Predictable formulae reveal whether a second disorder is present
Mnemonic
“Respiratory opposite, metabolic equal” identifies the primary problem; these rules then tell you whether compensation is appropriate:
- Metabolic acidosis: expected PaCO2 falls by about 1 to 1.3 kPa for every 10 mmol/l fall in bicarbonate. Winter’s formula in mmHg: expected PaCO2 = 1.5 x HCO3 + 8
- Metabolic alkalosis: carbon dioxide rises, but compensation is limited by hypoxic drive
- Acute respiratory change: bicarbonate moves by about 1 mmol/l per 1.3 kPa of carbon dioxide
- Chronic respiratory change: bicarbonate moves by about 4 mmol/l per 1.3 kPa
“Acute buffers, chronic compensates”: the immediate change is buffering only, while the kidney takes 2 to 5 days to compensate fully.
If the measured compensation differs from the expected, there is a second, independent disorder, which is the entire purpose of the rules.
Expansion
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | Winter’s formula: PaCO2 = (1.5 x HCO3) + 8, plus or minus 2 |
| Metabolic alkalosis | PaCO2 rises about 0.7 mmHg per 1 mmol/l rise in bicarbonate |
| Acute respiratory acidosis | Bicarbonate rises 1 mmol/l per 10 mmHg rise in PaCO2 |
| Chronic respiratory acidosis | Bicarbonate rises 4 mmol/l per 10 mmHg |
| Acute respiratory alkalosis | Bicarbonate falls 2 mmol/l per 10 mmHg fall |
| Chronic respiratory alkalosis | Bicarbonate falls 5 mmol/l per 10 mmHg |
Respiratory compensation begins within minutes and is complete in hours; metabolic compensation takes 3 to 5 days.
The value of these rules is in detecting mixed disorders. If the measured carbon dioxide is higher than Winter’s formula predicts, there is an additional respiratory acidosis, perhaps from tiring or sedation; if lower, there is an additional respiratory alkalosis, as in salicylate poisoning or sepsis.
Compensation never overcorrects, so a pH on the wrong side of normal always means a second primary process.