Arterial Blood Gas (ABG) Test: Proper Procedures and Precautions
Before performing a radial ABG, always conduct an Allen's test to confirm dual blood supply to the hand from both radial and ulnar arteries, and obtain informed consent discussing possible risks. 1, 2
Pre-Procedure Assessment and Preparation
Patient Selection and Timing
- Perform ABG sampling from arterial sources (not venous) in patients with shock, hypotension, or those on vasopressor therapy 3
- Obtain ABG in all critically ill patients to assess oxygenation, ventilation, and acid-base status 3, 2
- Do not perform ABG during acute exacerbations unless the patient has SpO2 ≤92%, is breathless, and unable to manage off oxygen 1
Allen's Test Protocol
- Mandatory before radial artery puncture to ensure collateral circulation from the ulnar artery 1, 2
- This prevents hand ischemia if radial artery thrombosis occurs post-procedure 1
Informed Consent
- Discuss risks including pain, bleeding, hematoma formation, arterial thrombosis, and nerve injury 1, 2
- Use local anesthesia for all ABG specimens except in emergencies 3, 2
Sampling Technique
Site Selection
- Radial artery is preferred for most patients 1
- In ECMO patients, sample from the right radial arterial line as this best represents cerebral perfusion 3
- Alternative sites include femoral or brachial arteries if radial access is not feasible 4
Collection Method
- Use pre-heparinized syringes to prevent clotting and ensure accurate results 4
- Samples can be obtained via arterial catheter or percutaneous needle puncture 4
- Only healthcare professionals with formal training and demonstrated proficiency should perform ABG sampling 4
Post-Procedure Monitoring
Immediate Actions
- Apply firm pressure to puncture site for at least 5 minutes (longer if patient is anticoagulated) 4
- Monitor for hematoma formation or signs of arterial occlusion 4
- Ensure sample is analyzed promptly to maintain accuracy 4
Specific Clinical Indications Requiring ABG
Respiratory Indications
- Oxygen saturation fall below 94% on room air or supplemental oxygen 2
- Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 2
- Within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in patients at risk for hypercapnic respiratory failure 3, 2
Metabolic Indications
- Suspected diabetic ketoacidosis 5
- Metabolic acidosis from renal failure, trauma, shock, or sepsis 5
- After return of spontaneous circulation following cardiopulmonary resuscitation 3
Monitoring During Oxygen Titration
- After oxygen titration is complete to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis 1, 2
- After each titration of flow rate in patients with baseline hypercapnia 1, 2
Critical Pitfalls to Avoid
Common Errors
- Never assume normal pulse oximetry rules out acid-base disturbances or hypercapnia 3, 2
- Pulse oximetry will appear normal in patients with normal PaO2 but abnormal pH or PaCO2 3, 2
- Do not rely solely on pulse oximetry when acid-base status and ventilation need assessment 2
Special Population Considerations
- In COPD patients with known CO2 retention, check ABG when starting oxygen therapy 3, 5, 2
- Patients with baseline hypercapnia require ABG monitoring after each flow rate titration to detect respiratory acidosis 1, 2
- If PaCO2 rises >1 kPa (7.5 mm Hg) during oxygen therapy, the patient may have clinically unstable disease requiring further medical optimization 1, 2
Alternative Sampling Methods
When Arterial Sampling is Not Practical
- Capillary blood gases (CBG) can be used for re-measuring PaCO2 and pH during oxygen titration 1
- Cutaneous capnography can measure PaCO2 alone but not pH 1
- In community settings, a combination of CBGs and oximetry (but not capnography alone) may be used as an alternative for initial assessment 1
- For most non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation 2