How it's calculated
Expected PaCO2 = (1.5 × HCO3⁻) + 8, ± 2 mmHg — the range of respiratory compensation expected for a primary metabolic acidosis. Comparing this predicted range against the patient's actual measured PaCO2 identifies a concurrent respiratory disorder.
Interpreting the comparison
| Measured PaCO2 vs expected range | Interpretation |
|---|---|
| Within range | Appropriate respiratory compensation — simple metabolic acidosis |
| Below range | Concurrent primary respiratory alkalosis (mixed disorder) |
| Above range | Concurrent primary respiratory acidosis / inadequate compensation (mixed disorder) |
Clinical use
- Used alongside an arterial blood gas to check whether the respiratory system is compensating appropriately for a primary metabolic acidosis.
- A measured PaCO2 outside the predicted range signals a mixed acid-base disorder rather than a single simple one, changing the differential diagnosis and management.
- Commonly applied in diabetic ketoacidosis, lactic acidosis, and toxic ingestions, where recognising a superimposed respiratory problem (e.g. fatigue-related hypoventilation) has direct clinical implications.
Frequently asked questions
What is Winters' formula used for?
Predicting the expected PaCO2 for appropriate respiratory compensation of a primary metabolic acidosis, so the actual measured PaCO2 can be checked against it.
What does it mean if the measured PaCO2 is higher than predicted?
The patient is not compensating as much as expected — either a concurrent primary respiratory acidosis or a failing compensatory response (e.g. respiratory muscle fatigue), both of which are clinically significant.
Does Winters' formula apply to respiratory acidosis or alkalosis?
No — it is specific to predicting compensation for a primary metabolic acidosis. Other compensation formulas apply to the other primary acid-base disorders.
References
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322.
- Berend K, et al. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371:1434-1445.