Asthma

Question 1

Define asthma

Answer 1

Immune mediated inflammatory disorder triggered by exposure to a variety of possible precipitants which precipitate immune mediated events to cause chronic small airway inflammation, increased mucus production and bronchoconstriction.

Question 2

Summarise the pathophysiology of asthma

Answer 2

1. Increased work of breathing
   - Small airway narrowing causes the normally active process of expiration to become active.
   - High airway resistance and hyperinflation stretch lungs and chest wall out to operate on a suboptimal position of their pressure-volume curves. Increase work to stretch them further in an attempt to ventilate adequately.
2. Gas-trapping
   - airway narrowing –> slow expiratory flow rates that require long expiratory times if the entire inspired volume is to be exhaled. If the next breath interrupts exhalation, then gas-trapping results.
   - Trapped gas creates intrinsic PEEP (AutoPEEP) above applied PEEP.
   - These leads to dynamic hyperinflation –> barotrauma and hypotension

Question 3

List the indications for endotracheal intubation in an asthmatic

Answer 3

1. Rising CO2 levels
2. Refractory hypoxaemia
3. Exhaustion
4. Deteriorating GCS
5. Haemodynamic instability

NB - Collaborate all clinical and biochemical data prior to the decision to intubate and ventilate

Question 4

Why should a large ETT be used in asthma

Answer 4

1. Reduced resistance and therefore work of breathing
2. Reduces risk of ETT occlusion by secretions

Question 5

Describe the ventilatory strategy in asthma

Answer 5

Controlled hypoventilation
1. Low Vt (less gas to exhale)
2. Reduced RR (Longer expiratory time)
3. Reduce Tinsp (Longer expiratory time)
4. Increase insp. flow rate ( longer exp. time)
5. Reduce CO2 production
- Sedation/Paralysis/Fever/Pain
6. Optimize PEEP (to minimise WOB but limit gas trapping)

Question 6

Why is the application of external PEEP controversial in Asthma

Answer 6

Optimal applied PEEP allows lung to operate at optimal position on the PV compliance curve minimising work of breathing.

However, applied PEEP added to autoPEEP can worsen gas trapping and lead to dynamic hyperinflation.

If evidence of dynamic hyperinflation, set PEEP < 5cmH20

Question 7

List and categorise the complications of mechanical ventilation in the asthmatic

Answer 7

Hypotension
- Sedation
- Dynamic Hyperinflation
- Pneumothorax

Arrythmias
- Electromechanical dissociation

Rapid development of dynamic hyperinflation

Pneumothorax
- Tension pneumothorax
- Worsening DHI in contralateral lung

Acute necrotising myopathy
- NMBA + Steroids
- Weakness + EMG findings + CK
- Slow recovery (up to 12 months)
- Minimize parenteral corticosteroids and early introduction of nebulized agents

Question 8

What is acute necrotising myopathy

Answer 8

It is a complication of asthmatic patients who undergo prolonged mechanical ventilation

• NMBA + Steroids
• Weakness + EMG findings + CK
• Slow recovery (up to 12 months)
• Minimize parenteral corticosteroids and early introduction of nebulized agents