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.

Definition

Asthma is chronic airway inflammation with bronchial hyper-responsiveness, producing variable symptoms and variable expiratory airflow limitation. NICE NG245 requires a compatible clinical history plus one objective test. The options are a raised blood eosinophil count or exhaled nitric oxide, bronchodilator reversibility on spirometry, peak flow variability, or bronchial hyper-responsiveness on challenge testing.

Epidemiology

Asthma affects people of all ages and commonly coexists with eczema and allergic rhinitis. The risk of a severe attack rises with poor control, frequent reliever use, a previous severe or near-fatal attack, recent hospital admission, smoking or vaping, occupational exposure and treatment non-adherence.

Pathophysiology

Airway narrowing raises expiratory resistance, so the lungs empty incompletely and gas is trapped behind airways that close early. Hyperinflation flattens the diaphragm and multiplies the work of breathing. Ventilation and perfusion become mismatched, producing hypoxaemia before any carbon dioxide problem. Mucus plugs occlude small airways and are not cleared by a bronchodilator, which is one reason a severe attack settles slowly even on correct treatment.

First principles

Variable inflammation gives variable obstruction

Eosinophilic and mast-cell inflammation sensitises airway smooth muscle. Allergen, viral infection, exercise, cold air, smoke or a workplace exposure then produces mucosal oedema, mucus and bronchoconstriction. Calibre therefore changes hour to hour, which is why symptoms are episodic, worse at night, and absent between attacks. Long-standing inflammation remodels the airway wall and leaves obstruction that no bronchodilator reverses.¹

Symptoms suggest asthma; a test confirms it

Wheeze, cough, breathlessness and chest tightness are common and non-specific, and a normal chest between episodes proves nothing. NICE NG245 therefore requires objective support before the diagnosis is recorded. In adults the first test is an inflammatory marker, blood eosinophils or fractional exhaled nitric oxide (FeNO). It is quick, and unlike spirometry it needs no effort from a breathless patient.¹

The reliever does not treat the disease

A short-acting beta2 agonist (SABA) relaxes smooth muscle within minutes and leaves the inflammation untouched, so the next attack is unaffected. NICE NG245 recommendation 1.6.3 says not to prescribe a SABA at any age without a concomitant inhaled corticosteroid (ICS). From age 12 the reliever itself carries the steroid: low-dose ICS with formoterol taken as needed is anti-inflammatory reliever (AIR) therapy.¹

Life-threatening asthma is a conjunction, not a checklist

Older teaching listed silent chest, cyanosis, exhaustion and a low peak flow as independent life-threatening features. The 2025 amendment to SIGN 158 replaced it: SpO2 below 92% plus one listed feature, in a patient who already meets acute severe criteria. The physiology explains the strangest entry on that list. A frightened, obstructed patient hyperventilates, so PaCO2 should be low; a normal PaCO2 means the respiratory muscles are failing.^2,3

Presentation

Episodic wheeze, cough, breathlessness or chest tightness that varies over time or with triggers such as allergen, exercise, cold air, viral infection, smoke or a workplace exposure. Symptoms and examination are often normal between episodes.^1,3

Cardinal features

Wheeze, cough, breathlessness or chest tightness that varies over time
Night-time, early-morning, seasonal or exercise-related symptoms
Triggers including allergen, viral infection, cold air, smoke, air pollution or occupational exposure
Personal or family history of eczema or allergic rhinitis
Expiratory polyphonic wheeze, although the chest is often clear between episodes
Symptoms that improve with treatment, or away from a workplace exposure

Red flags

Silent chest, cyanosis, feeble respiratory effort, exhaustion, arrhythmia, hypotension or altered consciousness
SpO2 below 92% on air or on oxygen, which is itself the trigger for a blood gas
Peak flow below 30% of best or predicted, or 30% to 33% given the current source discrepancy; manage either as life threatening
A normal or rising PaCO2, or PaO2 below 8 kPa, in a patient who is working hard to breathe
Previous near-fatal asthma, an attack despite oral corticosteroid, or a recent hospital admission
These prompt immediate senior help; the formal life-threatening definition needs SpO2 below 92% plus one of them in acute severe asthma
Sudden unilateral wheeze, stridor, chest pain, haemoptysis, or fever with focal signs, suggesting a different diagnosis

Investigations

Structured history and examination

Establish whether the pattern is variable and trigger-related, find occupational and environmental exposures, review inhaler use and adherence, and look for an alternative diagnosis. During an attack this is also the severity assessment.

Expected finding: A variable, trigger-related pattern supports asthma, and a normal chest between episodes does not exclude it. Stridor, focal signs, daily sputum, haemoptysis or a poor treatment response should redirect the work-up.

