Winter Formula for Compensatory Respiratory Response in Metabolic Acidosis
The Winter formula predicts the expected compensatory decrease in PaCO2 during metabolic acidosis: PaCO2 = (1.5 × HCO3) + 8 ± 2, where HCO3 is measured in mEq/L and PaCO2 in mmHg.
Formula Application and Interpretation
The Winter formula is used to assess whether respiratory compensation for metabolic acidosis is appropriate, not for calculating fluid administration in dehydration 1.
Expected Compensation Range
- Calculate expected PaCO2 using the formula: PaCO2 = (1.5 × serum bicarbonate) + 8, with a normal range of ±2 mmHg 1
- If measured PaCO2 is higher than predicted: indicates concurrent respiratory acidosis or inadequate respiratory compensation 1
- If measured PaCO2 is lower than predicted: suggests concurrent respiratory alkalosis or excessive compensation 1
Clinical Context in Dehydration
When evaluating dehydrated patients with metabolic acidosis, the Winter formula helps distinguish pure metabolic acidosis from mixed acid-base disorders 1.
Acid-Base Assessment in Dehydration
- Metabolic acidosis is common in moderate to severe dehydration, particularly with diarrheal illness 1, 2
- **Serum bicarbonate <17 mEq/L** has 77% sensitivity for predicting 6-10% dehydration and 94% sensitivity for >10% dehydration 3
- Monitor pH and CO2 content every 2-3 hours during acute rehydration in severe cases 1
Distinction from Fluid Calculation Formulas
The Winter formula should not be confused with fluid deficit calculations used in dehydration management 1, 2.
Actual Fluid Calculation Methods
- Holliday-Segar formula calculates maintenance water needs: 100 ml/kg/day for first 10 kg, plus 50 ml/kg/day for 10-20 kg, plus 25 ml/kg/day for weight >20 kg 1
- Moderate dehydration (6-9% deficit) requires 100 mL/kg ORS over 2-4 hours 2
- Severe dehydration requires initial isotonic saline at 10-20 mL/kg/hour, not exceeding 50 mL/kg over first 4 hours in pediatric patients 1