Kidney Week

Abstract: TH-PO0392

New Formula to Calculate the Proper Respiratory Compensation for Metabolic Alkalosis

Session Information

• Fluid, Electrolyte, and Acid-Base Disorders: Clinical - 1
  November 06, 2025 | Location: Exhibit Hall, 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

• Apodaca, Javier, University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Javier Apodaca, MD

• Obeidat, Yasin, University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Yasin Obeidat, MD

• Hodgins, Spencer, University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Spencer Hodgins, MD

• Mulhern, Jeffrey, University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Jeffrey Mulhern, MD

• Landry, Daniel L., University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Daniel L. Landry, DO, FASN

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

Brian Harris Nathanson, PhD

• Braden, Gregory Lee, University of Massachusetts Chan Medical School - Baystate Regional Campus, Springfield, Massachusetts, United States

Gregory Lee Braden, MD

Background

Controversy has existed for over 50 years regarding the best formula for calculating the proper respiratory compensation for metabolic alkalosis (MA). Older studies with few patients calculated an increase in pCO2 at 6 mmHg for every 10 mEq/L of bicarbonate (HCO3) greater than 26 mEq/L, which was re-written by Madias to be 40 + 0.7 (HCO3 - 24) = pCO2. Javaheri used 34 observations in 26 patients in 1987 and created the formula pCO2 = 0.7(HCO3) + 21. Recent textbook chapters by Hamm (pCO2= HCO3 + 15) and Emmet (pCO2= HCO3 + 10) have used unpublished observations to support these formulas.

Methods

We have reviewed all literature from 1970 to 2024 in published cases and series of MA and utilized this data to calculate a new formula. In addition to Javaheri's observations, we found 59 more cases of uncomplicated MA for a total of 93 data points. Patients with CHF, neurologic myopathies, any respiratory, mixed acid-base, or congenital spine disorders were excluded. We performed linear regression (LR) analyses with HCO3 on the X-axis and pCO2 on the Y-axis for each patient and all formulas. We also compared pCO2 to pO2, pCO2 to serum potassium (K), and pCO2 to serum K less or equal to 3 mEq/L by LR.

Results

We found the best formula to be pCO2= 0.65(HCO3) + 23. Formulas are shown in Figure 1. We performed LR of pCO2 vs. pO2 and pCO2 vs. serum K level. The pO2 decreased as pCO2 increased (r= 0.28, p <0.001). The pCO2 decreased as the serum K increased (r= 0.37, p <0.01). There was no significant relationship between pCO2 and severe hypokalemia (r=0.05, p= 0.18), which suggests that hypokalemia does not affect respiratory compensation.

Conclusion

We found that the best formula inclusive of all known published data for appropriate respiratory compensation for metabolic alkalosis is pCO2= 0.65(HCO3) + 23, from HCO3 of 30 to 120 mEq/L. The Javaheri and Madias formulas closely approximate this and are statistically the same. Hamm and Emmet’s formulas are less accurate, especially with serum HCO3 of 40 mEq/L or greater.

Figure 1

Funding

• Clinical Revenue Support

Digital Object Identifier (DOI)

• doi: 10.1681/ASN.2025v0asxqd5