Arterial Blood Gas Interpretation
On this page
Direct answer
pH 7.20, PaCO2 24 mmHg, bicarbonate 10 mmol per litre — a gas like this is read through a fixed five-step sequence: the pH names acidaemia or alkalaemia, the direction of PaCO2 and bicarbonate change names the primary disturbance, the compensation formula judges whether a second disorder hides, the anion gap unmasks mixed metabolic pictures, and the alveolar-arterial gradient explains any hypoxaemia. Normal values are pH 7.35 to 7.45, PaCO2 35 to 45 mmHg and bicarbonate 22 to 26 mmol per litre, with Winter's formula foremost among the expected-compensation calculations.
What you must remember
- Step 1 and 2: pH below 7.35 is acidaemia, above 7.45 alkalaemia; if PaCO2 moves in the direction that would cause the pH change, the primary problem is respiratory, otherwise metabolic.
- Compensation formulas: metabolic acidosis — expected PaCO2 equals 1.5 times the bicarbonate plus 8, plus or minus 2 (Winter's formula); metabolic alkalosis — roughly 0.7 times bicarbonate plus 20, plus or minus 5; acute respiratory acidosis adds 1 mmol of bicarbonate per 10 mmHg PaCO2 rise, chronic 3.5 to 4; acute respiratory alkalosis falls 2 per 10, chronic 4 to 5 — compensation never overcorrects the pH.
- Anion gap equals sodium minus chloride plus bicarbonate, normal 8 to 12 mmol per litre; add 2.5 mmol for every 1 g per dL of albumin below 4 g per dL; a high gap marks lactic acidosis, ketoacidosis, renal failure and toxins, a normal gap diarrhoea, renal tubular acidoses and ureteric diversion.
- The delta ratio (rise in anion gap divided by fall in bicarbonate) exposes mixed pictures: around 1 in pure high-gap acidosis, above 2 with coexisting metabolic alkalosis, below 1 with an additional normal-gap acidosis.
- The urine anion gap (sodium plus potassium minus chloride) separates normal-gap acidoses: negative indicates gastrointestinal bicarbonate loss, positive a renal tubular acidosis.
- Alveolar-arterial gradient equals alveolar minus arterial oxygen tension, where alveolar oxygen equals inspired oxygen fraction times (atmospheric pressure minus 47) minus PaCO2 divided by 0.8; hypoxaemia with a normal gradient means hypoventilation or low inspired oxygen, a widened gradient means mismatch, shunt, diffusion defect or right-to-left flow.
- Clinical anchors: lactate above 2 mmol per litre flags tissue hypoperfusion, and repeat sampling after intervention confirms the response.
One gas, five steps, worked aloud
Take pH 7.20, PaCO2 24, bicarbonate 10, sodium 140, chloride 100. Step one: acidaemia. Step two: the low bicarbonate in an acidotic patient names a primary metabolic acidosis. Step three, Winter's formula: expected PaCO2 equals 1.5 times 10 plus 8 — 23, plus or minus 2 — and the measured 24 sits inside, so compensation is appropriate. Step four, the anion gap: 30 — high — widened further by adding 2.5 for the albumin gram below 4. The delta ratio — gap rise 18 over bicarbonate fall 14, about 1.3 — sits near a pure high-gap acidosis; above 2 flags a coexisting metabolic alkalosis, below 1 an extra normal-gap acidosis. Step five, hypoxaemia: the alveolar-arterial gradient separates hypoventilation and low inspired oxygen (normal, the drowsy overdose vignette) from mismatch, shunt, diffusion defect and right-to-left flow (widened). Lactate above 2 flags hypoperfusion; a repeat gas after intervention closes the loop.
Where students slip
The commonest error is naming compensation a disorder — calling the low PaCO2 of compensated metabolic acidosis a respiratory alkalosis; Winter's is the most tested calculation in medicine papers. Albumin correction is forgotten in the hypoalbuminaemic patient, mislabelling a gap acidosis as normal. In hypoxaemia, the gradient separates hypoventilation (normal) from shunt or mismatch (widened), and the 100 per cent oxygen test separates shunt, which persists, from mismatch, which corrects.
Frequently asked questions
What is Winter's formula?
Expected PaCO2 equals 1.5 times the serum bicarbonate plus 8, plus or minus 2 mmHg — testing whether respiratory compensation for a metabolic acidosis is appropriate.
How is the anion gap calculated and corrected?
Sodium minus chloride and bicarbonate (normal 8 to 12 mmol per litre), adding 2.5 mmol for each 1 g per dL of albumin below 4 g per dL.
What does the urine anion gap tell in normal-gap acidosis?
A negative value indicates adequate urinary ammonium and gastrointestinal bicarbonate loss; a positive value indicates a renal tubular acidosis.
When is hypoxaemia associated with a normal A-a gradient?
With hypoventilation (sedatives, obesity-hypoventilation, neuromuscular weakness) or a low inspired oxygen tension at altitude.
How much does bicarbonate rise in chronic respiratory acidosis?
About 3.5 to 4 mmol per litre for every 10 mmHg rise in PaCO2, against about 1 mmol acutely — a favourite renal-pulmonary crossover.
Why calculate the delta ratio?
To unmask a second disorder — a coexisting metabolic alkalosis or normal-gap acidosis hidden behind a high-gap acidosis.