Acute Severe Asthma Criteria
All four options (A, B, C, and D) can suggest acute severe asthma, but options B (respiratory rate >25), C (pulse >110 bpm), and D (PEF <50% predicted) are the most specific criteria according to British Thoracic Society guidelines, while option A (O2 saturation <95%) indicates severe hypoxemia requiring immediate intervention. 1
Defining Features of Acute Severe Asthma
The British Thoracic Society establishes clear criteria for acute severe asthma in adults 1:
- Respiratory rate >25 breaths/min (Option B) 1
- Pulse >110 beats/min (Option C - note the question states "carotid pulse >100 bpm" which is close but the guideline threshold is >110) 1
- Peak expiratory flow <50% of predicted or best (Option D - the question states <300 L/min, which would be <50% for most adults) 1
- Inability to complete sentences in one breath 1
Oxygen Saturation Considerations
Option A (O2 saturation <95%) is clinically significant but not specifically listed in the formal criteria for acute severe asthma. 1 However, the guidelines emphasize:
- Maintaining SaO2 >92% is the target during treatment 1
- Severe hypoxia (PaO2 <8 kPa/60 mmHg) is a marker of life-threatening asthma, not just severe asthma 1
- An O2 saturation <95% would warrant immediate oxygen therapy and aggressive treatment 1
Critical Distinction: Severe vs. Life-Threatening
Life-threatening features include 1:
- PEF <33% predicted or best
- Silent chest, cyanosis, or feeble respiratory effort
- Bradycardia or hypotension
- Exhaustion, confusion, or coma
- Normal or elevated PaCO2 in a breathless patient (indicating impending respiratory failure) 2
Common Pitfall to Avoid
Do not be falsely reassured by a patient who appears calm or has "normal" vital signs in one parameter. 1 The guidelines explicitly warn that patients with severe or life-threatening attacks may not appear distressed and may not exhibit all abnormalities 1. The presence of any single criterion should alert the clinician to acute severe asthma 1.
Practical Application
When evaluating the question options:
- Option B (RR >25): Directly matches guideline criteria 1
- Option C (Pulse >100): Close to guideline threshold of >110 bpm 1
- Option D (PEF <300 L/min): Likely represents <50% predicted for most adults 1
- Option A (O2 sat <95%): Indicates hypoxemia requiring treatment but not a formal criterion for "severe" classification 1
The most objective and reliable measure is peak expiratory flow (Option D), as the guidelines emphasize that failure to make objective measurements is a major factor in preventable asthma deaths. 1