Acute Asthma: BTS/SIGN Severity and Escalation

Acute asthma on the ward is graded before it is treated: moderate, acute severe, life-threatening or near-fatal, using the table in the BTS/SIGN British guideline on the management of asthma (SIGN 158). The grade sets the treatment, the monitoring and who you call. At the CT1 anaesthetics clinical judgement station the marks are for spotting life-threatening features early, treating in the right order, and escalating before the patient tires rather than after.

Key takeaways

• Acute severe asthma is any one of: PEF 33 to 50 per cent of best or predicted, respiratory rate 25 or more, heart rate 110 or more, or inability to complete sentences in one breath (SIGN 158, 2024 edition, amended May 2025).
• Life-threatening asthma is an SpO2 below 92 per cent plus any one of altered conscious level, exhaustion, arrhythmia, hypotension, cyanosis, silent chest, poor respiratory effort, PEF below 33 per cent, PaO2 below 8 kPa, or a normal PaCO2 of 4.6 to 6.0 kPa.
• Near-fatal asthma is a raised PaCO2 and/or mechanical ventilation with raised inflation pressures. Hypercapnia means emergency anaesthetic and critical care involvement.
• First-hour treatment is oxygen titrated to an SpO2 of 94 to 98 per cent, high-dose nebulised beta-2 agonist repeated at 15 to 30 minute intervals, and prednisolone 40 to 50 mg daily continued for at least five days.
• Add nebulised ipratropium 0.5 mg four to six hourly in acute severe or life-threatening attacks; a single intravenous magnesium sulphate dose of 1.2 to 2 g over 20 minutes is given only after senior consultation.
• NICE NG245 (November 2024) covers diagnosis, monitoring and chronic management and states that it does not cover acute attacks; the acute pathway stays in SIGN 158.

How do you recognise acute asthma on the ward?

You recognise acute asthma by grading it against the SIGN 158 severity table at the bedside, not by a general impression of wheeze. The table is the answer the panel wants to hear.

Moderate acute asthma is increasing symptoms with a PEF above 50 to 75 per cent of best or predicted and no features of acute severe asthma. Acute severe asthma needs only one of the four features listed above.

Life-threatening asthma is the grade candidates get wrong since the May 2025 amendment. The current table requires an SpO2 below 92 per cent plus any one clinical sign or measurement. A silent chest is the sign that should frighten you most.

In practice, a normal PaCO2 in a breathless asthmatic is not reassuring. It means the patient can no longer blow off carbon dioxide and is tiring. A rising PaCO2 is near-fatal asthma.

What is the A to E and first-hour management of acute asthma?

The first hour of acute asthma is oxygen, high-dose nebulised bronchodilator and steroid, given in that order while you assess A to E and take an arterial blood gas in anyone with life-threatening features.

Airway and breathing

• Sit the patient up, and give controlled oxygen to all hypoxaemic patients, titrated to maintain an SpO2 of 94 to 98 per cent. SIGN 158 says not to delay oxygen while waiting for a pulse oximeter.
• Give a nebulised beta-2 agonist such as salbutamol. SIGN 158 states that most attacks respond to bolus nebulisation, with repeat doses at 15 to 30 minute intervals or continuous salbutamol at 5 to 10 mg per hour if the initial response is inadequate.
• Add nebulised ipratropium bromide 0.5 mg four to six hourly for acute severe or life-threatening asthma, or a poor initial response to the beta-2 agonist.
• Measure PEF before and after each nebuliser, and take an arterial blood gas if the SpO2 is below 92 per cent or there are life-threatening features.

Circulation, disability and exposure

• Intravenous access, bloods including potassium, and an ECG for the tachycardic or arrhythmic patient.
• A chest radiograph if you suspect pneumothorax, consolidation, or the patient is not improving.
• Steroid early. SIGN 158 says the earlier steroids are given in the attack, the better the outcome: prednisolone 40 to 50 mg daily, or hydrocortisone 100 mg six hourly if the patient cannot swallow and retain tablets.
• Check the conscious level and how exhausted the patient looks. Drowsiness is a life-threatening feature, not a sign of settling.

What is the SIGN 158 severity threshold that changes management?

The threshold that changes management in acute asthma is the move from acute severe to life-threatening, because it triggers the arterial blood gas, the ipratropium, the senior call and the discussion about magnesium and critical care.

According to SIGN 158, a single intravenous dose of magnesium sulphate, 1.2 to 2 g infused over 20 minutes, may be considered for acute severe asthma with a PEF below 50 per cent that has not responded well to inhaled bronchodilator. The guideline is explicit that it should only be used after consultation with senior medical staff.

