Respiratory alkalosis
Assess a primary PaCO₂ reduction and avoid mistaking its bicarbonate response for a separate metabolic acidosis.
Read the reduced PaCO₂ in context
A primary fall in PaCO₂ produces respiratory alkalosis. Bicarbonate falls in response, with a larger change in a sustained disturbance. A low PaCO₂ may also be appropriate respiratory compensation for metabolic acidosis; the clinical setting and compensation checks separate these possibilities.
The degree of bicarbonate reduction is interpreted against the selected course. A pH in the reference range does not erase a chronic respiratory disorder.
Expected bicarbonate response
Acute HCO₃⁻ = 24 − (1–2) × ΔCO₂
Chronic HCO₃⁻ = 24 − (4–5) × ΔCO₂
At PaCO₂ 30 mmHg
Acute HCO₃⁻ is expected around 22–23 mmol/L; chronic HCO₃⁻ around 19–20. These bands describe different histories.
The same separately labelled 2 mmol/L screening margin applies as in respiratory acidosis. Very low PaCO₂, recent ventilation changes and uncertain timing limit simple linear comparisons.
Investigate the stimulus to ventilation
- Assess oxygenation and the clinical probability of pulmonary disease, including pulmonary embolism when relevant.
- Consider fever, pain, systemic illness, medication or toxin exposure and central nervous system disease.
- Review pregnancy or liver-disease context and ventilator settings where applicable. Use a suitable reference framework for pregnancy.
- Do not assume anxiety explains hyperventilation before considering the clinical alternatives.
The differential explains possibilities. It neither establishes the cause nor directs a treatment dose.
When metabolic acidosis also exists
A corrected gap above its interval or bicarbonate below the expected course-specific response should trigger an additional metabolic assessment. With established metabolic acidosis, a PaCO₂ below Winter’s range supports a concurrent respiratory alkalosis.
A mixed gas with normal-range pHSources
MSD Manual Professional — Respiratory alkalosis · MSD Manual Professional — Primary changes and compensation · RCPA Manual — Alkalosis
Adult educational interpretation at 37°C. Apply arterial rules to arterial samples, use the laboratory’s reference intervals, and review the result alongside the clinical history.