Normal values
- pH
- 7.35–7.45
- PaCO₂
- 4.5–6.0 kPa (35–45 mmHg)
- PaO₂
- 10–13 kPa on air
- HCO₃⁻
- 22–26 mmol/L
- BE
- −2 to +2
- Lactate
- <2 mmol/L
Steps
- 1. Oxygenation: PaO₂ < 8 = type 1 RF; +PaCO₂ > 6 = type 2
- 2. pH: acidaemia (<7.35) or alkalaemia (>7.45)
- 3. CO₂: respiratory cause? Same direction = primary metabolic
- 4. HCO₃: metabolic cause?
- 5. Compensation: appropriate?
- 6. Anion gap if metabolic acidosis: Na − (Cl + HCO₃), normal 8–12
Causes
- Resp acidosis: COPD, opioids, NM disease
- Resp alkalosis: anxiety, PE, sepsis, salicylates
- Met acidosis HAG: MUDPILES, lactate, ketones
- Met acidosis NAG: diarrhoea, RTA
- Met alkalosis: vomiting, diuretics
