Taking and reading an ABG

Act now

1. 1

   Consent; pre-heparinised syringe; local anaesthetic if time allows; needle at 30–45° to the pulse.

3. 2

   Firm pressure for 5 minutes afterwards (longer if anticoagulated).

5. 3

   Analyse within 10–15 minutes.

Assess

• Indication: hypoxia, suspected CO₂ retention, acid–base disturbance, sepsis or shock.
• Anticoagulants or low platelets; prefer the radial artery; check collateral flow per local practice.

Investigations

• Repeat the gas 30–60 minutes after any change to oxygen or ventilation, or sooner if deteriorating.

Manage

• Normal: pH 7.35–7.45; PaCO₂ 4.7–6.0 kPa; PaO₂ 10–13 kPa on air; HCO₃⁻ 22–26 mmol/L; base excess −2 to +2; lactate under 2 mmol/L.
• Type 1 respiratory failure: PaO₂ under 8 kPa with normal or low PaCO₂. Type 2: PaO₂ under 8 kPa with PaCO₂ over 6 kPa.
• Rough check: expected PaO₂ (kPa) is about FiO₂ (%) minus 10.
• Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). High: lactate, ketones, renal failure, toxins (salicylate, methanol, ethylene glycol). Normal: diarrhoea, renal tubular acidosis, large volumes of 0.9% saline.

Escalate when