PassFinals
Respiratory

Asthma

Chronic airway inflammation leaves bronchial smooth muscle hyper-responsive, so airway calibre and expiratory airflow vary over time, with triggers, and with treatment.

In a nutshell

Asthma is variable airway inflammation causing variable expiratory airflow limitation; confirm it with an objective test, not symptoms alone. In an acute attack give oxygen to 94% to 98%, nebulised salbutamol 2.5 to 5 mg and prednisolone 40 to 50 mg.

Classic presentation

A young adult with episodic nocturnal wheeze and cough, worse after exercise or a cat, with eczema and hay fever, and a normal chest today.

Key points

  • Life-threatening asthma needs SpO2 below 92% plus one feature, in a patient who already has acute severe asthma. That "plus" is the 2025 amendment to SIGN 158.
  • Acute severe asthma is any one of: peak expiratory flow (PEF) 33% to 50%, respiratory rate 25 or more, heart rate 110 or more, or unable to finish a sentence.
  • Near-fatal asthma is a raised PaCO2, or ventilation with raised inflation pressures. Moderate is PEF 50% to 75% with no acute severe features.
  • Steroid dose is the asthma-versus-COPD discriminator: prednisolone 40 to 50 mg for at least 5 days in asthma, 30 mg for 5 days in COPD.
  • Oxygen target is 94% to 98% in asthma, not the 88% to 92% used in COPD, and the nebuliser is oxygen-driven at 6 L/min.
  • Low-dose and moderate-dose describe the maintenance inhaled corticosteroid (ICS) in maintenance and reliever therapy (MART). NICE publishes the microgram bands per drug in a separate table.
  • Spirometry confirms adult asthma if forced expiratory volume in one second rises 12% and 200 mL after bronchodilator. PEF variability of 20% or more also confirms.
  • Discharge with PEF below 75% of best or predicted, or diurnal variability above 25%, predicts early relapse. Tell the GP practice within 24 hours.

First-line investigation

Adults: blood eosinophil count or fractional exhaled nitric oxide (FeNO), diagnostic at 50 ppb or more. In an acute attack, PEF with SpO2, respiratory rate, heart rate and speech.

Management

Treat the attack

  • Oxygen titrated to SpO2 94% to 98%, without waiting for oximetry. Nebulised salbutamol 2.5 to 5 mg every 15 to 30 minutes, oxygen-driven at 6 L/min.7,5
  • Add nebulised ipratropium 500 micrograms 4 to 6 hourly for acute severe or life-threatening asthma, or a poor response. Not needed in milder attacks or after stabilisation.7,8
  • Prednisolone 40 to 50 mg orally daily for at least 5 days, or hydrocortisone 100 mg intravenously six-hourly. Do not stop the inhaled corticosteroid.7,9,10
  • Record PEF 15 to 30 minutes after starting treatment, and before and after each nebuliser. Blood gas if SpO2 is below 92% or any life-threatening feature.4,3

Poor response

  • Acute severe asthma with PEF below 50% responding poorly: magnesium sulfate 1.2 to 2 g intravenously over 20 minutes, single dose, after senior discussion.7,13
  • Continuous nebulised salbutamol 5 to 10 mg/hour rather than a bigger bolus; a 10 mg bolus is not more effective. Nebulised magnesium is not recommended in adults.7
  • Aminophylline only on senior advice: 5 mg/kg intravenously over 20 minutes, omitted if already on theophylline, then 0.5 to 0.7 mg/kg/hour. Check levels daily.7,6
  • Critical care for ventilatory support, falling PEF, worsening hypoxia, hypercapnia, falling pH, exhaustion, drowsiness or respiratory arrest. Notify anaesthetics early.7

Admit, discharge, follow up

  • Admit any life-threatening or near-fatal feature, and any severe feature persisting after treatment. PEF above 75% at one hour may allow discharge from the emergency department.3
  • Exceptions to that discharge rule: significant symptoms, adherence concerns, isolation, psychological problems, disability, previous near-fatal asthma, an attack despite oral steroid, night presentation, pregnancy.3
  • Inform the practice within 24 hours. GP or asthma nurse review within two working days; specialist asthma nurse or respiratory physician at about one month.14
  • Before discharge: inhaler technique, PEF recording and a written personalised asthma action plan. The BTS Asthma 4 bundle adds clinical review within 4 weeks.14,15

Chronic control

  • Never a short-acting beta2 agonist (SABA) alone. Aged 12 and over, newly diagnosed: low-dose ICS/formoterol as needed. Low-dose MART if highly symptomatic or presenting with a severe attack.1
  • Aged 5 to 11: twice-daily paediatric low-dose ICS plus SABA as needed. Under 5: an 8 to 12 week trial of paediatric low-dose ICS.1
  • Before stepping up, check adherence from prescription records, inhaler technique, smoking or vaping, exposures and the diagnosis. Review 8 to 12 weeks after any change.1
  • Step up to low-dose MART, then moderate-dose MART. If still uncontrolled, check FeNO and blood eosinophils and refer to a specialist if either is raised.1
  • If neither is raised, trial a leukotriene receptor antagonist or a long-acting muscarinic antagonist for 8 to 12 weeks. Step down once control is sustained.1

Pregnancy, work and montelukast

  • Review asthma in early pregnancy and postpartum and continue asthma medicines; good control protects parent and baby.1
  • Ask about symptoms at work and away from work, and refer suspected occupational asthma to an occupational asthma specialist.1
  • Montelukast: warn about neuropsychiatric reactions and stop it if new sleep, mood or behaviour symptoms appear. Sleep disturbance, depression and aggression affect up to 1 in 100.16

