Reference ranges

pH

7.35–7.45

PaCO₂

35–45 mmHg (4.7–6.0 kPa)

PaO₂

75–100 mmHg on air (10–13 kPa)

HCO₃⁻

22–26 mmol/L

Base excess

−2 to +2

Lactate

<2 mmol/L

Anion gap

8–12 mmol/L (with K) or 6–12 (without)

SpO₂

94–98% (88–92% in CO₂ retainers)

Step-wise approach

• • 1. Oxygenation — is the patient hypoxic? Type 1 vs Type 2 RF
• • 2. pH — acidaemia (<7.35) or alkalaemia (>7.45)
• • 3. Primary disturbance — CO₂ moves with pH = respiratory; HCO₃ opposite = metabolic
• • 4. Compensation — appropriate? (apply rules)
• • 5. Anion gap if metabolic acidosis
• • 6. Delta–delta if HAGMA
• • 7. Synthesis — name the disorder, identify cause, treat