In a patient with acute or worsening respiratory distress, when should an arterial blood gas be obtained and how should it be interpreted and managed?

Arterial Blood Gas Analysis in Acute Respiratory Distress

When to Obtain an ABG

Obtain an arterial blood gas immediately in any patient with acute or worsening respiratory distress, particularly if SpO2 < 94%, if the patient requires supplemental oxygen, or if there is risk of hypercapnic respiratory failure. 1, 2

Critical Indications Requiring Immediate ABG

• All critically ill patients including those in peri-arrest situations, shock/hypotension, major trauma, sepsis, or anaphylaxis require immediate ABG measurement 1, 2
• SpO2 < 90% on presentation or any unexpected fall in SpO2 below 94% in patients breathing air or oxygen 1, 2
• Suspected hypercapnic respiratory failure, particularly in patients with severe/moderate COPD, chest wall/spinal disease, neuromuscular disease, severe obesity, cystic fibrosis, or bronchiectasis 3, 2
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in previously stable patients with chronic hypoxemia 2, 4
• Unexplained confusion or agitation, as this may indicate hypoxemia and/or hypercapnia 3
• Metabolic concerns such as suspected diabetic ketoacidosis or metabolic acidosis from renal failure 2, 4

Common Pitfall

A normal SpO2 does not exclude the need for ABG—pulse oximetry cannot detect hypercapnia, respiratory acidosis, metabolic acidosis, or anemia-related hypoxia 2, 4. In carbon monoxide poisoning, pulse oximetry readings are falsely normal and ABG is mandatory 2.

How to Interpret ABG Results

Step 1: Assess Oxygenation Status

• PaO2 < 8.0 kPa (60 mmHg) indicates hypoxemia requiring oxygen therapy 3
• Consider the FiO2 when interpreting—adequate oxygenation should be achieved without excessive oxygen delivery 4

Step 2: Identify Acid-Base Status

• pH < 7.35 with PaCO2 > 6.0 kPa indicates respiratory acidosis with hypercapnia 3, 1
• pH < 7.35 with normal or low PaCO2 suggests metabolic acidosis requiring investigation and treatment of underlying cause 1
• Use systematic approach: assess pH first, then determine if primary disturbance is respiratory (PaCO2) or metabolic (HCO3) 5, 6

Step 3: Risk Stratify for Hypercapnic Respiratory Failure

Patients at risk include those with:

• Severe or moderate COPD, especially with previous respiratory failure or on long-term oxygen 3
• Severe chest wall/spinal disease, neuromuscular disease, severe obesity 3
• Cystic fibrosis or bronchiectasis 3

Management Based on ABG Results

For Patients NOT at Risk of Hypercapnic Respiratory Failure

Target SpO2: 94-98% 3, 1

• Start oxygen therapy to achieve target saturation 3
• Repeat ABG at 30-60 minutes after starting oxygen therapy 1, 4
• If target saturation not achieved, seek senior review and treat urgently 3
• Consider reducing FiO2 if PaO2 > 8.0 kPa to avoid hyperoxia 3

For Patients AT RISK of Hypercapnic Respiratory Failure

Target SpO2: 88-92% 3, 1

• Start with controlled low-flow oxygen: 24% Venturi mask at 2-3 L/min or 28% Venturi mask at 4 L/min 1
• Alternatively, use 1-2 L/min nasal oxygen 3
• Mandatory repeat ABG within 60 minutes of starting oxygen or any change in FiO2 2, 4
• Monitor after each titration of oxygen flow rate to detect worsening hypercapnia 2, 4

Critical Thresholds for Escalation

Seek immediate senior review and consider NIV or invasive ventilation if: 3, 1

• pH < 7.35 with PaCO2 > 6.0 kPa (respiratory acidosis despite oxygen therapy) 3, 1
• pH < 7.25 with hypercapnia—intubate immediately 1
• Respiratory rate > 25 breaths/min with SpO2 < 90% despite optimal therapy 1
• PaCO2 > 6.5 kPa with pH < 7.35 despite optimal medical therapy 1

Special Situations

Carbon monoxide poisoning: Give maximum oxygen via reservoir mask (15 L/min) regardless of oximetry readings, as pulse oximetry cannot differentiate carboxyhaemoglobin from oxyhaemoglobin 1

Critically ill patients: Commence 15 L/min oxygen via reservoir mask immediately, then titrate based on ABG results 3

Monitoring Requirements

• First repeat ABG: 30-60 minutes after starting oxygen therapy in at-risk patients 1, 4
• Second repeat ABG: 1 hour after achieving intended therapeutic flow rate 1
• Subsequent ABGs: After any change in inspired oxygen concentration, or with unexpected deterioration in NEWS score or fall in SpO2 ≥3% 2
• Use recognized physiological track-and-trigger systems (e.g., NEWS) for ongoing monitoring 3

Technical Considerations

• Use local anesthesia for all ABG specimens except emergencies 2, 4
• Perform Allen's test before radial artery puncture to ensure dual blood supply to hand 2, 4
• Arterial samples are mandatory in critically ill patients—venous blood gas cannot assess oxygenation adequacy and shows unreliable correlation in circulatory failure 1