How is an arterial blood gas (ABG) used?

How Arterial Blood Gas (ABG) is Used

ABG is the gold standard diagnostic test for assessing respiratory failure, acid-base disturbances, oxygenation adequacy, and ventilation status by measuring arterial pH, PaO2, PaCO2, and bicarbonate levels. 1, 2

Primary Clinical Applications

Assessment for Long-Term Oxygen Therapy (LTOT)

• Patients with resting SpO2 ≤92% should be referred for ABG assessment to determine LTOT eligibility. 1
• Initial LTOT assessment requires ABG sampling (Grade A recommendation), with two measurements at least 3 weeks apart during clinical stability to confirm need. 1
• Patients with peripheral edema, polycythemia (hematocrit ≥55%), or pulmonary hypertension may warrant ABG assessment at SpO2 ≤94% to identify PaO2 ≤8 kPa. 1
• After oxygen titration, ABG must be repeated to confirm adequate oxygenation (target PaO2 ≥8 kPa) without precipitating respiratory acidosis or worsening hypercapnia. 1

Monitoring Hypercapnia and Respiratory Acidosis

• Patients with baseline hypercapnia require ABG monitoring after each oxygen flow rate titration to detect respiratory acidosis development. 1
• A respiratory acidosis (pH <7.35) and/or PaCO2 rise >1 kPa (7.5 mmHg) during LTOT assessment indicates clinically unstable disease requiring medical optimization. 1
• If respiratory acidosis develops on two separate occasions despite apparent stability, domiciliary oxygen should only be prescribed with nocturnal ventilatory support. 1

Acute Care and Emergency Settings

• In acutely hypoxemic patients at risk of hypercapnic respiratory failure (severe COPD, chest wall disease, neuromuscular disease, obesity), target SpO2 88-92% and obtain ABG to guide oxygen therapy. 1
• For patients not at risk of hypercapnia with SpO2 <94%, target SpO2 94-98% and check ABG if respiratory acidosis is suspected (pH <7.35 with PaCO2 >6.0 kPa). 1
• ABG is essential for determining need for mechanical ventilation (invasive or non-invasive) in patients with acute respiratory failure. 1

Procedural Considerations

Technique and Safety

• Perform Allen's test before radial ABG sampling to confirm dual blood supply from radial and ulnar arteries. 1
• Obtain informed consent with discussion of risks including hematoma, pain, and arterial injury. 1
• The distal forearm radial approach has higher first-pass success (74.7%) compared to anatomical snuffbox approach (60.7%) for ABG sampling. 3

Alternatives to ABG

• Capillary blood gas (CBG) can substitute for ABG when re-measuring PaCO2 and pH during oxygen titration (Grade A). 1
• Cutaneous capnography may replace ABG for PaCO2 measurement alone (not pH) during oxygen titration (Grade A). 1
• In community settings where ABG is impractical, CBG combined with oximetry (not capnography alone) can be used for LTOT assessment, though this may result in some patients receiving LTOT unnecessarily. 1

Important Caveats

Venous Blood Gas Limitations

• Venous blood gas (VBG) is NOT an acceptable substitute for ABG in diagnosing respiratory failure or hypercarbia. 4
• VBG has high sensitivity (97.6%) but very poor specificity (36.9%) for respiratory failure diagnosis, resulting in unacceptably high false-positive rates. 4
• While venous lactate may substitute for arterial lactate (96% agreement), venous pH and base deficit fall outside clinically acceptable limits in 27.4% and 23.5% of cases respectively. 5

Clinical Context Matters

• ABG should not be routinely ordered at time of acute exacerbation; wait for 8 weeks of clinical stability before formal LTOT assessment. 1
• Evidence suggests 33-66% of ABG samples in critical care lack clear clinical indication, contributing to iatrogenic anemia, unnecessary costs, and environmental burden. 6
• ABG is essential for assessing acid-base status (pH), oxygenation (PaO2), ventilation (PaCO2), and metabolic compensation (HCO3), which together guide diagnosis and treatment of critically ill patients. 2, 7

Special Populations

• For air travel, patients with sea-level hypoxemia should increase oxygen flow by 1-2 L/min to prevent PaO2 falling below 55 mmHg (7.0 kPa). 1
• During anesthesia for patients with severe COPD (FEV1 <1.0 L), prolonged expiration time is critical, and postoperative ABG guides need for mechanical ventilation if respiratory acidosis or severe hypoxemia develops. 1