When is an arterial blood gas (ABG) analysis recommended for patients with respiratory distress or failure?

When to Perform Arterial Blood Gas Analysis in Respiratory Distress or Failure

ABG analysis should be obtained in all critically ill patients with respiratory distress or failure, particularly when oxygen saturation falls below 94%, when there is clinical deterioration requiring increased oxygen, or when patients have risk factors for hypercapnic respiratory failure. 1, 2

Critical Indications Requiring Immediate ABG

• All critically ill patients require ABG measurement, with samples obtained from arterial rather than venous sources 1
• Peri-arrest situations or patients requiring immediate medical intervention mandate ABG testing 1
• After return of spontaneous circulation following cardiopulmonary resuscitation, obtain ABG to guide ongoing oxygen therapy 1, 2
• Major trauma, sepsis, shock, and anaphylaxis all require ABG measurement 1

Respiratory-Specific Indications

Acute Deterioration

• Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen requires ABG measurement 1, 2
• Deteriorating oxygen saturation or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) necessitates ABG 1, 2
• Previously stable patients who deteriorate clinically and require increased FiO2 to maintain constant oxygen saturation should have ABG analysis 1, 2

High-Risk Populations

• Any patient with risk factors for hypercapnic respiratory failure (COPD, obesity hypoventilation, neuromuscular disease) who develops acute breathlessness, deteriorating oxygen saturation, drowsiness, or other features of carbon dioxide retention requires ABG measurement 1
• Acute asthma, pneumonia, and acute respiratory exacerbations requiring oxygen therapy should prompt ABG measurement 1
• Major head injury cases require ABG prior to securing the airway 1

Monitoring During Oxygen Therapy

Initial Assessment

• Within 60 minutes of starting oxygen therapy in patients with COPD or other risk factors for hypercapnic respiratory failure, obtain ABG 1, 2
• After oxygen titration is complete, perform ABG to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis or worsening hypercapnia 1, 2

Ongoing Monitoring

• Within 60 minutes of any change in inspired oxygen concentration in at-risk patients, obtain ABG 1, 2
• After each titration of oxygen flow rate in patients with baseline hypercapnia, perform ABG to detect respiratory acidosis 1, 2
• For COPD patients aged 50 years or more, check ABG before giving FiO2 of more than 28% via Venturi mask or 2 L/min via nasal cannulae 1

Non-Invasive Ventilation (NIV) Context

• ABG measurement is needed prior to and following starting NIV 3
• After 1-2 hours of NIV, measure ABG in most patients 3
• After 4-6 hours if the earlier sample showed little improvement, repeat ABG 3
• If there has been no improvement in PaCO2 and pH after this period despite optimal ventilator settings, NIV should be discontinued and invasive ventilation considered 3

Metabolic Indications

• Patients with breathlessness who may have metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure require ABG measurement 1, 2

Track and Trigger Systems

• When there is an unexpected change in "track and trigger" systems such as a sudden rise in NEWS or an unexpected fall in oxygen saturation of 3% or more, obtain ABG 1

Critical Pitfalls to Avoid

Common Misconceptions

• A normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy 1, 2
• Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2, or with low oxygen content due to anemia 1, 2
• In carbon monoxide poisoning, pulse oximetry readings may be falsely normal, necessitating ABG regardless of oximeter readings 1

Technical Considerations

• Local anesthesia should be used for all ABG specimens except in emergencies 1, 2
• Patients undergoing radial ABG should have an Allen's test performed first to ensure dual blood supply to the hand 1, 2
• Informed consent should be obtained for the procedure with discussion of possible risks 1

Alternative Sampling Methods

• For most non-critical patients requiring blood gas sampling, arterialised earlobe blood gases may be used as an alternative to obtain accurate pH and PCO2 measurements, though PO2 is less accurate in these samples 1, 2
• When using earlobe blood gas samples, oximetry should be monitored carefully and a repeat arterial specimen taken if there are concerns about accuracy 1