^1,3

Blood eosinophil count or fractional exhaled nitric oxide (FeNO) in adults

NICE NG245 recommendation 1.2.1 puts an inflammatory marker first in adults, because it is quick and needs no respiratory manoeuvre.

Expected finding: Diagnose asthma if the eosinophil count is above the laboratory reference range, or FeNO is 50 ppb or more. A negative result does not end the pathway if suspicion persists.

Spirometry with bronchodilator reversibility

Used when eosinophils and FeNO have not confirmed asthma, under NG245 recommendation 1.2.2. It shows obstruction and shows that the obstruction reverses.

Expected finding: In adults, diagnose asthma if the forced expiratory volume in one second (FEV1) rises by at least 12% and 200 mL after a bronchodilator. A rise of at least 10% of predicted normal FEV1 also counts. In children aged 5 to 16 a 12% rise from baseline is enough.

Serial peak expiratory flow, then bronchial challenge

Use twice-daily peak expiratory flow (PEF) for 2 weeks when spirometry is unavailable or delayed, under NG245 recommendation 1.2.3. Refer for bronchial challenge only when the other tests have not confirmed asthma and suspicion persists.

Expected finding: Diagnose asthma if PEF variability, expressed as amplitude percentage mean, is 20% or more. A single reading is never diagnostic. Demonstrated bronchial hyper-responsiveness on challenge testing confirms the diagnosis.

Objective severity assessment during an acute attack

Band the attack on PEF, SpO2, respiratory rate, heart rate, speech, conscious level and respiratory effort. Clinical impression alone under-calls severity, because a patient with a severe or life-threatening attack may not look distressed.

Expected finding: Use predicted PEF only when the recent best, within two years, is unknown. Current official renderings conflict: the SIGN 158 PDF prints below 33%, while the SIGN 244 Right Decisions adult page prints below 30%. Treat 30% to 33% as life threatening rather than allowing the publication discrepancy to delay escalation.

^2,3

Blood gas, chest radiograph and bloods in an acute attack

Pulse oximetry cannot detect hypercapnia, so a saturation above 92% is reassurance about oxygen only. Blood tests catch the treatment's own effects.

Expected finding: Take an arterial gas if SpO2 is below 92% on air or oxygen, or any life-threatening feature is present. Chest radiography is not routine: request it for suspected pneumothorax or pneumomediastinum, suspected consolidation, life-threatening asthma, poor response, or a need for ventilation. Measure potassium and glucose, because beta2 agonists, steroids and theophylline all lower potassium and hypoxia potentiates the effect. If aminophylline continues beyond 24 hours, check the theophylline concentration against a target of 10 to 20 mg/L (55 to 110 micromol/L).

