Acute severe asthma not responding to nebulisers
The case
A 24-year-old man presents to the emergency department with acute severe asthma triggered by a viral upper respiratory tract infection 2 days ago. He normally uses salbutamol only when needed. On arrival he is drowsy but rousable, with a silent chest, respiratory rate 35/min, heart rate 130/min, blood pressure 110/70 mmHg and oxygen saturation 92% on 12 L/min via non-rebreather mask. Arterial blood gas: pH 7.28, PaCO2 6.2 kPa (rising from 5.1 kPa one hour earlier), PaO2 9.5 kPa, bicarbonate 24 mmol/L. Chest X-ray shows no pneumothorax. Three doses of nebulised salbutamol and ipratropium, intravenous hydrocortisone 200 mg and intramuscular adrenaline (epinephrine) have been given without improvement. What is the most urgent next intervention?
Options
- AIntravenous magnesium sulfate over 20 minutes
- BNebulised salbutamol 10 mg/hour
- CIntravenous salbutamol bolus then infusion
- DHeliox via non-invasive support
- EKetamine infusion before intubation
Think it through before you read on. Which single option is best, and why are the other four wrong?
Show the answer and explanation
Answer
A. Intravenous magnesium sulfate over 20 minutes
Why this is the right answer
This is life-threatening asthma. The clues are drowsiness, a silent chest, oxygen saturation below 92% and, most importantly, a PaCO2 that is rising. In a severe asthma attack a young patient normally breathes fast and the PaCO2 is low. A normal or rising PaCO2 means the patient is tiring and respiratory arrest is close.
He has already had the first-line treatments: high-flow oxygen, repeated nebulised salbutamol with ipratropium, and systemic corticosteroid. The Australian Asthma Handbook (National Asthma Council, 2024) lists intravenous magnesium sulfate as the next step for severe or life-threatening asthma that does not respond to initial bronchodilators [VERIFY].
The usual adult dose is 10 mmol (about 2.5 g) intravenously over 20 minutes. Magnesium relaxes bronchial smooth muscle by blocking calcium entry and adds a different mechanism to beta-agonist therapy. It is safe, cheap and works within minutes.
At the same time, the intensive care team should be called and preparations made for intubation. Ketamine is the preferred induction agent if intubation is needed, but intubation itself is high-risk in asthma because of dynamic hyperinflation and hypotension, so drug therapy that may avoid it comes first.
Always check for a pneumothorax before escalating, which has been done here.
Why the other options are wrong
- B
Continuous nebulised salbutamol is reasonable, but he has already failed three nebulisers; magnesium adds a new mechanism.
- C
Intravenous salbutamol is an option in refractory asthma but has weaker evidence than magnesium and causes lactic acidosis and tachycardia.
- D
Heliox is an unproven adjunct and does not replace magnesium.
- E
Intubation is the last resort; magnesium should be tried first while ICU is called.
High-yield takeaway
A rising PaCO2 in acute asthma means impending arrest: give intravenous magnesium 10 mmol over 20 minutes and call intensive care.
Reference: Australian Asthma Handbook (National Asthma Council Australia), Acute asthma in adults (2024)
Common questions
Why is a normal PaCO2 dangerous in acute asthma?
A patient in severe bronchospasm should be blowing off CO2. A normal or rising level means they are exhausted and about to stop ventilating.
What is the adult dose of intravenous magnesium for asthma?
10 mmol (about 2.5 g) intravenously over 20 minutes, given once.
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