What are the indications for mechanical ventilation in severe asthma?

Indications for Mechanical Ventilation in Severe Asthma

Mechanical ventilation in severe asthma is indicated for patients who present with apnea, coma, persistent or increasing hypercapnia, exhaustion, severe distress, and depression of mental status. 1 Clinical judgment remains essential in determining the need for immediate endotracheal intubation in these critically ill patients.

Primary Indications for Mechanical Ventilation

• Respiratory arrest or impending respiratory failure:
  • Apnea
  • Coma
  • Persistent or increasing hypercapnia despite maximal therapy
  • Exhaustion
  • Severe respiratory distress
  • Altered mental status/depression of consciousness

Clinical Assessment Parameters

• Ventilatory parameters:
  • Deteriorating peak flow despite maximal therapy
  • Worsening or persistent hypoxia (PaO₂ <8 kPa) despite 60% inspired oxygen
  • Hypercapnia (PaCO₂ >6 kPa)
  • Onset of exhaustion with feeble respiratory effort
  • Confusion or drowsiness
  • Respiratory arrest

Decision-Making Algorithm

1. Initial assessment: Evaluate severity of asthma attack and response to initial treatment

   • If peak expiratory flow remains <33% of predicted or best value after initial nebulization
   • If patient shows signs of exhaustion or altered mental status

2. Trial of non-invasive ventilation:

   • Consider non-invasive positive-pressure ventilation (NIPPV) for patients with:
     • Acute respiratory failure
     • Alert mental status
     • Adequate spontaneous respiratory effort
   • NIPPV may delay or eliminate the need for endotracheal intubation 1

3. Proceed to intubation if:

   • Patient fails to improve with NIPPV
   • Patient has apnea, coma, or severe altered mental status
   • Persistent or worsening hypercapnia
   • Severe exhaustion
   • Inability to protect airway

Important Considerations During Mechanical Ventilation

• Ventilator settings for severe asthma:

  • Use lower respiratory rates (to allow for longer expiratory time)
  • Use smaller tidal volumes (6-8 mL/kg)
  • Use shorter inspiratory times (adult inspiratory flow rate 80-100 L/min)
  • Use longer expiratory times (inspiratory to expiratory ratio 1:4 or 1:5)
  • Accept permissive hypercapnia (pH >7.20) to reduce risk of barotrauma 1

• Monitoring for complications:

  • Auto-PEEP (intrinsic PEEP) from air trapping
  • Barotrauma (pneumothorax, pneumomediastinum)
  • Hypotension from decreased venous return
  • Ventilator dyssynchrony

Management of Complications

• For auto-PEEP and air trapping:

  • Briefly disconnect patient from ventilator to allow passive exhalation
  • For hypotension due to auto-PEEP, assist exhalation by pressing on chest wall after disconnection
  • Decrease respiratory rate or tidal volume to minimize auto-PEEP
  • Consider paralytic agents if auto-PEEP persists despite adequate sedation 1

• For deterioration during mechanical ventilation:

  • Check for DOPE: tube Displacement, tube Obstruction, Pneumothorax, Equipment failure
  • Evaluate for auto-PEEP
  • Consider tension pneumothorax and treat if present 1

Special Considerations

• Use rapid sequence intubation performed by an expert in airway management
• Use the largest endotracheal tube available (usually 8 or 9 mm) to decrease airway resistance
• Confirm tube placement with clinical examination and waveform capnography
• Obtain chest radiograph after intubation
• Continue bronchodilator therapy through the endotracheal tube
• Provide adequate sedation to optimize ventilation and decrease ventilator dyssynchrony 1

Rescue Therapies for Refractory Cases

• For patients with severe asthma unresponsive to conventional mechanical ventilation:
  • Consider inhaled anesthetics (sevoflurane, isoflurane) which may have direct bronchodilator effects 1
  • In extremely rare circumstances of persistent respiratory failure despite aggressive measures, extracorporeal membrane oxygenation (ECMO) may be considered 1

Mechanical ventilation in severe asthma carries significant risks but should be initiated promptly when indicated to prevent morbidity and mortality. The focus should be on avoiding complications while maintaining adequate gas exchange until bronchodilator therapy can reverse the underlying bronchospasm.