Learning Objective:
Understand and apply Winter’s formula to predict the expected PaCO₂ during compensatory respiratory response in simple metabolic acidosis.

Formula:

Explanation:

• = predicted arterial CO₂ pressure (mm Hg)
• = plasma bicarbonate concentration (mEq/L)
• The formula predicts how the lungs compensate for metabolic acidosis by hyperventilation (decreasing PaCO₂).
• ±2 mm Hg accounts for physiologic variation.

Interpretation:

• If measured PaCO₂ ≈ predicted PaCO₂ → appropriate respiratory compensation
• If PaCO₂ is higher → concurrent respiratory acidosis
• If PaCO₂ is lower → concurrent respiratory alkalosis

Example:

Given:

✅ Answer: Predicted PaCO₂ = 24–28 mm Hg
→ If measured PaCO₂ is within this range, respiratory compensation is appropriate.