^3,4,5,6

Management

Step	Detail	Source
Grade the attack: moderate and acute severe	Moderate: PEF 50% to 75% best or predicted, no severe features. Acute severe: any one of PEF 33% to 50%, respiratory rate at or above 25 per minute, heart rate at or above 110, or inability to complete a sentence. Use predicted values only if the best PEF within two years is unknown.^2,3	SIGN 158, section 9, Table 15, levels of severity of acute asthma attacks in adults
Life-threatening and near-fatal asthma, as amended in 2025	The 2025 amendment made this conjunctive. Life-threatening asthma is SpO2 below 92% plus one feature in acute severe asthma: altered consciousness, exhaustion, arrhythmia, hypotension, cyanosis, silent chest, poor effort, PaO2 below 8 kPa or normal PaCO2. Sources conflict on PEF below 30% versus 33%; treat 30% to 33% as life threatening. Near-fatal means raised PaCO2 or ventilation with raised pressures.^2,3	SIGN 158, revised edition November 2024 with corrections May 2025; its own revision table records one change to section 9, the 2025 amendment to the life-threatening asthma criteria
Treat the attack: oxygen, bronchodilators and steroid	Oxygen titrated to SpO2 94% to 98%, given without waiting for pulse oximetry. Nebulised salbutamol 2.5 to 5 mg repeated every 15 to 30 minutes, oxygen-driven at 6 L/min. Add nebulised ipratropium 500 micrograms 4 to 6 hourly for acute severe or life-threatening asthma, or a poor response. Prednisolone 40 to 50 mg orally daily for at least 5 days.^7,5,8,9	SIGN 158, treatment of acute asthma in adults, with BNF monographs for salbutamol, ipratropium and prednisolone. The BNF records that the ipratropium dose for severe or life-threatening acute asthma is unlicensed.
Steroid: the parenteral route and stopping it	Parenteral hydrocortisone 400 mg daily (100 mg six-hourly) is equivalent; higher doses are no better. Do not stop inhaled corticosteroid while oral steroid runs. After recovery, stop the steroid abruptly without tapering, provided the patient takes an inhaled corticosteroid and is not on maintenance steroid. If oral treatment is impractical, SIGN 158 offers intramuscular methylprednisolone 160 mg.^7,10	SIGN 158, treatment of acute asthma in adults, with the BNF hydrocortisone monograph. The intramuscular methylprednisolone dose is taken from the guideline text; there is no BNF monograph dose for this indication in the source pack.
The asthma-versus-COPD contrast, examined constantly	Asthma takes prednisolone 40 to 50 mg for at least 5 days and an oxygen target of 94% to 98%, with oxygen-driven nebulisers. A COPD exacerbation takes prednisolone 30 mg for 5 days, and 88% to 92% for anyone at risk of hypercapnic respiratory failure. The steroid dose and the saturation target are the two differences.^11,12,9	NICE NG115 recommendation 1.3.16 for the COPD steroid dose, and the BNF oxygen treatment summary for the two target saturation ranges
Poor response: magnesium and continuous nebulisation	For acute severe asthma with PEF below 50% that responds poorly to inhaled bronchodilators, give magnesium sulfate 1.2 to 2 g by intravenous infusion over 20 minutes. Single dose only, and after senior discussion. Repeat doses are unassessed and risk hypermagnesaemia with muscle weakness. Consider continuous nebulised salbutamol 5 to 10 mg/hour; a 10 mg bolus is not more effective.^7,13	SIGN 158, treatment of acute asthma in adults, with the BNF magnesium sulfate monograph. The BNF records severe acute asthma as an unlicensed use of magnesium sulfate.
Aminophylline and critical care referral	Aminophylline adds little to inhaled bronchodilators plus steroid, and increases arrhythmias and vomiting. Use it only after senior discussion: 5 mg/kg intravenously over 20 minutes, omitted if already on oral theophylline, then 0.5 to 0.7 mg/kg/hour. Refer to critical care for ventilatory support, falling PEF, worsening hypoxia, hypercapnia, falling pH, exhaustion, drowsiness or respiratory arrest.^7,6	SIGN 158, treatment of acute asthma in adults, and indications for admission to intensive care, with the BNF aminophylline monograph
Monitor while treating	Record PEF 15 to 30 minutes after starting treatment, then by response, and before and after each nebulised beta2 agonist. Keep SpO2 94% to 98%. Take a blood gas if SpO2 is below 92% or any life-threatening feature is present. Repeat it within an hour if PaO2 was below 8 kPa, PaCO2 was normal or raised, or the patient deteriorates.^4,3	SIGN 158, further investigation and monitoring in acute asthma in adults; repeat gases again at 4 to 6 hours if there is no improvement
Admission, and discharge from the emergency department	Admit any life-threatening or near-fatal feature, and any severe feature persisting after initial treatment. PEF above 75% of best or predicted one hour after treatment allows discharge from the emergency department, with exceptions. These are significant symptoms, adherence concerns, social isolation, psychological problems, disability or learning difficulty, previous near-fatal asthma, an attack despite adequate oral steroid, night presentation, or pregnancy.³	SIGN 158, criteria for admission, acute asthma in adults
Discharge from the ward	No single measurement fixes the timing. The patient should be on reducing beta2 agonist, preferably no more than four-hourly, and on treatment they can continue at home. Discharge with PEF below 75% of best or predicted, and diurnal variability above 25%, predicts early relapse. Before discharge, trained staff cover inhaler technique, PEF recording and a written action plan.¹⁴	SIGN 158, hospital discharge and follow up. The guideline sets no fixed nebuliser-free interval before discharge.
Follow-up after an attack	Inform the patient's practice within 24 hours. Arrange GP or asthma nurse review within two working days, and specialist asthma nurse or respiratory physician review at about one month. The British Thoracic Society (BTS) Asthma 4 bundle adds, for those aged 16 and over: medication review, an action plan, tobacco dependence support, and clinical review within 4 weeks.^14,15	SIGN 158, hospital discharge and follow up, and the BTS Asthma 4 asthma attack care bundle
Start age-appropriate ICS-containing treatment	Aged 12 and over, newly diagnosed: low-dose ICS/formoterol as needed, anti-inflammatory reliever therapy. If highly symptomatic or presenting with a severe attack, start low-dose maintenance and reliever therapy (MART) and treat the attack. Aged 5 to 11: twice-daily paediatric low-dose ICS with a SABA as needed. Under 5: an 8 to 12 week trial of paediatric low-dose ICS.¹	NICE NG245 recommendations 1.7.1, 1.7.2, 1.8.1 and 1.9.1
Correct poor control before stepping up	At every asthma review check adherence from prescription records, inhaler technique, device suitability, smoking or vaping, occupational and environmental exposure, rhinitis, reflux, obesity, anxiety and the diagnosis itself. Check FeNO if available: a raised level suggests poor adherence or too little ICS. Review 8 to 12 weeks after starting or changing treatment.¹	NICE NG245 recommendations 1.5.1, 1.6.2, 1.6.4 and 1.11.1
Step up, refer, and step down	Aged 12 and over: low-dose MART, then moderate-dose MART. Still uncontrolled on moderate-dose MART: check FeNO and blood eosinophils, and refer if either is raised. If neither is raised, trial a leukotriene receptor antagonist (LTRA) or long-acting muscarinic antagonist (LAMA) for 8 to 12 weeks. Refer anyone uncontrolled on high-dose ICS. Step down once control is sustained.¹	NICE NG245 recommendations 1.7.3, 1.7.4, 1.7.5 and 1.7.11
Pregnancy, occupational asthma and severe asthma	Review asthma in early pregnancy and postpartum, and continue asthma medicines: good control protects both the pregnant person and the baby. Ask about symptoms at work and on days away from work, and refer suspected occupational asthma to an occupational asthma specialist. Refer persistent poor control on appropriate inhaled treatment for severe-asthma assessment, including biologic therapy.¹	NICE NG245 recommendations 1.4.2, 1.7.11 and 1.12.1
Counsel about montelukast	If montelukast is used, warn the patient or carer about neuropsychiatric reactions and ask about new sleep disturbance, nightmares, mood or behaviour change, agitation, depression, hallucinations or suicidal thinking. Stop it if these appear. Sleep disturbance, depression and aggression affect up to 1 in 100 users; hallucinations and suicidal thinking up to 1 in 10,000.¹⁶	MHRA Drug Safety Update, volume 17, issue 9, April 2024: Montelukast, reminder of the risk of neuropsychiatric reactions
Do not use these in an acute attack	Routine antibiotics are not indicated; a precipitating infection is usually viral. Oral leukotriene receptor antagonists are not supported in an acute attack. Nebulised magnesium is not recommended in adults. Heliox is for trials only, and nebulised furosemide shows no benefit. Pulsus paradoxus is an inadequate severity indicator and should not be used.^7,3	SIGN 158, treatment of acute asthma in adults, and initial assessment of symptoms, signs and measurements

