What are the indications for blood gas testing, including Arterial Blood Gas (ABG) and Venous Blood Gas (VBG), in the Emergency Department (ED)?

Indications for Blood Gas Testing in the Emergency Department

Blood gas testing in the Emergency Department is indicated for all critically ill patients, patients with respiratory distress, unexplained hypoxemia, suspected acid-base disturbances, and those at risk for hypercapnic respiratory failure. 1

Primary Indications for Blood Gas Testing

• All critically ill patients 1
• Unexpected or inappropriate fall in SpO₂ below 94% in patients breathing air or oxygen 1
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 1
• Previously stable patients who deteriorate clinically and require increased FiO₂ to maintain constant oxygen saturation 1
• Patients with risk factors for hypercapnic respiratory failure who develop acute breathlessness, deteriorating oxygen saturation, drowsiness, or other features of carbon dioxide retention 1
• Patients with breathlessness at risk of metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure 1
• Any unexpected change in vital signs such as a sudden rise in NEWS score or unexpected fall in oxygen saturation of 3% or more 1

Specific Clinical Scenarios Requiring Blood Gas Testing

Respiratory Conditions

• COPD exacerbations: ABG is indicated as spirometry alone cannot reliably identify patients with significant hypoxemia 2
• Severe asthma exacerbations: ABG is recommended for patients with FEV₁ or PEF ≤25% of predicted value after initial treatment 1
• Multiple rib fractures: Especially in patients with ≥3 ribs, flail chest, or pulmonary contusion 3

Critical Conditions

• Shock or hypotension (systolic BP <90 mmHg): Initial blood gas should be arterial 1
• Suspected respiratory failure: Particularly when evaluating PaCO₂ in patients with severe distress 1
• Patients requiring oxygen therapy: To guide appropriate oxygen administration 1

Choice Between Arterial vs. Venous Blood Gas

• Arterial blood gas (ABG) is required for:

  • Critically ill patients or those with shock/hypotension 1
  • Accurate assessment of oxygenation status (PaO₂) 4
  • Initial assessment of severe respiratory distress 1

• Venous blood gas (VBG) can be used for:

  • pH and HCO₃⁻ measurements (good correlation with ABG) 5
  • Follow-up measurements after initial ABG 1
  • Note: VBG PCO₂ values typically run 7-8 mmHg higher than arterial values 5

Special Considerations

COPD Patients

• All patients with suspected COPD exacerbations should have blood gas analysis in the ED 2
• Repeat blood gases after 30-60 minutes for all COPD patients even if initial PCO₂ was normal 1
• Target oxygen saturation of 88-92% for patients with COPD or other risk factors for hypercapnic respiratory failure 1

Asthma Patients

• Blood gas analysis is indicated for patients with severe distress or FEV₁/PEF ≤25% of predicted value 1
• Arterial blood gas measurements help detect hypoventilation in severe asthma exacerbations 1
• pH <7.25 and PaCO₂ >60 mmHg may indicate need for ventilatory support 6

Practical Considerations

• Local anesthesia should be used for all ABG specimens except in emergencies 1
• The presence of a normal SpO₂ does not eliminate the need for blood gas measurements, especially in patients on supplemental oxygen 1
• For most patients who are not critically ill, either arterial or arteriolized earlobe blood gases may be used to obtain accurate pH and PCO₂ 1

Common Pitfalls to Avoid

• Relying solely on pulse oximetry: Oximetry cannot detect hypercapnia or acid-base disturbances 1
• Delaying treatment for blood gas results: Laboratory studies must not delay treatment initiation 1
• Applying asthma criteria to COPD patients: Spirometric criteria that eliminate the need for ABGs in asthma cannot be safely applied to COPD patients 2
• Missing metabolic disorders: Blood gas analysis is essential for diagnosing mixed respiratory and metabolic acid-base disorders 4

By following these evidence-based indications for blood gas testing in the ED, clinicians can appropriately assess respiratory, circulatory, and metabolic disorders while avoiding unnecessary testing.