When should an arterial blood gas (ABG) test be performed?

When to Perform Arterial Blood Gas (ABG) Testing

All critically ill patients require ABG measurement, and initial sampling should be arterial (not venous) in patients with shock, hypotension (systolic BP <90 mmHg), or any peri-arrest situation. 1, 2, 3

Critical Care Indications

ABG testing is mandatory in the following high-acuity situations:

• All critically ill patients including major trauma, sepsis, shock, and anaphylaxis 1, 2, 3
• Shock or hypotension (systolic blood pressure <90 mmHg) - arterial sampling is required, not venous 1, 2
• Post-cardiac arrest - after return of spontaneous circulation to guide oxygen therapy 2, 3
• Carbon monoxide poisoning - pulse oximetry will be falsely normal 3
• Major head injury - prior to securing the airway 3

Respiratory Deterioration

ABG should be obtained when patients show signs of respiratory compromise:

• Unexpected fall in SpO2 below 94% in patients breathing room air or supplemental oxygen 1, 2, 3
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) 1, 2, 3
• Increased oxygen requirements - previously stable patients who deteriorate clinically and require higher FiO2 to maintain constant oxygen saturation 1, 2, 3
• Risk factors for hypercapnic respiratory failure with acute breathlessness, deteriorating saturation, drowsiness, or other features of CO2 retention 1, 3
• Acute asthma, pneumonia, or acute respiratory exacerbations requiring oxygen therapy 3

Metabolic Disturbances

ABG is essential for detecting acid-base abnormalities:

• Suspected diabetic ketoacidosis 1, 2, 3
• Metabolic acidosis from renal failure 1, 2, 3
• Any breathless patient at risk for metabolic conditions affecting acid-base balance 1, 2, 3

Oxygen Therapy Monitoring

A critical pitfall is assuming normal pulse oximetry negates the need for ABG - it does not, especially in patients on supplemental oxygen. 1, 2, 3

ABG monitoring is required:

• Within 60 minutes of starting oxygen therapy in patients with COPD or risk factors for hypercapnic respiratory failure 3
• Within 60 minutes of any change in inspired oxygen concentration in at-risk patients 3
• After oxygen titration to confirm adequate oxygenation without precipitating respiratory acidosis or worsening hypercapnia 1, 2, 3
• Before giving FiO2 >28% via Venturi mask or >2 L/min via nasal cannulae in COPD patients aged ≥50 years 3
• After each titration of oxygen flow rate in patients with baseline hypercapnia 1, 2, 3

COPD and Hypercapnia-Prone Patients

For patients with COPD or conditions causing fixed airflow obstruction:

• Prior to oxygen therapy availability - aim for SpO2 88-92% using 24% Venturi mask at 2-3 L/min or 28% at 4 L/min or nasal cannulae at 1-2 L/min 1
• Recheck blood gases after 30-60 minutes on oxygen 1
• Adjust target to 94-98% if PCO2 is normal (unless history of previous ventilatory support) 1

Long-Term Oxygen Therapy (LTOT) Assessment

For home oxygen assessment:

• Two ABG measurements at least 3 weeks apart during clinical stability are required before confirming need for LTOT 1, 3
• After oxygen titration is complete to determine adequate oxygenation without respiratory acidosis 1, 2

Technical Considerations

• Use local anesthesia for all ABG specimens except emergencies 1, 2, 3
• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand 1, 3
• Obtain informed consent with discussion of risks 1, 3
• For non-critical patients, arterialized earlobe blood gases may substitute for arterial sampling to measure pH and PCO2 (though PO2 is less accurate) 1, 3

Critical Pitfalls to Avoid

• Normal SpO2 does not rule out abnormal pH, PCO2, or anemia - pulse oximetry will appear normal in patients with normal PO2 but abnormal acid-base status 1, 2, 3
• Pulse oximetry is falsely normal in carbon monoxide poisoning - ABG is mandatory regardless of oximeter readings 3
• Failing to repeat ABG after oxygen changes in patients at risk for CO2 retention can miss dangerous hypercapnia 1, 2
• Relying solely on pulse oximetry when acid-base status and ventilation assessment are needed 2, 3