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

Differentials

Chronic obstructive pulmonary disease

Older, with a smoking or exposure history, and persistent rather than variable airflow limitation. Asthma and COPD can coexist.

Inducible laryngeal obstruction

Inspiratory noise, throat rather than chest tightness, abrupt onset and offset, and no response to asthma treatment. Confirmed in specialist care.

Bronchiectasis or chronic suppurative lung disease

Daily productive cough, recurrent chest infections, haemoptysis or persistent focal chest signs.

Heart failure

Orthopnoea, paroxysmal nocturnal dyspnoea, oedema and a raised JVP rather than trigger-related variable obstruction.

Inhaled foreign body or other focal airway disease

Sudden onset or unilateral wheeze, especially in a child, needing urgent assessment for an alternative diagnosis.

Complications

Acute severe, life-threatening or near-fatal asthma
Respiratory failure, cardiac arrest and death
Hypokalaemia from beta2 agonists, steroids and theophylline combined
Missed alternative diagnosis or unrecognised occupational sensitisation
Airway remodelling and persistent airflow limitation
Adverse effects of repeated systemic or high-dose inhaled corticosteroids
Montelukast-associated neuropsychiatric reactions

Prognosis

Most people achieve good control on an inhaled corticosteroid-containing regimen with correct technique, adherence and a written action plan. Asthma still kills, and risk concentrates in people with poor control, repeated attacks, heavy reliever use and missed follow-up. A previous near-fatal attack is one of the named reasons not to discharge someone from the emergency department even when their peak flow has recovered.

Guidelines

Asthma: diagnosis, monitoring and chronic asthma management (BTS, NICE, SIGN) (NG245) (NICE, 2024)
British guideline on the management of asthma (SIGN 158), revised edition with 2025 corrections (SIGN and British Thoracic Society, 2024)
Asthma pathway (BTS, NICE, SIGN) (SIGN 244) (Healthcare Improvement Scotland, NICE and British Thoracic Society)

References

Evidence checked: 2026-08-07

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.