Acute asthma is a core exam emergency because it requires you to grade severity instantly and escalate treatment accordingly — and because the signs of a life-threatening attack are counterintuitive. This guide covers the acute assessment and management as examiners frame them, drawing on the BTS/SIGN acute asthma standard, with chronic diagnosis and management now in the 2024 BTS/NICE/SIGN guideline (NG245). Follow current guidance and local protocols in practice; this reflects guidance as of mid-2026.
What an acute attack is
Acute asthma is an episode of worsening bronchoconstriction, airway inflammation and mucus plugging, commonly triggered by a respiratory infection, allergen exposure, exercise, cold air, or poor adherence to preventer therapy. The history should establish the trigger, the speed of deterioration, the usual level of control and treatment, any previous severe attacks or intensive-care admissions, and reliever overuse — all of which flag a patient at higher risk of a life-threatening attack.
Grading severity
The whole answer turns on classifying the attack, because severity drives treatment and disposition. In adults:
- Moderate: peak expiratory flow (PEF) 50 to 75% of best or predicted, speech normal, respiratory rate below 25, pulse below 110.
- Acute severe: any one of PEF 33 to 50%, respiratory rate 25 or more, pulse 110 or more, or inability to complete sentences in one breath.
- Life-threatening: any one of PEF below 33%, oxygen saturations below 92%, a silent chest, cyanosis, poor respiratory effort, exhaustion, confusion or reduced consciousness, arrhythmia, or hypotension — and, critically, a normal or rising carbon dioxide.
- Near-fatal: a raised carbon dioxide and/or the need for mechanical ventilation.
The carbon dioxide trap
A point examiners love: in an acute attack the patient hyperventilates, so the carbon dioxide should be low. A normal or rising carbon dioxide means the patient is tiring and is a sign of a life-threatening attack — not reassurance. Equally, patients with severe or life-threatening asthma may not appear distressed, and a silent chest (no air movement to generate a wheeze) is an ominous sign, not a sign of improvement. Other features that mark a patient at risk of a fatal attack include previous near-fatal asthma or intensive-care admission, multiple recent hospital attendances, heavy reliever use, and adverse psychosocial circumstances — all of which should lower the threshold for admission and senior involvement.
Investigations
Measure PEF and oxygen saturations, but never let this delay treatment. Take an arterial blood gas if there are life-threatening features or saturations below 92%. A chest X-ray is not routine — reserve it for suspected pneumothorax or consolidation, a life-threatening attack, or failure to respond. Bedside observations — respiratory rate, heart rate, oxygen saturation and PEF as a percentage of best or predicted — drive the severity grading, and serial PEF measurements track the response to treatment.
Management
Treat in parallel with assessment:
- Oxygen to maintain saturations of 94 to 98%, using a high-concentration mask in the critically unwell.
- High-dose inhaled salbutamol: a spacer for moderate attacks, an oxygen-driven nebuliser for severe or life-threatening attacks, and continuous nebulisation for a poor response.
- Steroids early: oral prednisolone, or intravenous hydrocortisone if the patient cannot swallow, for every acute attack, continued for at least five days. Steroids take some hours to act, which is exactly why they are given early rather than held back.
- Ipratropium bromide: add nebulised ipratropium for acute severe or life-threatening attacks, or a poor response to salbutamol.
- Magnesium sulfate: a single intravenous dose is considered in acute severe asthma not responding to initial treatment, or in life-threatening asthma, as a senior decision.
- Escalation: involve critical care early for life-threatening or near-fatal features, or for deterioration.
Monitor the response with repeat PEF, oxygen saturations and clinical assessment, and reassess severity after initial treatment. Before discharge, the patient should be stable on their usual inhalers with a PEF above 75% of best or predicted, have a clear oral steroid course, have their inhaler technique and adherence checked, and have follow-up arranged — most asthma deaths occur outside hospital, and a recent attack is itself a risk factor for another.
High-yield exam points and traps
- A normal or rising carbon dioxide in an acute attack signals exhaustion and a life-threatening attack.
- A silent chest is life-threatening, not reassuring.
- Give oral steroids to every patient with an acute attack — they reduce admissions and relapse.
- PEF and arterial blood gas must not delay oxygen and bronchodilators.
- The severity categories are defined by the single worst feature — one acute-severe feature makes the attack acute severe, and any single life-threatening feature makes it life-threatening.
- Patients with severe attacks may look deceptively well.
- Always ask about previous intensive-care admission and reliever overuse — both predict a dangerous attack.
- A patient too breathless to speak, or with falling saturations, needs immediate senior and critical-care input.
A few common questions
What defines life-threatening asthma? Any one of PEF below 33%, saturations below 92%, a silent chest, cyanosis, poor respiratory effort, exhaustion, confusion, arrhythmia, hypotension, or a normal or rising carbon dioxide.
Why is a normal carbon dioxide concerning in an asthma attack? A normal carbon dioxide is concerning because a distressed patient should be hyperventilating and blowing it off; a normal or rising level means they are tiring — a life-threatening sign.
What is the initial drug treatment? High-dose inhaled salbutamol with oxygen, plus early oral or intravenous steroids; ipratropium and magnesium are added for severe or life-threatening attacks.
When is magnesium sulfate used? A single intravenous dose is considered for acute severe asthma not responding to initial treatment, or for life-threatening asthma, under senior guidance; it is given as a single dose rather than repeatedly.
Do you need a chest X-ray? Not routinely — only for suspected pneumothorax or consolidation, a life-threatening attack, or failure to respond to treatment, and it should never delay emergency treatment.
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