Physiologic Risks of Intubating Asthmatics
Intubation in asthmatic patients carries significant physiologic risks and should be performed semielectively before respiratory arrest occurs, with consultation from a physician expert in ventilator management due to the complicated and risky nature of ventilating patients with severe asthma. 1
Key Physiologic Risks
Dynamic hyperinflation and auto-PEEP: Asthmatic patients have prolonged expiratory time requirements due to bronchoconstriction, which can lead to incomplete exhalation, air trapping, and auto-PEEP when mechanically ventilated 2
Hypotension: Intravascular volume should be maintained or replaced because hypotension commonly accompanies the initiation of positive pressure ventilation in asthmatic patients 1
Barotrauma: High ventilator pressures associated with mechanical ventilation in asthmatics significantly increase the risk of pneumothorax, pneumomediastinum, and subcutaneous emphysema 1, 2
Worsening bronchospasm: The process of intubation itself can trigger further bronchospasm through mechanical stimulation of the airways 3
Cardiovascular collapse: The combination of auto-PEEP, reduced venous return, and hypotension can lead to cardiovascular collapse during or immediately after intubation 2, 3
Pre-Intubation Considerations
Patient selection: Intubation should be considered in patients with persistent or increasing hypercapnia, exhaustion, and depressed mental status 1
Immediate intubation: Patients presenting with apnea or coma should be intubated immediately 1
Timing: Intubation should be performed semielectively before respiratory arrest occurs, as delaying intubation until cardiorespiratory arrest significantly increases mortality 1, 4
Location: Intubation should be performed in the ED with subsequent transfer to an appropriate intensive care unit 1
Intubation Technique
Endotracheal tube size: Use the largest endotracheal tube available (usually 8 or 9 mm) to decrease airway resistance 2
Expert assistance: The most expert available doctor (ideally an anaesthetist) should perform the intubation to minimize complications 1
Volume status: Ensure adequate intravascular volume before intubation to mitigate hypotension 1
Ventilation Strategy
Permissive hypercapnia: Use a "controlled hypoventilation" strategy that provides adequate oxygenation while minimizing airway pressures and the possibility of barotrauma 1, 2
Ventilator settings: Use slower respiratory rates (10-14 breaths/min), smaller tidal volumes (6-8 mL/kg), shorter inspiratory times (inspiratory flow rate 80-100 L/min), and longer expiratory times (I:E ratio 1:4 or 1:5) 2
Monitoring for auto-PEEP: Regularly assess for signs of auto-PEEP which can lead to complications such as hyperinflation, tension pneumothorax, and hypotension 2
Sedation Management
Adequate sedation: Provide sufficient sedation to prevent ventilator dyssynchrony, which can worsen air trapping 2, 5
Sedation options: Consider propofol infusion (5-50 mcg/kg/min) as the primary agent, with fentanyl for analgesia and midazolam as a complementary sedative if necessary 5
Paralytic agents: Consider paralytic agents if auto-PEEP persists despite adequate sedation 2
Emergency Management of Complications
Managing auto-PEEP: If auto-PEEP develops, quickly reduce high-end expiratory pressure by briefly disconnecting the patient from the ventilator circuit to allow PEEP to dissipate during passive exhalation 2
Tension pneumothorax: Be vigilant for signs of tension pneumothorax, which requires immediate decompression 2, 3
Pitfalls to Avoid
Conventional ventilator settings: Never use conventional ventilator settings designed for non-asthmatic patients, as these can worsen air trapping 2
High respiratory rates and large tidal volumes: Avoid these as they can lead to incomplete exhalation and auto-PEEP 2
Inadequate sedation: Insufficient sedation can lead to patient-ventilator dyssynchrony and worsen air trapping 5
Delayed expert consultation: Obtain expert consultation early for management of mechanical ventilation in severe asthma 2