystate

Background

In chronic respiratory acidosis, the expected renal compensatory increase in serum bicarbonate (HCO₃^-) is 0.35-0.4 mEq/L for every 1 mmHg PaCO₂ over 40 mmHg with the highest PaCO₂ being 70 mmHg. However, at extreme hypercarbia, at PaCO₂ >80 mmHg, limited research dating from the 1960s suggests the HCO₃^-/P_aCO₂ relationship flattens at a PaCO₂ > 70 mmHg and renal compensation is lessened.

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 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 (ARA), we performed a sensitivity analysis on 101 patients who had 603 ABGs 4-10 days apart to assure chronicity of respiratory acidosis. Generalized linear regression models were developed to predict HCO₃^-

Results

For the 2840 ABGs, the mean standard deviation (SD) was pH: 7.27 (0.05), P_aCO₂: 92.51 (10.95) mmHg (range 80-177), and HCO₃^-: 37.9 (4.5) mEq/L. In figure 1, the increase in serum bicarbonate was 0.26; 95% CI (0.25, 0.28) for each value of P_aCO² above 80 mmHg after adjusting for pH. The regression equation yielded: HCO₃^-= 88.64*pH+0.34*P_aCO₂-637.2. The sensitivity analysis produced the exact same findings.

Conclusion

At extreme hypercarbia, defined as P_aCO₂ >80 mmHg, the kidneys have limited ability to retain bicarbonate compared to established formulae for lesser increases in P_aCO₂. Our new equation predicts this response is a 0.26 mEq/L increase in HCO3- for each 1 mmHg in P_aCO₂ increase over 80 mmHg. Clinicians should now use this formula for determining mixed acid-base disorders in patients with extreme hypercarbia.