ABG compensation rules
Every acid-base disorder sets off a compensating response in the other system. Metabolic disorders are compensated by a change in PaCO2, and respiratory disorders by a change in HCO3−. [Merck Manual: acid-base disorders] This page gives the expected size of that response for each simple disorder, as the Merck Manual lists it.
Expected compensation
| Primary disorder | Expected compensation (Merck Manual) |
|---|---|
| Metabolic acidosis | PaCO2 falls 1.2 mmHg for every 1 mEq/L fall in HCO3−; or PaCO2 = (1.5 × HCO3−) + 8 (± 2), which is Winter’s formula |
| Metabolic alkalosis | PaCO2 rises 0.6 to 0.75 mmHg for every 1 mEq/L rise in HCO3−; PaCO2 should not rise above 55 mmHg in compensation |
| Respiratory acidosis | Acute: HCO3− rises 1 to 2 mEq/L for every 10 mmHg rise in PaCO2. Chronic: 3 to 4 mEq/L for every 10 mmHg |
| Respiratory alkalosis | Acute: HCO3− falls 1 to 2 mEq/L for every 10 mmHg fall in PaCO2. Chronic: 4 to 5 mEq/L for every 10 mmHg |
These are the values in the Merck Manual’s table of primary changes and compensations in simple acid-base disorders. [Merck Manual: acid-base disorders] Other references print slightly different numbers. Our app uses the bedside rules in Brandis, Acid-Base Physiology 9.3 (metabolic alkalosis: expected PaCO2 = 0.7 × HCO3 + 20, plus or minus 5; respiratory acidosis: HCO3 rises 1 (acute) to 4 (chronic) per 10 mmHg; respiratory alkalosis: HCO3 falls 2 (acute) to 5 (chronic) per 10 mmHg), which bracket the Merck figures above. [Brandis: Acid-Base Physiology 9.3] Winter’s formula is the same in both. Use the rule your program or institution uses.
Compensation is never complete
The Merck Manual says compensation “cannot return pH completely to normal and never overshoots.” [Merck Manual: acid-base disorders] Compensation also takes time. The lungs adjust over minutes to hours; the kidneys change how much bicarbonate they handle hours to days after the change. [Merck Manual: acid-base regulation]
That is why a gas with a pH inside 7.35 to 7.45 and abnormal PaCO2 and HCO3− is a judgment call. Nursing courses label it fully compensated. [RegisteredNurseRN: ROME] But the Merck Manual notes that in mixed disorders values may be deceptively normal, so check the compensation against the expected size. [Merck Manual: acid-base disorders]
If the compensation is the wrong size
If PaCO2 and HCO3− do not show the expected compensation, the Merck Manual says a second primary process should be suspected. [Merck Manual: acid-base disorders] For example, in respiratory acidosis a bigger rise in HCO3− than expected implies a primary metabolic alkalosis, and a smaller rise suggests either no time yet for compensation or a coexisting metabolic acidosis. [Merck Manual: acid-base disorders]
For metabolic alkalosis, a bigger rise in PaCO2 than expected implies a concomitant respiratory acidosis, and a smaller rise a respiratory alkalosis. [Merck Manual: acid-base disorders]
Where this fits in the method
This is step 5 of how to interpret an ABG, after you have named the primary disorder. To try the labels first, use the practice cases.
Sources
- Merck Manual Professional Edition, Acid-Base Disorders (Lewis; peer reviewed by Braunstein), last updated April 2025
- Merck Manual Professional Edition, Acid-Base Regulation (Lewis; peer reviewed by Braunstein), last updated March 2025
- RegisteredNurseRN, ROME Method ABG Interpretation: Uncompensated vs. Compensated (nursing education)
- Brandis, Acid-Base Physiology, 9.3 Bedside Rules for Assessment of Compensation (the rules our app uses)
Every fact on this page was checked against these sources on . noshu is not affiliated with Merck, the National Library of Medicine or any source named here, and this is a study aid, not clinical guidance: your program, your institution’s protocols and a qualified clinician decide what is done for a patient. If something here is wrong, tell us at a@noshu.net.