Lucan · Conditions

Acute severe asthma

Australian guidance: Australian Asthma Handbook — Acute Asthma Management

1. Definition and epidemiology

Acute severe asthma is a bronchospastic exacerbation of sufficient intensity to prevent the patient from speaking in whole sentences or to produce an oxygen saturation at or below 94% on room air. In Australia, asthma affects approximately 11% of the population and is responsible for over 400 deaths annually, with most deaths considered preventable. Indigenous Australians and those in socioeconomic disadvantage carry a disproportionate burden of asthma mortality.

2. Pathophysiology

The cardinal mechanisms in acute severe asthma are bronchoconstriction, airway oedema, and mucus hypersecretion acting in concert to produce variable expiratory airflow limitation. Allergens, respiratory viruses, cold air, exercise, and irritants trigger mast cell degranulation with histamine release and activation of the arachidonic acid cascade, generating prostaglandins and leukotrienes that sustain smooth muscle contraction and recruit eosinophils. Type 2 airway inflammation driven by interleukin (IL)-4, IL-5, and IL-13 underpins most adult asthma: eosinophil accumulation damages epithelial integrity, increases airway hyperresponsiveness, and drives mucus gland hypertrophy. The resultant dynamic hyperinflation raises intrinsic positive end-expiratory pressure, increases the work of breathing, and can precipitate respiratory muscle fatigue. Hypoxaemia arises primarily from ventilation-perfusion mismatch. Hypercapnoea in acute asthma is a late and ominous finding, signalling impending respiratory failure rather than compensatory physiology. (Source: Robbins Ch 15)

3. Clinical presentation and diagnostic criteria

Patients present with progressive dyspnoea, wheeze, chest tightness, and cough. In severe disease the patient cannot complete sentences, adopts an upright position with accessory muscle use, has intercostal and subcostal recession, and may display paradoxical thoracoabdominal movement in impending arrest. Wheeze alone is unreliable: a silent chest indicates near-total obstruction and is an emergency. Tachycardia and tachypnoea are universal; pulsus paradoxus greater than 25 mmHg reflects severe hyperinflation. Oxygen saturation at or below 94% on room air prompts immediate transfer to acute care. Key triggers to identify in the history include recent upper respiratory tract infections, allergen exposure (house dust mite, pollen, animal dander), non-steroidal anti-inflammatory drug or aspirin use, and significant psychosocial stressors. Severity is classified by symptom pattern, oxygen saturation, and spirometric response to initial bronchodilator therapy.

4. Investigations and AU-specific cutoffs

Initial assessment runs simultaneously with salbutamol delivery. Pulse oximetry is the first-line monitor; the Australian Asthma Handbook notes it may overestimate saturation in patients with high skin pigmentation or with acrylic nails. Spirometry (FEV1 or peak flow) should be performed once the patient is stable enough; an FEV1 below 60% predicted at one hour after initial bronchodilator treatment is a criterion for hospital admission. Blood gas analysis is mandatory in life-threatening presentations and is the only reliable means of detecting hypercapnoea. Chest X-ray excludes pneumothorax, lobar consolidation, and cardiac failure but is not needed in every case. For patients being assessed for chronic severe or refractory disease, additional investigations include blood eosinophil count (a count at or above 150 cells per microlitre on medium-dose inhaled corticosteroid (ICS) suggests refractory type 2 inflammation; a count at or above 1500 cells per microlitre warrants urgent specialist referral), fractional exhaled nitric oxide (FeNO; at or above 20 parts per billion on high-dose ICS suggests ongoing type 2 inflammation), and sputum eosinophils at or above 2%.

