Mixed Acid-Base Disorders and Compensation
On this page
Direct answer
A mixed acid-base disorder is two or more primary processes acting at once, and it is unmasked by checking whether the compensation expected for each primary change is appropriate. Winter's formula predicts PaCO2 as 1.5 times the bicarbonate plus 8, with a band of plus or minus 2, while the delta-delta ratio compares the rise in anion gap with the fall in bicarbonate to uncover a coexisting metabolic alkalosis or non-gap acidosis.
What you must remember
- Step one: pH gives the direction; PaCO2 and bicarbonate identify the primary respiratory or metabolic change, since the body compensates towards normal pH without fully restoring it.
- Winter's formula: expected PaCO2 in metabolic acidosis equals 1.5 times bicarbonate plus 8, plus or minus 2; a value outside the band signals a second disorder.
- Acute and chronic respiratory change: bicarbonate shifts about 1 mmol/L per 10 mmHg change in PaCO2 acutely, and about 3.5 to 4 mmol/L chronically.
- Anion gap: sodium minus the sum of chloride and bicarbonate, normally around 12 mmol/L; a raised gap points to lactic acidosis, ketoacidosis, uraemia or toxins.
- Delta-delta ratio: (anion gap minus 12) divided by (24 minus bicarbonate); a value between 1 and 2 suggests pure high-gap acidosis.
- Ratio below 1 or above 2: below 1 suggests a coexisting normal-gap acidosis, while above 2 suggests coexisting metabolic alkalosis or chronic respiratory acidosis.
- Metabolic alkalosis compensation: PaCO2 rises about 0.7 mmHg for each 1 mmol/L rise in bicarbonate, and hypoventilation beyond that signals a respiratory acidosis.
A raised gap with a PaCO2 that will not fall
Consider a patient with diabetes, sepsis and drowsiness, whose gas shows pH 7.22, PaCO2 46 mmHg and bicarbonate 18 mmol/L, with sodium 136 and chloride 96 mmol/L. The pH is acidaemic, and both bicarbonate and PaCO2 are abnormal in the acid direction, so two acidotic processes are likely. The primary metabolic acidosis is identified by the low bicarbonate. Winter's formula predicts a PaCO2 of 35 mmHg, with a band of 33 to 37, and the measured 46 mmHg lies well outside it, so a concurrent respiratory acidosis is present, probably from fatigue or sedation.
The anion gap is 136 minus 96 minus 18, which equals 22 mmol/L, so the metabolic acidosis is high-gap, likely ketoacidosis. The delta-delta ratio is about 1.7, which by itself would suggest a pure high-gap acidosis, but the ratio only interprets the metabolic component and cannot detect the respiratory process. The lesson is to use the ratio after Winter's formula, not instead of it.
Where the examiner tries to catch you
The classic trap is that a normal pH does not exclude a mixed disorder, since two opposing primary changes can cancel each other.
A second trap is compensation overshoot: the body never fully corrects pH, so a compensated pH usually stays abnormal in the direction of the primary disorder (chronic respiratory alkalosis can come close to normal), and a fully normal pH with abnormal PaCO2 and bicarbonate should prompt a search for a mixed disorder.
Candidates also forget that the anion gap is underestimated in hypoalbuminaemia, and the albumin-corrected gap adds about 2.5 mmol/L for each 1 g/dL that albumin falls below 4 g/dL.
Frequently asked questions
How is Winter's formula used in metabolic acidosis?
Winter's formula is used in metabolic acidosis to predict the PaCO2 that should be present if the respiratory system is compensating appropriately. The expected PaCO2 is 1.5 times the bicarbonate plus 8, plus or minus 2, and a measured value above or below this band indicates a concurrent respiratory disorder.
What does a delta-delta ratio above 2 indicate?
A delta-delta ratio above 2 in a high-gap metabolic acidosis indicates a coexisting metabolic alkalosis, such as vomiting in a patient with ketoacidosis, or a chronic respiratory acidosis. The ratio is calculated as (anion gap minus 12) divided by (24 minus bicarbonate), and it should be read only after the Winter band has been checked.
Can a normal pH exclude a mixed acid-base disorder?
A normal pH cannot exclude a mixed acid-base disorder, because two opposing primary processes, such as metabolic acidosis and respiratory alkalosis, can offset each other and leave the pH normal. Checking the bicarbonate, PaCO2 and anion gap together is therefore essential even when the pH looks reassuring.
How does hypoalbuminaemia affect the anion gap?
Hypoalbuminaemia lowers the measured anion gap because albumin is a major unmeasured anion, so a patient with a falling albumin can have a genuinely raised gap that looks normal. The albumin-corrected gap adds about 2.5 mmol/L for each 1 g/dL that albumin is below 4 g/dL, and this corrected value should be used.
What is the expected PaCO2 in metabolic alkalosis?
In metabolic alkalosis the PaCO2 rises by about 0.7 mmHg for every 1 mmol/L rise in bicarbonate, reflecting respiratory compensation. A PaCO2 that is higher than predicted suggests a concurrent respiratory acidosis, while a lower value suggests a concurrent respiratory alkalosis.