Respiratory acidosis
Interpret a primary PaCO₂ increase using the clinical time course and the expected bicarbonate response.
Start with carbon dioxide and the history
A primary PaCO₂ increase reflects inadequate alveolar ventilation for CO₂ production. Bicarbonate rises through buffering and, over time, renal adaptation. The pH may remain low or move into the reference interval.
The history selects acute, chronic, or evolving disease. A gas can be compatible with a chronic response without proving how long the disturbance has been present. Compare earlier results, respiratory status and recent ventilation changes.
Compare the expected bicarbonate
Acute HCO₃⁻ = 24 + (1–2) × ΔCO₂
Chronic HCO₃⁻ = 24 + (3–4) × ΔCO₂
At PaCO₂ 60 mmHg
The acute band is 26–28 mmol/L; the chronic band is 30–32. HCO₃⁻ 27 fits acute, while 31 fits chronic.
The app uses an additional 2 mmol/L screening margin before suggesting an extra process. Values just outside the formula band remain borderline. A higher bicarbonate can also fall within published variation in stable chronic hypercapnia; that possibility is flagged rather than automatically called metabolic alkalosis.
Look for additional processes and causes
Bicarbonate substantially below the selected expected band suggests added metabolic acidosis; substantially above suggests added metabolic alkalosis. When the course is unknown, the acute and chronic possibilities stay visible separately.
- Ventilatory drive: sedating medication, intoxication or central nervous system pathology.
- Ventilatory mechanics: airway obstruction, obstructive lung disease, neuromuscular weakness or restrictive disease.
- Supported ventilation: review the patient, airway and delivered ventilation rather than inferring a device problem from the gas alone.
Chronic response and changing physiology
A prospective study of 28 stable outpatients found a larger chronic bicarbonate response than the conventional 3–4 per 10 rule. Its approximate guide was HCO₃⁻ = 24 + 0.5 × (PaCO₂ − 40), with about ±2 mmol/L variation, for steady-state PaCO₂ up to 70 mmHg. The app uses this as a reason to preserve uncertainty when a higher chronic response is plausible.
A single steady-state equation cannot establish an acute-on-chronic change. The previous PaCO₂ and bicarbonate, timing, examination and ventilatory history are central. A high bicarbonate may be residual renal adaptation after a recent CO₂ change.
A venous PCO₂ is not entered as an arterial PaCO₂. The calculator retains numeric results for non-arterial samples while withholding arterial compensation conclusions.
Sources
MSD Manual Professional — Respiratory acidosis · MSD Manual Professional — Primary changes and compensation · González et al. — Chronic respiratory-acidosis response (2018) · AARC Clinical Practice Guideline — Blood gas analysis and haemoximetry (2013)
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.