Why Acute Asthma Management Is Examined
Acute asthma is a core acute medical scenario in OSCEs and clinical placements. Examiners test whether you can: classify severity accurately using objective criteria, prescribe bronchodilators and oxygen correctly, identify life-threatening features that require immediate escalation, and counsel on discharge — including inhaler technique and action plan.
Severity Classification — BTS/SIGN 2023
Moderate Acute Asthma
- PEFR 50-75% best or predicted
- No features of severe or life-threatening asthma
- Speech normal
Acute Severe Asthma
Any one of:
- PEFR 33-50% best or predicted
- Respiratory rate 25/min or above
- Heart rate 110 bpm or above
- Cannot complete sentences in one breath
Life-Threatening Asthma
Any one of:
- PEFR below 33% best or predicted
- SpO2 below 92%
- PaO2 below 8 kPa on ABG
- Silent chest (no wheeze — no air movement)
- Cyanosis
- Bradycardia or arrhythmia
- Exhaustion, confusion, altered consciousness
- Normal PaCO2 (3.5-6 kPa) — this is abnormal in severe asthma, as hyperventilation normally causes hypocapnia
Near-Fatal Asthma
- Raised PaCO2 (above 6 kPa) — CO2 retention indicates respiratory failure
- Requires ventilation
🧠 Mnemonic
Life-threatening features — CHAOS:
- C yanosis / CO2 normal or raised
- H eart rate low (bradycardia) or arrhythmia
- A ltered consciousness / exhaustion
- O xygen saturation below 92%
- S ilent chest / PEFR below 33%
Immediate Management
1. Oxygen
- Target SpO2 94-98%
- Controlled oxygen via nasal cannulae or simple face mask — not high-flow unless SpO2 below 94%
- (Unlike COPD, hyperoxia is not the primary concern in asthma — but unnecessary high-flow O2 should be avoided)
2. Bronchodilators
Salbutamol (first-line):
| Route | Dose | Notes |
|---|---|---|
| Nebulised | 2.5-5 mg back-to-back every 15-20 minutes | Driven by oxygen |
| IV | 250 micrograms over 10 minutes | If nebulised not tolerated |
| Inhaler (moderate only) | 2-4 puffs via spacer | As effective as nebuliser for moderate asthma |
Ipratropium bromide (add in severe/life-threatening):
- 0.5 mg nebulised every 4-6 hours combined with salbutamol
- Adds approximately 10% improvement in bronchodilation via anticholinergic mechanism
💎 Clinical Pearl
Spacer + MDI is as effective as a nebuliser for moderate asthma. In an OSCE you may be asked to counsel on inhaler technique — demonstrate the spacer method if available.
3. Corticosteroids
| Route | Drug | Dose | Duration |
|---|---|---|---|
| Oral (preferred) | Prednisolone | 40-50 mg OD | 5 days |
| IV | Hydrocortisone | 100 mg QDS | If cannot swallow |
Give as soon as possible — corticosteroids take 4-6 hours to work. Do not delay for investigation results.
4. Magnesium Sulphate (severe/life-threatening only)
- 1.2-2 g IV over 20 minutes (single dose)
- Causes bronchial smooth muscle relaxation via calcium antagonism
- Used when inadequate response to initial bronchodilators
5. IV Aminophylline / IV Salbutamol Infusion
- Third-line; requires ICU monitoring
- Risk of arrhythmia — discuss with senior before prescribing
ABG Interpretation in Acute Asthma
| Pattern | Significance |
|---|---|
| Low PaCO2 (below 4.5 kPa) + high pH | Expected — hyperventilation in mild/moderate attack |
| Normal PaCO2 (4.5-6 kPa) | Warning sign — patient tiring, losing hyperventilation drive |
| High PaCO2 (above 6 kPa) | Near-fatal — CO2 retention, impending respiratory failure — ICU |
Monitoring and Escalation
After initial treatment:
- Repeat PEFR 15-30 minutes after bronchodilators
- Reassess severity classification
- Repeat ABG if SpO2 not improving or concerns about CO2 retention
Escalate to ICU/HDU if:
- Life-threatening features not responding to treatment
- Near-fatal features (raised CO2)
- Deteriorating despite maximal therapy
- Patient exhausted or requiring intubation
Discharge Planning and OSCE Counselling
⚠️ Red Flag
Never discharge a patient with acute asthma within 1 hour of last bronchodilator and only if PEFR is above 75% with sustained improvement for at least 60 minutes.
Before discharge, ensure:
- 1PEFR above 75% predicted/personal best
- 2Oral prednisolone course prescribed and explained (5 days)
- 3Inhaler technique reviewed and corrected if needed
- 4Written personalised asthma action plan provided
- 5GP follow-up arranged within 2 working days
- 6Review of preventer inhaler — escalate step if needed
- 7Safety-net: return immediately if symptoms worsen
Frequently Asked Questions
"What is a silent chest and why is it dangerous?"
A silent chest means no wheeze is audible on auscultation despite ongoing respiratory distress. This occurs not because the bronchoconstriction has resolved, but because so little air is moving that no wheeze is generated. It is a life-threatening sign indicating near-total airway obstruction. Immediate escalation, IV bronchodilators, and anaesthetic review are required.
"Why is normal CO2 a warning sign in asthma?"
In an acute asthma attack, the physiological response is to increase respiratory rate, which causes CO2 to be blown off — resulting in a low PaCO2. If PaCO2 is normal or rising despite tachypnoea, it means the patient is no longer able to maintain the compensatory hyperventilation, indicating fatigue and impending respiratory failure.
"What are the safe prescribing points for IV magnesium sulphate?"
Check renal function before administering (dose-reduce in renal impairment — magnesium is renally excreted). Administer over 20 minutes minimum — rapid infusion causes flushing, hypotension, and cardiac arrest. Monitor for hypermagnesaemia signs: loss of deep tendon reflexes, respiratory depression, cardiac arrhythmias. Have calcium gluconate available as antidote.
"Should I give high-flow oxygen in asthma?"
No — target saturations are 94-98%, not 100%. There is evidence that hyperoxia in acute asthma is associated with worse outcomes due to hypoxic pulmonary vasoconstriction reversal, absorption atelectasis, and increased ventilation-perfusion mismatch. Use controlled oxygen to target saturations. High-flow oxygen is reserved for patients with SpO2 below 94%.
"What is the difference between reliever, preventer, and combination inhalers?"
Reliever (SABA — salbutamol): short-acting bronchodilator, used for symptom relief. Preventer (ICS — beclometasone, fluticasone): inhaled corticosteroid, reduces airway inflammation — must be used daily. Combination (LABA+ICS — salmeterol/fluticasone, formoterol/budesonide): long-acting bronchodilator plus steroid — step-up therapy. MART regime (Maintenance And Reliever Therapy) uses budesonide/formoterol as both preventer and reliever.
Related Posts
- Anaphylaxis Management OSCE — another acute emergency where bronchospasm is a key feature
- Basic Life Support OSCE — managing respiratory arrest as a complication of near-fatal asthma
- Blood Results Interpretation OSCE — ABG interpretation in the context of respiratory failure