Exam traps

  • Every revision resource still teaches the disjunctive list. Applying it over-calls life-threatening asthma in the exam and under-calls the severe patient in front of you.
  • A silent chest means airflow too low to make a sound. It is deterioration, not improvement.
  • A normal or rising PaCO2 in a distressed, tachypnoeic patient is a fatigue warning, because effective hyperventilation should lower it.
  • Pulse oximetry cannot detect hypercapnia, which is why SpO2 below 92% triggers a blood gas rather than reassurance.
  • Pulsus paradoxus is an inadequate severity indicator in acute asthma and should not be used. In cardiac tamponade it is a cardinal sign: different disease, different question.
  • FeNO of 50 ppb or more is the adult threshold; in children aged 5 to 16 it is 35 ppb. Do not carry over older 40 ppb teaching.
  • Non-invasive ventilation is not the answer to hypercapnia in acute asthma. That patient needs intubation, in intensive care.

Illustrations

Asthma airway narrowingDiagram contrasting a normal airway with an asthmatic airway showing bronchoconstriction, mucosal oedema and mucus, with a note that the obstruction is variable.PassFinals · original
Objective diagnosis pathwayFlow diagram from compatible history to eosinophils or FeNO, spirometry with bronchodilator reversibility, serial peak flow and specialist bronchial challenge.PassFinals · original
Acute asthma severity and escalationClinical severity diagram linking peak flow, speech, oxygenation, mental state, silent chest and carbon dioxide to urgent treatment and escalation.PassFinals · original

Key sources

  1. NICE NG245: Asthma: diagnosis, monitoring and chronic asthma management (BTS, NICE, SIGN) (NG245 recommendations 1.2.1 to 1.2.6, 1.4.2, 1.5.1, 1.6.2 to 1.6.4, 1.7.1 to 1.7.5, 1.7.11, 1.8.1, 1.9.1, 1.11.1, 1.12.1, 1.14.1 and 1.16.1, verified in the guidance PDF)Published 27 Nov 2024
  2. SIGN 158: British guideline on the management of asthma (Revised edition published November 2024 with corrections May 2025. Section 9, Table 15, and the revision table entry recording the 2025 amendment to the life-threatening asthma criteria)Updated 1 May 2025
  3. Asthma pathway (BTS, NICE, SIGN) SIGN 244: acute asthma in adults (Right Decisions rendering of SIGN 158 section 9: levels of severity, criteria for referral and admission, pulse oximetry, blood gases, chest X-ray and systolic paradox)
  4. Asthma pathway (BTS, NICE, SIGN) SIGN 244: further investigation and monitoring (Right Decisions rendering of SIGN 158 section 9: peak flow and blood gas intervals, oxygen saturation target, potassium, glucose and theophylline concentration)
  5. BNF: Salbutamol (Acute asthma, adult, by inhalation of nebulised solution: 2.5 to 5 mg repeated every 15 to 30 minutes, oxygen-driven where available; and monitoring of plasma potassium in severe asthma)
  6. BNF: Aminophylline (Severe acute asthma: slow intravenous injection then infusion at 500 to 700 micrograms/kg/hour, and the therapeutic plasma-theophylline range of 10 to 20 mg/L (55 to 110 micromol/L))
  7. Asthma pathway (BTS, NICE, SIGN) SIGN 244: treatment of acute asthma in adults (Right Decisions rendering of SIGN 158 section 9: oxygen, beta2 agonists, steroid therapy, ipratropium, magnesium sulphate, intravenous aminophylline, antibiotics, heliox, critical care and non-invasive ventilation)
  8. BNF: Ipratropium bromide (Severe or life-threatening acute asthma, adult, by inhalation of nebulised solution: 500 micrograms every 4 to 6 hours as required; the dose for this indication is recorded as unlicensed)
  9. BNF: Prednisolone (Severe or life-threatening acute asthma, adult, by mouth: 40 to 50 mg once daily for at least 5 days)
  10. BNF: Hydrocortisone (Severe or life-threatening acute asthma, adult, by intravenous injection: 100 mg every 6 hours until conversion to oral prednisolone is possible)
  11. NICE NG115: Chronic obstructive pulmonary disease in over 16s: diagnosis and management (Recommendation 1.3.16, "Offer 30 mg oral prednisolone daily for 5 days", verified in the guidance PDF. Cited here only for the deliberate contrast with asthma)Published 5 Dec 2018 | Updated 26 Jul 2019
  12. BNF: Oxygen (BNF treatment summary: target saturation 94% to 98% in most acutely ill patients, and 88% to 92% for patients at risk of hypercapnic respiratory failure)
  13. BNF: Magnesium sulfate (Severe acute asthma, adult, by intravenous infusion: 1.2 to 2 g over 20 minutes; recorded as an unlicensed use)
  14. Asthma pathway (BTS, NICE, SIGN) SIGN 244: hospital discharge and follow up (Right Decisions rendering of SIGN 158 section 9: timing of discharge, patient education before discharge, and follow-up intervals)
  15. British Thoracic Society: The Asthma 4, asthma attack care bundle (The four bundle actions for adults and adolescents aged 16 and over, including clinical review within 4 weeks)
  16. MHRA: Montelukast, reminder of the risk of neuropsychiatric reactions (Drug Safety Update, volume 17, issue 9, April 2024)Published 29 Apr 2024

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.