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3 Compensation and Mixed Acid–Base Disorders

Learn how to calculate expected acid–base compensation, recognize when measured values indicate an additional primary disorder, and interpret six worked cases.

How works

is the predictable response of the other system: the lungs respond to metabolic disturbances, while the kidneys respond to respiratory disturbances. It moves pH toward normal but does not itself correct the underlying cause, and it does not overcorrect the pH.

If the measured response falls outside the expected range, suspect an additional primary acid–base disorder rather than “overcompensation.” A near-normal pH does not rule out a mixed disorder, because opposing primary processes can move pH toward the normal range.

Expected by disorder

For respiratory disorders, compare the measured bicarbonate with the expected change from a baseline of PaCO2=40 mm HgPaCO_2 = 40\ \mathrm{mm\,Hg} and HCO3−=24 mEq/LHCO_3^- = 24\ \mathrm{mEq/L}. Respiratory is fast, while renal takes longer. Use the history to distinguish acute from chronic respiratory disorders.

The following rules are approximate estimates. Published rule-of-thumb ranges vary slightly, so interpret the calculated response in its clinical context.

Metabolic disorders

  • : Use to estimate the expected PaCO2PaCO_2:

Expected PaCO2=1.5×HCO3−+8±2 mm Hg\text{Expected } PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2\ \mathrm{mm\,Hg}

  • : PaCO2PaCO_2 rises by about 0.6 mm Hg0.6\ \mathrm{mm\,Hg} for each 1 mEq/L1\ \mathrm{mEq/L} rise in HCO3−HCO_3^- above 24 mEq/L24\ \mathrm{mEq/L}. A practical estimate is:

Expected PaCO2=40+0.6×(HCO3−−24) mm Hg\text{Expected } PaCO_2 = 40 + 0.6 \times (HCO_3^- - 24)\ \mathrm{mm\,Hg}

usually does not raise PaCO2PaCO_2 above about 55 mm Hg55\ \mathrm{mm\,Hg}.

Respiratory disorders

  • Acute : HCO3−HCO_3^- rises by about 1 mEq/L1\ \mathrm{mEq/L} for each 10 mm Hg10\ \mathrm{mm\,Hg} rise in PaCO2PaCO_2 above 40 mm Hg40\ \mathrm{mm\,Hg}.

  • Chronic : HCO3−HCO_3^- rises by about 33–4 mEq/L4\ \mathrm{mEq/L} for each 10 mm Hg10\ \mathrm{mm\,Hg} rise in PaCO2PaCO_2 above 40 mm Hg40\ \mathrm{mm\,Hg}.

  • Acute : HCO3−HCO_3^- falls by about 2 mEq/L2\ \mathrm{mEq/L} for each 10 mm Hg10\ \mathrm{mm\,Hg} fall in PaCO2PaCO_2 below 40 mm Hg40\ \mathrm{mm\,Hg}.

  • Chronic : HCO3−HCO_3^- falls by about 55–7 mEq/L7\ \mathrm{mEq/L} for each 10 mm Hg10\ \mathrm{mm\,Hg} fall in PaCO2PaCO_2 below 40 mm Hg40\ \mathrm{mm\,Hg}.

Recognizing an additional disorder

Calculate the expected response for the primary disorder, then compare it with the measured compensatory variable. A measured value within the expected range supports a simple disorder; a value outside that range suggests another primary process.

In , a measured PaCO2PaCO_2 higher than expected indicates additional . A PaCO2PaCO_2 lower than expected indicates additional . Apply the same comparison logic to other primary disorders: check whether the measured compensatory variable matches its predicted value.

Do not treat an out-of-range response as “overcompensation.” It suggests an additional primary disorder. Also, do not use a near-normal pH alone to conclude that the disturbance is simple.

Worked cases

For each case, compare the measured value with the expected response. The stated respiratory disorder is acute or chronic as specified.

  1. With HCO3−=12 mEq/LHCO_3^- = 12\ \mathrm{mEq/L} and measured PaCO2=27 mm HgPaCO_2 = 27\ \mathrm{mm\,Hg}, gives 1.5×12+8=26 mm Hg1.5 \times 12 + 8 = 26\ \mathrm{mm\,Hg}, with an expected range of 2424–28 mm Hg28\ \mathrm{mm\,Hg}. The measured value is within range, so is appropriate.

  2. With HCO3−=10 mEq/LHCO_3^- = 10\ \mathrm{mEq/L} and measured PaCO2=34 mm HgPaCO_2 = 34\ \mathrm{mm\,Hg}, the expected value is 1.5×10+8=23 mm Hg1.5 \times 10 + 8 = 23\ \mathrm{mm\,Hg}, or 2121–25 mm Hg25\ \mathrm{mm\,Hg} including the expected range. The measured value is too high, suggesting additional : ventilation has not lowered PaCO2PaCO_2 as much as expected, or a separate is present.

  3. With pH =7.40= 7.40, HCO3−=12 mEq/LHCO_3^- = 12\ \mathrm{mEq/L}, and PaCO2=20 mm HgPaCO_2 = 20\ \mathrm{mm\,Hg}, Winter’s expected PaCO2PaCO_2 is 26±2 mm Hg26 \pm 2\ \mathrm{mm\,Hg}. The measured value is lower than expected, indicating a second primary process: . The near-normal pH reflects the opposing effects and does not establish simple .

  1. With HCO3−=36 mEq/LHCO_3^- = 36\ \mathrm{mEq/L}, bicarbonate is 12 mEq/L12\ \mathrm{mEq/L} above baseline. The estimated PaCO2PaCO_2 is 40+(0.6×12)=47.2 mm Hg40 + (0.6 \times 12) = 47.2\ \mathrm{mm\,Hg}. A measured PaCO2PaCO_2 of 48 mm Hg48\ \mathrm{mm\,Hg} is close to the expected value, so is appropriate.

Respiratory disorders

  1. In acute with PaCO2=60 mm HgPaCO_2 = 60\ \mathrm{mm\,Hg}, PaCO2PaCO_2 has risen 20 mm Hg20\ \mathrm{mm\,Hg} above baseline. Acute predicts HCO3−HCO_3^- near 26 mEq/L26\ \mathrm{mEq/L}, about 2 mEq/L2\ \mathrm{mEq/L} above baseline. A measured HCO3−=32 mEq/LHCO_3^- = 32\ \mathrm{mEq/L} does not fit acute and suggests additional . If the duration were unknown, consider chronicity before concluding that there is a second disorder.

  2. In chronic with PaCO2=25 mm HgPaCO_2 = 25\ \mathrm{mm\,Hg}, PaCO2PaCO_2 has fallen 15 mm Hg15\ \mathrm{mm\,Hg}. The expected bicarbonate decrease is about 7.57.5–10.5 mEq/L10.5\ \mathrm{mEq/L} from baseline, giving an expected HCO3−HCO_3^- range of about 13.513.5–16.5 mEq/L16.5\ \mathrm{mEq/L}. A measured value of 10 mEq/L10\ \mathrm{mEq/L} is lower than expected and suggests additional .