# Acute Severe Asthma > Acute severe asthma for NEET-PG Medicine covering severity grading, peak flow criteria, magnesium, ventilation and features of near-fatal asthma. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/acute-severe-asthma - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-08 - Last updated: 2026-10-08 - Reference basis: Harrison's Principles of Internal Medicine; British Thoracic Society and SIGN guideline on asthma; Davidson's Principles and Practice of Medicine - How to cite: "Acute Severe Asthma", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/acute-severe-asthma ## Direct answer Acute severe asthma is an attack where peak expiratory flow is 33 to 50 per cent of predicted or best, unable to complete full sentences in one breath. Life-threatening asthma has peak flow below 33 per cent, saturation below 92 per cent, or a normal or rising PaCO2 signalling exhaustion. Treatment is oxygen titrated to a saturation of 94 to 98 per cent, repeated nebulised salbutamol with ipratropium, and early corticosteroids, adding intravenous magnesium if there is no response. ## What you must remember - **Acute severe criteria:** peak flow 33 to 50 per cent of predicted or best, respiratory rate 25 or more, heart rate 110 or more, and inability to complete sentences in one breath. - **Life-threatening criteria:** peak flow below 33 per cent, saturation below 92 per cent, normal PaCO2 of 4.6 to 6.0 kPa, silent chest, cyanosis, hypotension, exhaustion, confusion or arrhythmia. - **Near-fatal asthma:** a raised PaCO2 or a need for mechanical ventilation, which marks a high risk of a future fatal attack. - **Oxygen:** titrated to a saturation of 94 to 98 per cent, correcting hypoxia while avoiding hyperoxia. - **Bronchodilators:** salbutamol 5 mg nebulised repeatedly or continuously, with nebulised ipratropium bromide 500 micrograms added in the first hour of a severe attack. - **Magnesium and steroids:** intravenous magnesium sulfate 1.2 to 2 g over twenty minutes for attacks not responding to initial treatment; oral prednisolone 40 to 50 mg daily or intravenous hydrocortisone 100 mg six-hourly, given without delay because their effect takes hours. ## A 22-year-old whose attack is worsening after two hours A 22-year-old woman with known asthma has been wheezing for two hours despite home salbutamol, and she can speak only single words. Her peak flow is 40 per cent of predicted, so this is acute severe asthma. 1. **Start treatment at once:** Nebulised salbutamol with ipratropium is started with oxygen titrated to 94 to 98 per cent, while steroids are given without delay. 2. **Watch for life-threatening features:** If her saturation falls below 92 per cent, or peak flow drops below 33 per cent, life-threatening features are present and an arterial blood gas is taken. 3. **Read a normal PaCO2 as a warning:** A normal PaCO2 is alarming in this setting, because she should be hyperventilating, so the reading signals fatigue and the need for critical care review. 4. **Add magnesium and reassess:** Intravenous magnesium is added, and ventilation is considered when exhaustion, confusion or rising PaCO2 develops. 5. **Ventilate protectively:** If mechanical ventilation is needed, low tidal volumes and a long expiratory time limit dynamic hyperinflation, and some hypercapnia is accepted to avoid barotrauma. 6. **Check for pneumothorax:** A chest X-ray is checked for pneumothorax before and after ventilation, because it is a common complication in this group. ## Where the examiner tests the reassurance trap The commonest errors involve a finding that looks reassuring. - **Trap:** taking reassurance from a normal PaCO2 — **Correct:** in an acute attack it is a late and dangerous sign, not a good one. - **Trap:** reading a silent chest as a calm patient — **Correct:** almost no air entry places the patient in the most severe category. - **Trap:** adding aminophylline routinely — **Correct:** it adds little when high-dose salbutamol is already given and carries arrhythmia risk, so it is not used routinely. - **Trap:** judging control only in hospital — **Correct:** many asthma deaths worldwide occur before hospital arrival, so the examiner may ask about a written action plan, home peak flow monitoring and early escalation. Peak flow should be recorded before and after bronchodilator, since the response itself guides the next step. ## Frequently asked questions ### What peak flow value defines life-threatening asthma? A peak expiratory flow below 33 per cent of predicted or best value defines life-threatening asthma in adults, especially when accompanied by a normal or raised PaCO2. Such patients need immediate senior review and critical care support. ### Why is a normal PaCO2 worrying in acute severe asthma? A normal PaCO2 during an acute severe attack means ventilation is failing to clear carbon dioxide, which signals exhaustion rather than recovery. Because the patient should be hyperventilating, a normal or rising PaCO2 is an alarming sign. ### When is intravenous magnesium given in acute asthma? Intravenous magnesium sulfate is given to adults with acute severe asthma who do not respond to initial nebulised bronchodilators, or who have life-threatening features. Its bronchodilator effect is modest, so it is an add-on rather than a replacement for salbutamol. ### Which features mark near-fatal asthma? Near-fatal asthma is defined by a raised PaCO2, a need for mechanical ventilation with raised inflation pressures, or both. Confusion and exhaustion are life-threatening features that often accompany it. Patients who have had a near-fatal attack need an individual action plan and early specialist review.