Nebulised magnesium is not recommended in adults. Intravenous aminophylline is also a senior decision, and SIGN 158 notes it is unlikely to add benefit and increases arrhythmias and vomiting.

That said, no threshold replaces looking at the patient. A candidate who reports 'PEF 40 per cent, so acute severe' but does not mention the patient is too breathless to speak has missed the point of the table.

What is the definitive management of acute asthma?

Definitive management of acute asthma is continued steroid for at least five days or until recovery, bronchodilator weaned by response, treatment of any trigger such as infection, and a review of why the attack happened before discharge.

SIGN 158 says to continue prednisolone at 40 to 50 mg daily for at least five days, or longer until recovery. Steroid tablets are as effective as injected steroid provided they can be swallowed and retained.

Objective monitoring continues throughout: PEF, oxygen saturation kept at 94 to 98 per cent, and repeat blood gases in anyone who was hypoxic or hypercapnic. Improvement is a sustained rise in PEF, not a single good reading.

The panel also likes a sentence about prevention. SIGN 158 devotes a section to lessons from asthma deaths, and a patient with a near-fatal attack should be kept under specialist supervision with a relative involved in the plan.

When do you escalate acute asthma, and who do you call?

You escalate acute asthma at the first life-threatening feature, and you call the medical registrar, the on-call anaesthetist and critical care together rather than in sequence. Hypercapnia is the point at which SIGN 158 says emergency specialist or anaesthetic intervention is needed.

SIGN 158 lists the indications for intensive care or high-dependency admission: patients needing ventilatory support, and those with acute severe or life-threatening asthma failing to respond, shown by deteriorating PEF, persisting or worsening hypoxia, hypercapnia, a falling pH, exhaustion or feeble respiration, drowsiness or confusion, or respiratory arrest.

As the anaesthetic CT1 you may be the person called. Say what you would want to know on the phone: the grade, the gas, what has been given and when, and whether the patient is tiring. Then go and see the patient.

In practice, intubating a patient with near-fatal asthma is a consultant-level decision with a high risk of cardiovascular collapse. SIGN 158 asks for a clinician with airway and critical care skills to be involved as early as possible, which is the escalation you describe.

What is the human-factors and reflective-practice angle in acute asthma?

The human-factors angle in acute asthma is that the sickest patients look quiet: a silent chest, a normal carbon dioxide and a drowsy patient are all easy to misread as improvement in a busy bay.

Name the trap out loud. A patient who stops wheezing after a nebuliser may have opened up, or may be moving no air. The difference is the PEF, the saturation and the effort, and a good answer says you would re-examine rather than assume.

Reflective practice at the station means a real example. Most candidates have seen an asthmatic who was under-graded on arrival. Say what was missed, what you did differently next time, and how you now hand over the grade rather than the wheeze.

Working under pressure is the third marked domain. Describe how you would allocate tasks: a nurse on the nebulisers and observations, you on the gas and the phone, the registrar deciding on magnesium.

What are the common mistakes candidates make with acute asthma?

The commonest mistake with acute asthma at interview is grading it from a single number, then treating the number rather than the patient. The next is escalating late because the patient 'looks calmer'.

• Quoting the old life-threatening list. Since May 2025 the SIGN 158 table requires an SpO2 below 92 per cent plus one further feature, and candidates who learned an earlier version quote it wrongly.
• Giving intravenous magnesium as a first-line drug. It is a single dose, for acute severe asthma with a poor response, after senior consultation.
• Withholding oxygen for fear of hypercapnia. That caution belongs to COPD; in asthma the target is 94 to 98 per cent.
• Delaying steroid until the diagnosis is certain. SIGN 158 says earlier is better and tablets are as effective as injections.
• Treating a normal PaCO2 as reassuring, or a quiet chest as recovery.
• Calling for help only when intubation seems imminent, rather than at the first life-threatening feature.

How this comes up at the CT1 anaesthetics interview

Acute asthma comes up at the CT1 anaesthetics interview as a ward call in the clinical judgement station, which ANRO marks on clinical judgement and decision making, reflective practice and working under pressure across a 15-minute station worth 50 marks.

Clinical judgement is the grading and the order of treatment. Decision making is the moment you say 'this is life-threatening, I am calling the registrar and anaesthetics now'. Reflective practice is the example you bring. Working under pressure is how you run the bay while you wait.

The anaestheticinterview question bank has 340 questions with AI-marked spoken practice, and its asthma scenarios follow exactly this shape: the panel adds a falling PEF, then a normal gas, and listens for whether your answer changes.