Winters Formula (expected PaCO2)
Winters Formula (expected PaCO2) - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
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What this is
Compute the Winters formula for expected respiratory compensation in metabolic acidosis: expected PaCO2 = 1.5 x HCO3 + 8 +/- 2 (in mmHg). Sophie Well takes the measured serum bicarbonate, returns the predicted PaCO2 window, and compares it against the measured PaCO2 to flag a concurrent respiratory acidosis (PaCO2 above the window) or respiratory alkalosis (PaCO2 below the window). The corollary that PaCO2 should approximately equal the last two digits of the arterial pH is also displayed.
When to use it
Use this whenever an ABG shows metabolic acidosis (low HCO3, low pH) and you need to decide whether the patient is appropriately compensating, retaining CO2 (mixed metabolic and respiratory acidosis, often a respiratory-failure or post-arrest picture), or hyperventilating beyond compensation (mixed metabolic acidosis and respiratory alkalosis, classic for salicylate toxicity and early sepsis). Failure of compensation is one of the earliest signals that a patient is tiring or has a second acid-base process driving the gas - it changes management urgency.
References
Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med 1967;66(2):312-322.
Worked example: Expected PaCO2 27-31; appropriate respiratory compensation
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