Synopsis
Interpret blood gases by checking sample context, identifying the primary disturbance, testing compensation and the anion gap, and connecting the result to an actionable clinical explanation.
- Check whether the sample is arterial or venous and record oxygen delivery before interpreting it.
- Acidaemia means pH below 7.35; alkalinaemia means pH above 7.45.
- Examine carbon dioxide and bicarbonate even when the pH falls within the reference range.
Reasoning priorities
Establish whether the values answer the intended clinical question.
Check patient identity, sampling time, oxygen delivery and arterial versus venous origin. Air contamination and delayed analysis can distort results and warrant repeat sampling if inconsistent.
Worked reasoning
An acutely ill adult has pH 7.10, bicarbonate 12 mmol/L, PaCO2 5.3 kPa, sodium 140 mmol/L and chloride 100 mmol/L; albumin is normal.
- Identify acidaemia with low bicarbonate, making metabolic acidosis a major component. Calculate the anion gap as 140 − (100 + 12), giving 28 mmol/L.
- Calculate expected PaCO2 as 1.5 multiplied by 12 plus 8, giving 26 mmHg with an approximate range of 24–28 mmHg, equivalent to about 3.2–3.7 kPa.
- Convert the measured 5.3 kPa to approximately 40 mmHg. This is above the expected compensatory range, identifying an additional respiratory acidosis rather than adequate compensation.
- Assess and support ventilation urgently while testing lactate, ketones and renal function to explain the raised gap; investigate toxic exposure when the history or results support it.
- Verify the arithmetic and repeat clinical and gas assessment after intervention. Improved pH alone is insufficient unless ventilation, perfusion and the identified underlying process also improve.