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Asthma (Acute)

Asthma Pathway (BTS, NICE, SIGN) [SIGN 244]

Background Information

Asthma exacerbation: a typically reversible episode of lower airway obstruction (bronchospasm) characterised by an acute or subacute worsening of baseline symptoms and lung function in patients with asthma. In some cases, a patient may present for the first time in an exacerbation. [Ref]

Main causes of  exacerbations include: [Ref]
  • Asthma triggers 
  • Poor adherence to ICSInhaled Corticosteroid-containing medication 

 

NB: Asthma exacerbations may occur in the absence of known risk factors / triggers

Symptoms [Ref]
  • Increasing breathlessness, wheeze, cough, chest tightness
  • Symptoms often occur at rest, disturb sleep or limit activity 
  • Rapid progression / worsening 

O/E
  • ↑ RR, ↑ HR, ↓ SpO2
  • Increased work of breathing 
  • Auscultation
    • Widespread expiratory wheeze 
    • Silent chest
  • Agitation, drowsiness, or confusion (ominous signs of impending respiratory failure)

Other 
  • ↓ PEF or FEV1 (compared to personal best or predicted)
  • ABG findings 
    • Initial → Respiratory alkalosis  ± hypoxaemia
    • Late → Hypercapnia / Respiratory acidosis 

 

In asthma exacerbation, a normal / elevated / rising PaCO₂ is a red flag:

  • Normally, patients with acute asthma hyperventilate → low PaCO₂ (respiratory alkalosis).

  • If PaCO₂ is normal or rising, this means the patient is tiring, airflow obstruction is worsening, and ventilatory failure is imminent.

  • Favourable prognosis for most patients with rapid symptom resolution after acute management [Ref]
  • Relapse rate: ~7-15% after ED discharge  
    • History of prior exacerbations → Strongest predictor of future episodes 
  • Poor prognostic indicators [Ref]
    • Hx of prior severe exacerbations (esp. requiring hospitalisation/ICU admission)
    • Poor baseline lung function 
    • Poor asthma control 
    • Inadequate ICS therapy (including poor adherence)
    • Smoking

Main complication (severe/life-threatening exacerbations) → respiratory failure  +/- acute respiratory acidosis 

Adult Guidelines

Severity Criteria
Moderate No features of severe acute asthma, and:
  • PEF >50-75% (best or predicted)
  • Increasing symptoms
Severe Any of the following:
  • PEF 33-50% (best or predicted)
  • Respiratory rate ≥25/min
  • Pulse ≥110/min
  • Inability to complete a sentence in one breath
Life-threatening Any of the following:
  • PEF <33% (best or predicted)
  • SpO2 <92%
  • PaO2 <8 kPa
  • Normal PaCO2 (4.6-6.0 kPa) - indicates respiratory muscle fatigue
  • Low pH
 
  • Cyanosis
  • Exhaustion or altered conscious level
  • Arrhythmia or hypotension
  • Silent chest
Near-fatal Any of the following:
  • PaCO2 >6.0 kPa
  • Requiring mechanical ventilation with raised inflation pressures


 

You can remember life-threatening features using the '33,92,CHEST' mnemonic

  • 33 → PEF <33%
  • 92 → SpO2 <92%
  • C → Cyanosis 
  • H → Hypotension 
  • E → Exhaustion / poor respiratory effort (normal PaCO2)
  • S → Silent chest (poor air entry) 
  • T → Tachycardia or bradycardia (arrhythmias)

Refer all the following severity to hospital:
  • Severe
  • Life-threatening asthma
  • Near-fatal

Admission criteria:
  • Life-threatening or near-fatal asthma attack
  • Severe features persist after initial treatment
  • Pregnant
  • Presentation at night
  • Asthma attack despite oral corticosteroid prior presentation
  • Previous near-fatal asthma attack

Offer all the following:
  • β2 agonist bronchodilator (e.g. salbutamol) via spacer
    • If no improvement → nebulised salbutamol 5mg (preferably oxygen-driven)
 
