How to Check Compensation Status on an ABG

Step-by-Step Logic

1. Identify primary disorder (from pH + the changed parameter)
2. Apply the compensation formula → get the EXPECTED value
3. Compare EXPECTED vs MEASURED
   - MEASURED = EXPECTED → Appropriate compensation (simple disorder)
   - MEASURED > EXPECTED → Additional acidifying process present
   - MEASURED < EXPECTED → Additional alkalinizing process present

The 4 Primary Disorders & Their Compensation Formulas

1. Metabolic Acidosis

Expected PaCO2 = (1.5 × [HCO3-]) + 8 ± 2

• If measured PaCO2 = 29 → simple metabolic acidosis ✓
• If measured PaCO2 = 38 → mixed metabolic acidosis + respiratory acidosis
• If measured PaCO2 = 20 → mixed metabolic acidosis + respiratory alkalosis

2. Metabolic Alkalosis

Expected PaCO2 = 0.7 × ([HCO3-] − 24) + 40 ± 2

3. Respiratory Acidosis

Acute:   For every ↑10 mm Hg in PaCO2 above 40 → HCO3- ↑ by 1 mEq/L
Chronic: For every ↑10 mm Hg in PaCO2 above 40 → HCO3- ↑ by 4-5 mEq/L

4. Respiratory Alkalosis

Acute:   For every ↓10 mm Hg in PaCO2 below 40 → HCO3- ↓ by 2 mEq/L
Chronic: For every ↓10 mm Hg in PaCO2 below 40 → HCO3- ↓ by 4-5 mEq/L

Quick Reference Summary Table

Key Rules to Remember

1. Compensation never fully corrects pH - the pH always stays on the side of the primary disorder. If the pH is fully normal, suspect a mixed disorder.
2. Metabolic disorders are compensated by the lungs (fast - minutes to hours).
3. Respiratory disorders are compensated by the kidneys (slow - 3-5 days for full chronic compensation).
4. Acute vs chronic matters only for respiratory disorders - the degree of expected renal compensation differs significantly.
5. If measured compensation is more than expected → a second alkalinizing process is present alongside.
6. If measured compensation is less than expected → a second acidifying process is present alongside.

Identifying Mixed Acid-Base Disorders

The Gold Standard Nomogram (Harrison's, 2025)

4 Detection Tools - Use All of Them Systematically

Tool 1: The Opposite-Direction Rule (Quick Screen)

"PaCO2 and HCO3- always change in the SAME direction in simple disorders (compensation). If they move in OPPOSITE directions, a mixed disorder is present by definition."

• Morgan & Mikhail's Clinical Anesthesiology, 7e

Tool 2: Compare Measured vs Expected Compensation

Tool 3: The Delta-Delta Ratio (ΔΔ) - Used in High Anion Gap Acidosis

ΔΔ = ΔAG / ΔHCO3-
   = (Measured AG - 12) / (24 - Measured HCO3-)

Tool 4: The pH Clue - When pH is Normal But Parameters Are Abnormal

A normal pH with abnormal PaCO2 and HCO3- is a red flag for a mixed disorder, especially one where acidosis and alkalosis are cancelling each other out.

Common Clinical Mixed Disorders

Worked Example: DKA + Vomiting

• Measured PaCO2 = 38 mmHg > 32.5 → concomitant respiratory acidosis (not compensating adequately) Step 4 - ΔΔ: (35 - 12) / (24 - 15) = 23/9 = 2.6 → > 2.0
• This means a metabolic alkalosis is also present (from vomiting) that raised baseline HCO3-

This is clinically the diabetic vomiting case described in Fishman's Pulmonary Diseases and Disorders - the unmeasured anion concentration added to plasma HCO3- gives a value >30 mEq/L, proving a pre-existing alkalosis.

Key Rules Summary

1. Compensation never overshoots - if pH is fully corrected or swings past normal, it is mixed.
2. PaCO2 and HCO3- in opposite directions = always mixed.
3. Always calculate the AG, even with a normal pH - a hidden mixed disorder may be present.
4. Use ΔΔ whenever you find a high AG acidosis - a second metabolic disorder is frequently hiding.
5. Clinical context is essential - vomiting, renal failure, liver disease, sepsis, and certain toxins classically produce specific mixed patterns.