When should intubation be considered in an asthmatic patient?

When to Intubate an Asthmatic Patient

Intubate an asthmatic patient when there is deteriorating peak flow with worsening or persistent hypoxia despite 60% oxygen, hypercapnia (PaCO2 >6 kPa/45 mmHg), exhaustion with feeble respirations, confusion, drowsiness, coma, or respiratory arrest. 1

Life-Threatening Indicators Requiring ICU Transfer and Preparation for Intubation

The British Thoracic Society guidelines establish clear criteria for when a patient should be transferred to intensive care with a physician prepared to intubate 1:

• Deteriorating peak expiratory flow despite aggressive medical management 1
• Persistent or worsening hypoxia (PaO2 <8 kPa/60 mmHg) despite 60% inspired oxygen 1
• Rising hypercapnia (PaCO2 >6 kPa/45 mmHg) - particularly dangerous as a normal or elevated CO2 in a breathless asthmatic indicates impending respiratory failure 1
• Exhaustion with feeble respirations - the patient is tiring out and cannot maintain the work of breathing 1
• Altered mental status: confusion, drowsiness, or coma 1
• Respiratory or cardiac arrest - immediate intubation required 1

Critical Pre-Intubation Considerations

Timing is everything: Intubation should be performed semi-electively before respiratory arrest occurs, as delaying until cardiorespiratory arrest significantly increases mortality 2. The most expert available physician (ideally an anesthesiologist) should perform the procedure 2.

Physiologic Dangers of Intubation in Asthmatics

Intubating asthmatics carries unique risks that must be anticipated 2:

• Cardiovascular collapse from the combination of auto-PEEP, reduced venous return, and hypotension during or immediately after intubation 2
• Hypotension commonly accompanies initiation of positive pressure ventilation - ensure adequate intravascular volume before intubating 2
• Barotrauma risk including pneumothorax, pneumomediastinum, and subcutaneous emphysema from high ventilator pressures 2

Arterial Blood Gas Markers of Impending Failure

Always measure arterial blood gases in patients with acute severe asthma admitted to hospital 1. The following ABG findings indicate a very severe, life-threatening attack requiring ICU-level care 1:

• Normal (5-6 kPa/38-45 mmHg) or elevated PaCO2 in a breathless asthmatic - this is paradoxical and indicates the patient can no longer hyperventilate to compensate 1
• Severe hypoxia (PaO2 <8 kPa/60 mmHg) despite oxygen therapy 1
• Low pH or high H+ indicating respiratory acidosis 1

Clinical Features Preceding the Need for Intubation

Recognize these physical examination findings as harbingers of respiratory failure 1:

• Silent chest - absence of breath sounds despite respiratory effort indicates severe bronchospasm 1
• Cyanosis despite oxygen therapy 1
• Bradycardia or hypotension - ominous signs in acute asthma 1
• Peak flow <33% predicted or best 1

Intubation Technique Specifics

When intubation becomes necessary 2:

• Use the largest endotracheal tube available (usually 8-9 mm) to decrease airway resistance 2
• Ensure adequate intravascular volume before intubation to mitigate hypotension 2
• Have the most experienced operator perform the procedure 2

Post-Intubation Ventilation Strategy

Use a "controlled hypoventilation" strategy to provide adequate oxygenation while minimizing airway pressures and barotrauma 2:

• Slower respiratory rates (10-14 breaths/min) 2
• Smaller tidal volumes (6-8 mL/kg) 2
• Shorter inspiratory times with high inspiratory flow rates (80-100 L/min) 2
• Longer expiratory times (I:E ratio 1:4 or 1:5) to allow complete exhalation and prevent air trapping 2
• Sufficient sedation to prevent ventilator dyssynchrony, which worsens air trapping 2

Common Pitfalls to Avoid

• Never delay intubation until cardiac arrest occurs - mortality increases dramatically 2, 3
• Never use conventional ventilator settings designed for non-asthmatic patients, as these worsen air trapping 2
• Never use high respiratory rates or large tidal volumes - these lead to incomplete exhalation and auto-PEEP 2
• Do not underestimate the cardiovascular instability that accompanies intubation in severe asthmatics 2
• Avoid inadequate sedation which leads to patient-ventilator dyssynchrony 2

Prognosis

While complications occur in approximately 45% of mechanically ventilated asthmatics, mortality is relatively low (6%) when intubation is performed in a controlled manner before cardiac arrest 3. However, patients requiring mechanical ventilation for asthma have a 14% mortality at 3 years and require very close medical follow-up 4.