5. Management

Management follows a simultaneous assess-and-treat model. Salbutamol via pressurised metered-dose inhaler (pMDI) with spacer is the preferred initial bronchodilator. For a severe exacerbation, 12 actuations are delivered using the tidal breathing technique (one actuation into spacer, four tidal breaths, repeated) and repeated every 20 minutes for the first hour if dyspnoea persists. Nebulisation (5 mg salbutamol) is reserved for situations where a pMDI and spacer are unavailable or when the presentation is life-threatening. Supplemental oxygen is started if saturation falls below 92%, titrated to a target of 93 to 95% in otherwise healthy adults to avoid worsening hypercapnoea; patients with concurrent chronic obstructive pulmonary disease, morbid obesity, or neuromuscular disease have a target of 88 to 92%. Systemic corticosteroids are commenced at presentation regardless of severity: prednisone or prednisolone 37.5 to 50 mg orally each morning for 5 to 10 days in adults, with no taper required for short courses. Dexamethasone 16 mg orally for 2 days is an alternative in the emergency department. If there is no improvement or the patient deteriorates, ipratropium bromide 8 actuations (21 micrograms per actuation) via pMDI and spacer every 20 minutes for one hour is added, followed by 4 to 6 hourly doses for 24 hours; this reduces hospitalisation rates compared with short-acting beta-2 agonist (SABA) alone. Intravenous magnesium sulfate 0.2 mmol per kg (maximum 10 mmol) infused over 20 minutes is the second-line agent for severe or life-threatening asthma unresponsive to initial measures. Admission is indicated if hypoxia was present at arrival, FEV1 remains below 60% predicted at one hour, work of breathing is unresolved at 1 to 2 hours, or there is a history of prior intensive care unit admission for asthma.

6. Pharmacology and AU PBS notes

Salbutamol (Ventolin, Asmol) is the first-line SABA and is available without restriction on the PBS for all patients with asthma. Ipratropium bromide (Atrovent) is the first-line short-acting muscarinic antagonist for acute severe disease in the ED setting, available on PBS. Prednisone and prednisolone are PBS-listed for acute asthma without restriction. Intravenous magnesium sulfate is a hospital formulary item and has no PBS listing. For patients discharged from the ED, the preferred maintenance regimen is an ICS-formoterol combination (budesonide-formoterol, brand name Symbicort; or beclometasone-formoterol, brand name Fostair) used as both maintenance and reliever therapy (MART strategy). These are PBS-listed under Streamlined Authority for asthma when ICS alone is insufficient. Patients previously prescribed ICS-formoterol as-needed-only should be stepped up to medium-dose MART at discharge rather than reverted to salbutamol alone. For refractory type 2 eosinophilic asthma, four biologic agents are PBS-accessible under Section 100 Highly Specialised Drugs with specialist-initiated authority: benralizumab (Fasenra, anti-IL-5 receptor alpha), mepolizumab (Nucala, anti-IL-5), dupilumab (Dupixent, anti-IL-4 receptor alpha), and omalizumab (Xolair, anti-IgE). All four require documented failure of high-dose ICS-LABA with good adherence and correct technique.

7. Complications and monitoring

Life-threatening complications include tension pneumothorax (to be excluded by chest X-ray), respiratory arrest, hypoxic cardiac arrest, and salbutamol toxicity (worsening tachycardia, metabolic acidosis, hypokalaemia). Observe admitted patients with continuous pulse oximetry. Serum electrolytes should be checked given salbutamol-induced transcellular potassium shift. Before discharge, the patient must be observed for at least four hours after symptoms resolve. Written discharge instructions covering reliever and preventer regimens, an updated written asthma action plan, and the oral corticosteroid schedule are mandatory. Follow-up with a general practitioner is arranged within 3 to 5 days, and a comprehensive asthma review (including inhaler technique, trigger identification, and consideration of specialist referral) is scheduled at two weeks. Patients with multiple emergency presentations within 12 months or repeated corticosteroid courses warrant urgent respiratory specialist referral. For those on long-term high-dose ICS, bone mineral density and fasting blood glucose are monitored given the risk of osteoporosis and type 2 diabetes. Dispensing of 12 or more salbutamol canisters per year (average 6.6 actuations per day) is associated with significantly increased risk of asthma death and should prompt urgent specialist review.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.