What are the optimal ventilator settings for severe asthma to avoid air trapping?

Optimal Ventilator Settings for Severe Asthma to Avoid Air Trapping

For patients with severe asthma requiring mechanical ventilation, use a ventilation strategy with 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) to minimize air trapping and auto-PEEP. 1

Initial Ventilator Settings

• Use the largest endotracheal tube available (usually 8 or 9 mm) to decrease airway resistance 1
• Set tidal volume to 6-8 mL/kg of ideal body weight to minimize barotrauma 1
• Use a slower respiratory rate (10-14 breaths/min) to allow for complete exhalation 1
• Set inspiratory flow rate high (80-100 L/min) to shorten inspiratory time 1
• Maintain inspiratory to expiratory (I:E) ratio of 1:4 or 1:5 to provide adequate expiratory time 1
• Implement permissive hypercapnia (mild hypoventilation) to reduce the risk of barotrauma 1

Monitoring for Auto-PEEP

• Regularly assess for signs of auto-PEEP (breath stacking) which can lead to complications such as hyperinflation, tension pneumothorax, and hypotension 1
• Monitor ventilator flow and pressure curves for evidence of incomplete exhalation 1
• Evaluate for auto-PEEP if the patient's condition deteriorates or if ventilation becomes difficult 1
• Check for the common causes of acute deterioration using the mnemonic DOPE (tube Displacement, tube Obstruction, Pneumothorax, Equipment failure) 1

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 1
• For significant hypotension due to auto-PEEP, assist with exhalation by pressing on the chest wall after disconnection of the ventilator circuit 1
• To minimize auto-PEEP, further decrease the respiratory rate or tidal volume or both 1
• Consider paralytic agents if auto-PEEP persists and the patient displays ventilator dyssynchrony despite adequate sedation 1

Sedation Management

• Provide adequate sedation to optimize ventilation, decrease ventilator dyssynchrony, and minimize barotrauma 1, 2
• Consider propofol infusion (5-50 mcg/kg/min) as the primary sedative agent 2
• Add fentanyl (25-50 mcg IV every 1-2 hours) for analgesia and midazolam (1-2 mg IV every 2-4 hours) as needed for complementary sedation 2
• Monitor sedation level using the Richmond Agitation-Sedation Scale (RASS), targeting a score of -2 to 0 2

Continued Bronchodilator Therapy

• Continue to administer inhaled albuterol treatments through the endotracheal tube as delivery of inhaled medications may have been inadequate before intubation 1
• Use oxygen to nebulize beta-agonists such as salbutamol 5 mg or terbutalina 10 mg 3

Special Considerations

• Obtain expert consultation for management of mechanical ventilation in severe asthma 1
• Consider extracorporeal membrane oxygenation (ECMO) in exceedingly rare circumstances when aggressive treatment fails to provide adequate gas exchange 1
• Avoid any sedation in non-intubated asthmatic patients as it can depress the respiratory center 3, 2

Pitfalls to Avoid

• Never use conventional ventilator settings designed for non-asthmatic patients, as these can worsen air trapping 1
• Avoid high respiratory rates and large tidal volumes which can lead to incomplete exhalation and auto-PEEP 1
• Do not underestimate the severity of asthma exacerbations; perform objective measurements 3
• Be vigilant for pneumothorax development, especially in patients with difficult ventilation or sudden deterioration 1

By following these ventilator strategies specifically designed for severe asthma, clinicians can minimize the risk of air trapping and its associated complications, ultimately improving patient outcomes.