  • Prednisolone 40-50mg (continue for minimum 5 days)

Offer all the following:
  • Oxygen therapy - only if SpO2 <94%
 
  • Bronchodilator therapy 
    • Nebulised salbutamol 5mg (preferably oxygen-driven)
    • +/- Nebulised ipratropium 0.5mg (usually given routinely if life-threatening attack or add-on if poor response to salbutamol)
 
  • Steroid
    • Oral prednisolone 40-50mg daily (continue for minimum 5 days)
    • If cannot swallow oral tablet (e.g. due to vomiting) → IV hydrocortisone 400mg daily

The following should only be used after consultation with senior medical staff:
  • IV magnesium sulfate 1.2-2g over 20 min (single dose)
  • IV amiophylline (no robust evidence)
  • IV salbutamol

 

IV magnesium sulfate is commonly used to treat severe / life-threatening / near-fatal asthma attacks. However, aminophylline and salbutamol is rarely used in practice, only in very refractory cases and under specialist care.

Transfer to ICU for intubation is indicated if there is:
  • Hypercapnia
  • Falling pH or rising hydrogen on ABG
  • Persisting or worsening hypoxia
  • Exhaustion, altered conscious state
  • Poor respiratory effort or respiratory arrest

 

Unlike in COPD, non-invasive ventilation has NO ROLE in asthma. If medical management failed, escalation to ICU for intubation is necessary.

Measure the following to monitor treatment:
  • PEF
  • Pulse oximetry
  • Blood gas

There is no explicit guidance on when to discharge but the following should be met:
  • PEF >75% (best or predicted), and
  • On medical therapy that can be continued at home (i.e. oral tablets, inhalers) (no oxygen therapy, nebulisers, IV therapy etc.)

Actions to be performed before discharging the patient:
  • Provide asthma education
    • Inhaler technique
    • PEF record keeping
    • Written PEF and symptom-based personalised asthma action plan
 
  • Follow up
    • With GP within 2 days
    • With respiratory clinic within 1 month

Children Guidelines

Essentially the same as adults, apart from age-specific heart rate and respiratory rate which has been removed for simplification.

The main difference is that SpO2 <92% puts the patient in the severe category (in adult, SpO2 <92% would put the patient in the life-threatening category)
 
Severity Criteria
Moderate No features of severe acute asthma, and:
  • PEF >50-75% (best or predicted)
  • Increasing symptoms
Severe Any of the following:
  • SpO2 <92%
  • PEF 33-50% (best or predicted)
  • Inability to complete a sentence in one breath
Life-threatening SpO2 <92% + any of the following:
  • PEF <33% (best or predicted)
  • Silent chest
  • Poor respiratory effort
  • Confusion
  • Cyanosis
 

ALL patients:
  • β2 agonist bronchodilator (e.g. salbutamol) via spacer (1 puff every 60 sec up to maximum of 10 puffs)
    • If no improvement → nebulised salbutamol 5mg
  • Oral prednisolone 30-40mg daily (continue for minimum 5 days)

Offer all the following:
  • Oxygen therapy - only if SpO2 <94%
 
  • Bronchodilator therapy 
    • Nebulised salbutamol 5mg (preferably oxygen-driven)
    • +/- Nebulised ipratropium 0.5mg (usually given routinely if life-threatening attack or add-on if poor response to salbutamol)
 
  • Steroid
    • Oral prednisolone 30-40mg daily (continue for minimum 5 days)
    • If cannot swallow oral tablet (e.g. due to vomiting) → IV hydrocortisone 4mg/kg daily

Only consider the following after consulting specialist:
  • IV magnesium sulfate (40mg/kg/day)
  • IV amiophylline(no robust evidence)
  • IV salbutamol (no robust evidence)

 

IV magnesium sulfate is commonly used to treat severe / life-threatening / near-fatal asthma attacks. However, aminophylline and salbutamol is rarely used in practice, only in very refractory cases and under specialist care.

References

Author: Adams Lau
Reviewer:
Last edited: 29/07/25