What indicates acute severe asthma in a patient with a history of asthma, previous best peak flow of 400 L/min, and current complaint of increasing wheeze?

Acute Severe Asthma Criteria

Based on British Thoracic Society guidelines, respiratory rate >25 breaths/min (Option B) is the correct answer that indicates acute severe asthma in this patient. 1, 2, 3

Understanding the Severity Classification

The British Thoracic Society defines acute severe asthma by the presence of any of the following features: 1, 3

• Respiratory rate >25 breaths/min 1, 2, 3
• Heart rate >110 beats/min (not >100 as stated in option C) 1, 2, 3
• Peak flow <50% of predicted or best 1, 2, 3
• Inability to complete sentences in one breath 3, 4

Why Each Answer is Correct or Incorrect

Option A: O₂ saturation <95% - INCORRECT

• While hypoxia is concerning, O₂ saturation <95% alone does not define acute severe asthma by guideline criteria 1
• Life-threatening asthma is indicated by severe hypoxia (PaO₂ <8 kPa/60 mmHg) despite oxygen therapy, not simply saturation <95% 1, 2

Option B: Respiratory rate >25 - CORRECT

• This is an explicit criterion for acute severe asthma per British Thoracic Society guidelines 1, 2, 3
• This parameter is easily measurable and reliably indicates severity 3

Option C: Carotid pulse >100 bpm - INCORRECT

• The threshold for acute severe asthma is heart rate >110 bpm, not >100 bpm 1, 2, 3
• While tachycardia >100 is concerning, it doesn't meet the specific guideline definition 1

Option D: Peak flow <300 L/min - INCORRECT for acute severe classification

• With a personal best of 400 L/min, a peak flow of 300 L/min represents 75% of personal best 1
• Acute severe asthma requires PEF <50% of predicted or best (which would be <200 L/min in this patient) 1, 2, 3
• A PEF of 300 L/min (75% of best) indicates suboptimal control but does not meet severe criteria 1
• Life-threatening asthma would be PEF <33% of best (<133 L/min in this patient) 1, 2, 3

Critical Clinical Pitfall

Physicians often underestimate asthma severity when relying solely on clinical judgment without objective measurements. 3, 5 Always measure peak flow immediately in any patient presenting with acute asthma symptoms, as clinical assessment alone is frequently inaccurate. 3, 5, 4

Immediate Management Implications

When any feature of acute severe asthma is present, immediate treatment includes: 1, 3

• High-dose inhaled β-agonists (salbutamol 5 mg or terbutaline 10 mg via oxygen-driven nebulizer) 1, 3
• Systemic corticosteroids (prednisolone 30-60 mg orally or hydrocortisone 200 mg IV) 1, 3
• Reassess PEF 15-30 minutes after initial treatment 1, 3
• Consider hospital admission if features persist after initial treatment 1, 3