Makindo Medical Notes

📌 Note: ABG sampling, particularly from the radial artery, can be painful. Offer 1 ml of 1% lidocaine at the puncture site to reduce pain without altering results.

Indications

• Assessment of respiratory failure and guiding therapy.
• Post-cardiac arrest management (baseline and ongoing monitoring).
• Acid–base assessment in critically unwell patients.

What ABG Provides

• pH (acid–base balance).
• Partial pressure of oxygen (PaO₂).
• Partial pressure of carbon dioxide (PaCO₂).
• Bicarbonate (HCO₃⁻).
• Base excess and calculated anion gap.

Preparation

• Wait 15–20 minutes after any O₂ therapy change for steady state.
• Explain procedure, risks (pain, bleeding, thrombosis, infection).
• Gain consent. Consider chaperone.

Contraindications

• Poor collateral circulation (check with modified Allen test).
• Severe coagulopathy or anticoagulation (relative).
• Severe peripheral vascular disease.

Equipment

• 2–3 ml heparinised syringe with cap.
• 20–22G needle (radial); larger for femoral access.
• Antiseptic wipes, gauze, tape, sterile gloves.
• Sharps bin.

Procedure

1. Wash hands, wear gloves, explain to patient.
2. Identify radial artery. Perform modified Allen test.
3. Position wrist extended, palm up. Palpate pulse.
4. Clean site with antiseptic.
5. Expel excess heparin from syringe. Insert needle at 30–45° (bevel up).
6. Bright red blood should pulsate into syringe under pressure.
7. Obtain 1–3 ml, then withdraw needle and apply firm pressure ≥5 min (longer if anticoagulated).
8. Cap syringe, remove air bubbles, send sample rapidly (on ice if delay).
9. Document FiO₂ at time of sampling.

Arterial vs Venous Blood

• Arterial: bright red, fills syringe under pressure.
• Venous: darker, slow flow, may require suction.

Normal Ranges

• pH: 7.35 – 7.45
• PaO₂: 11 – 13 kPa (82 – 97 mmHg)
• PaCO₂: 4.7 – 6.0 kPa (35 – 45 mmHg)
• HCO₃⁻: 22 – 26 mmol/L
• Base excess: –2 to +2
• Anion gap: 8 – 12 (without K⁺)

Stepwise Interpretation

1. Check pH: acidaemia (<7.35) or alkalaemia (>7.45)?
2. Identify primary process:
   • Low HCO₃⁻ → metabolic acidosis.
   • High HCO₃⁻ → metabolic alkalosis.
   • High PaCO₂ → respiratory acidosis.
   • Low PaCO₂ → respiratory alkalosis.
3. Check compensation:
   • Metabolic acidosis → Winter’s formula: PaCO₂ ≈ 1.5 × HCO₃⁻ + 8 (±2).
   • Metabolic alkalosis → PaCO₂ ≈ 0.7 × (HCO₃⁻ – 24) + 40.
   • Acute resp. acidosis: HCO₃⁻ ↑ ~1 per 10 PaCO₂ ↑.
   • Chronic resp. acidosis: HCO₃⁻ ↑ ~3–4 per 10 PaCO₂ ↑.
   • Acute resp. alkalosis: HCO₃⁻ ↓ ~2 per 10 PaCO₂ ↓.
   • Chronic resp. alkalosis: HCO₃⁻ ↓ ~4–5 per 10 PaCO₂ ↓.
4. If metabolic acidosis: calculate anion gap.
   • High AG → DKA, lactic acidosis, uraemia, toxins (methanol, ethylene glycol, salicylates).
   • Normal AG → diarrhoea, renal tubular acidosis, saline excess.
5. Match to clinical context.

Common Patterns & Causes

• Metabolic acidosis: DKA, sepsis/lactate, renal failure, diarrhoea.
• Metabolic alkalosis: vomiting, NG losses, diuretics, hyperaldosteronism.
• Respiratory acidosis: COPD exacerbation, opiates, severe asthma, neuromuscular disease.
• Respiratory alkalosis: anxiety, sepsis, pregnancy, pulmonary embolism.

Teaching Pearls

• Always record FiO₂ with the ABG.
• VBG useful for acid–base trend; ABG required for oxygenation.
• Lactate guides sepsis resuscitation.
• Correct AG for low albumin: each 10 g/L drop ↓ AG by ~2.5–3.

Related topics: Metabolic Acidosis | ABG Analysis