How do you use arterial blood gas (ABG) to set target oxygen saturation (SpO2) for a patient with asthma?

Using ABG to Set Target Oxygen Saturation in Asthma Patients

In patients with acute asthma, aim for an oxygen saturation of 94-98% and use arterial blood gas measurements to guide oxygen therapy and detect complications. 1

Initial Assessment and Oxygen Therapy Protocol

1. Initial oxygen therapy approach:

   • Start with nasal cannulae at 2-6 L/min or simple face mask at 5-10 L/min
   • If SpO₂ is below 85%, use a reservoir mask at 15 L/min
   • Target saturation range: 94-98% 1

2. When to obtain ABG measurements:

   • At presentation for all patients with severe asthma
   • When SpO₂ < 92% despite oxygen therapy
   • If there are signs of deterioration or increased work of breathing
   • If peak expiratory flow (PEF) is < 25% of predicted (indicates risk of hypercapnia) 2

Interpreting ABG Results in Asthma

Normal ABG Pattern in Asthma

• Most patients with acute asthma present with:
  • Hypoxemia (low PaO₂)
  • Normal or low PaCO₂ (due to hyperventilation)
  • Respiratory alkalosis (high pH) 3

ABG-Based Decision Algorithm

1. If ABG shows normal or low PaCO₂ with respiratory alkalosis:

   • Continue targeting SpO₂ 94-98%
   • This is the expected pattern in asthma
   • Monitor for clinical improvement

2. If ABG shows rising PaCO₂ (compared to baseline or previous ABG):

   • This indicates worsening respiratory failure and fatigue
   • Consider escalation to high-dependency or intensive care
   • Prepare for possible non-invasive or invasive ventilation 1

3. If ABG shows hypercapnia (PaCO₂ > 45 mmHg):

   • This is a serious sign of respiratory muscle fatigue
   • Adjust target saturation to 88-92% if patient has risk factors for hypercapnic respiratory failure
   • Urgent senior medical review required
   • Consider mechanical ventilation 1, 4

4. If ABG shows metabolic acidosis (pH < 7.35 with normal/low PaCO₂):

   • Maintain SpO₂ 94-98%
   • Investigate and treat the cause of metabolic acidosis 1

Monitoring Protocol After Initial ABG

1. Repeat ABG measurements:

   • Within 1 hour if initial ABG showed abnormalities
   • If oxygen requirements increase
   • If clinical deterioration occurs
   • After any significant change in oxygen therapy 1, 4

2. Continuous monitoring:

   • Use continuous pulse oximetry for all patients receiving oxygen
   • Monitor respiratory rate, heart rate, and work of breathing
   • Tachypnea and tachycardia are more sensitive indicators of deterioration than cyanosis 1

Special Considerations

• Pitfall: Relying solely on SpO₂ without ABG can miss rising PaCO₂ levels, as patients may maintain normal oxygen saturation while developing hypercapnia

• Caveat: Pulse oximetry may be less accurate in conditions like poor peripheral perfusion, which can occur in severe asthma with significant vasoconstriction 4

• Warning sign: Normal PaCO₂ in a patient with severe asthma may actually indicate impending respiratory failure, as these patients typically have respiratory alkalosis 3

• Practical tip: Use peak expiratory flow (PEF) as a screening tool - patients with PEF ≥ 25% predicted are unlikely to have significant hypercapnia or acidosis, potentially reducing unnecessary ABGs 2

Adjusting Oxygen Therapy Based on ABG Results

• If PaO₂ remains low despite oxygen therapy:

  • Increase oxygen flow rate
  • Consider changing to a higher-concentration delivery device
  • If using nasal cannulae at maximum flow, switch to simple mask or reservoir mask 1

• If hypercapnia develops during oxygen therapy:

  • Reduce oxygen to target SpO₂ 88-92%
  • Obtain urgent senior medical review
  • Consider non-invasive ventilation if appropriate 1, 4

Remember that hypoxemia is a constant finding in acute severe asthma and may take a week or longer to return to normal levels, even with appropriate treatment 3. Serial ABG measurements are essential for monitoring progress and guiding therapy in all patients with acute severe asthma.