If the compensation is less or greater than expected (or predicted by the equation), a second acid-base disorder will be present in addition to the primary disorder. Such a situation is called a mixed disorder. For example:
-If PaCO2 is too low, there is an additional respiratory alkalosis; If PaCO2 is too high, there is an additional respiratory acidosis.
-If the HCO3– is too low, there is an additional metabolic acidosis; If the HCO3– is too high, there is an additional metabolic alkalosis.
NB: Because a person cannot hypoventilate and hyperventilate at the same time, one cannot simultaneously have a respiratory acidosis and respiratory alkalosis.
Related article: Acid-Base Nomogram.
| Compensation for Acid-Base Disorders | |||
| Primary Disorder |
Compensatory Process | Expected Compensation | |
| Metabolic acidosis | A decrease in pH (within the arteries and CSF) leads to increased stimulation of the medullary chemoreceptors. This leads to an increase in RR and/or TV which leads to increased exhalation of CO2 | ↓PaCO2 = 1.2 x ΔHCO3– or PaCO2 = (1.5 x HCO3– ) + 8 ± 2 (Winter’s Formula, see here on MDCalc.com) also, PaCO2 ≈ the last 2 digits of the pH |
|
| Metabolic alkalosis |
An increase in pH (within the arteries and CSF) leads to a decreased stimulation of the medullary chemoreceptors. This leads to a decrease in RR and/or TV which leads to decreased exhalation of CO2 (or increased retention of CO2). | ↑PaCO2 = 0.7 x ΔHCO3– | |
| Respiratory acidosis | Acute | Whole-body buffering in blood, without significant renal compensation | ↑HCO3– = 0.1 x ΔPaCO2 |
| Chronic | increased H+secretion by the kidneys (which increases the serum [HCO3–]). Also increased reabsorption of bicarb in the kidneys. | ↑HCO3– = 0.35 x ΔPaCO2 | |
| Respiratory alkalosis | Acute | Whole-body buffering in blood, without significant renal compensation | ↓HCO3– = 0.2 x ΔPaCO2 |
| Chronic | Increased H+retention (.i.e. ↓ H+ excretion) by the kidneys, which decreases serum HCO3. Also ↓ reabsorption of bicarb in the kidneys. | ↓HCO3– = 0.4 x ΔPaCO2 | |
RR= Respiratory rate; TV=Tidal volume
Note: The change (Δ) in bicarb, ΔHCO3–, or the change in the partial pressure of carbon dioxide, ΔPaCO2, is the difference between expected normal value and the value that is measured. E.g. in a patient w/ metabolic acidosis, the ΔHCO3– is the difference between bicarb as seen on the ABG and what would be considered normal bicarb for the pt ( e.g. 24 mEq/L). That is, ΔHCO3– = (24 – HCO3–). In the same way, ΔPaCO2 = (40 – PaCO2). You are just looking for the difference, so if the result is negative, you drop the negative sign.
The ↓PaCO2 or ↑PaCO2 or ↑HCO3– or ↓HCO3– in the expected compensation formulas, the part before the equal to sign, essential means change (Δ). It is calculated the same way as the Δ. For example, ↓PaCO2 = (40 – PaCO2 ); ↑HCO3– = (HCO3– – 24).
References / Resources
https://www.amboss.com/us/knowledge/Acid-base_disorders
Secondary responses to altered acid-base status: the rules of engagement. J Am Soc Nephrol. 2010 Jun;21(6):920-3. https://jasn.asnjournals.org/content/21/6/920