Overview
This lecture introduces Winter's formula, a tool used to assess if the respiratory system is properly compensating for metabolic acidosis, and provides an example calculation.
Metabolic Acidosis Basics
- Metabolic acidosis is defined by arterial pH < 7.35 and arterial bicarbonate (HCO₃⁻) < 22 mEq/L.
- Values may vary by institution, but these thresholds are used for this lesson.
Purpose of Winter's Formula
- Winter's formula determines if respiratory compensation for metabolic acidosis is appropriate.
- Respiratory compensation involves the lungs expelling CO₂ to lower blood acidity.
Winter's Formula Structure
- Formula: Expected PCO₂ = (1.5 × [HCO₃⁻]) + 8 (±2).
- The expected PCO₂ gives a range; add and subtract 2 to account for variability.
- Use arterial blood gas values for calculations, not venous.
Example Calculation
- Example patient: Low HCO₃⁻ due to diarrhea, resulting in metabolic acidosis.
- Substitute HCO₃⁻ = 14 mEq/L into formula: (1.5 × 14) + 8 = 29; range is 27–31 mmHg.
- Actual PCO₂ = 20 mmHg, which is below the expected range.
Interpretation of Results
- PCO₂ below range: Inadequate compensation; indicates primary respiratory alkalosis alongside metabolic acidosis.
- Causes for low PCO₂: Hyperventilation, certain drugs, fever, altitude adjustment, or anxiety.
- PCO₂ above range (not in this example): Indicates combined metabolic and respiratory acidosis.
Key Terms & Definitions
- Metabolic Acidosis — Condition where blood pH is low due to decreased HCO₃⁻.
- Winter's Formula — Tool to estimate expected PCO₂ for appropriate respiratory compensation.
- PCO₂ — Partial pressure of carbon dioxide in arterial blood.
- Compensation — The body's attempt to correct acid-base imbalances via respiratory or metabolic changes.
Action Items / Next Steps
- Practice Winter's formula calculations with different HCO₃⁻ values.
- Review causes and signs of mixed acid-base disorders.