In a 14‑year‑old with bronchial asthma during an acute exacerbation, what peak expiratory flow values are expected relative to predicted or personal best?

Peak Expiratory Flow in Acute Asthma Exacerbation: 14-Year-Old Child

In a 14-year-old with bronchial asthma during an acute exacerbation, expect peak expiratory flow (PEF) to be <50% of predicted or personal best, with values <33% indicating life-threatening severity requiring immediate ICU transfer. 1, 2

Severity Classification by PEF Values

Acute Severe Asthma

• PEF <50% of predicted or personal best defines acute severe asthma in children and mandates immediate aggressive treatment 1, 2
• This threshold triggers the need for high-flow oxygen (40-60%), nebulized salbutamol 5 mg, and intravenous hydrocortisone 1, 2
• Children at this level are typically too breathless to talk, with respiratory rate >50 breaths/min and heart rate >140 beats/min 1, 2

Life-Threatening Asthma

• PEF <33% of predicted or personal best indicates life-threatening severity requiring ICU transfer with a physician prepared to intubate 1, 2, 3
• At this level, expect associated features including poor respiratory effort, silent chest, cyanosis, exhaustion, or altered consciousness 1, 2, 3
• The British Thoracic Society emphasizes that ANY life-threatening feature should trigger maximum therapy regardless of PEF value 2

Critical Clinical Pitfalls

The "Silent Chest" Warning

• Absence of wheezing does NOT mean improvement—it may indicate the patient is deteriorating toward respiratory arrest with airflow too minimal to generate wheezing 3
• A silent chest with feeble respiratory effort represents life-threatening obstruction even if the child appears less distressed 2, 3

PEF Measurement Limitations in Children

• PEF has high specificity (82-95%) but poor sensitivity (51-57%) for detecting airway obstruction, meaning normal PEF reliably excludes severe disease, but abnormal PEF doesn't fully capture severity 4
• PEF poorly reflects small airway dysfunction (correlation with FEF50 only r=0.49), so it provides an incomplete picture of obstruction 5
• Many inner-city patients report inaccurate "personal best" values—45% had measured PEF exceeding their reported personal best during follow-up 6

Monitoring Protocol During Treatment

Initial Assessment

• Measure PEF immediately upon presentation if the child can perform the maneuver (may not be possible in severe distress) 1, 2
• An initial PEF <50% predicts hospitalization with 81% sensitivity and specificity 7

Serial Measurements

• Repeat PEF 15-30 minutes after starting treatment to assess response 1, 2
• Chart PEF before and after each nebulized β-agonist dose, minimum 4 times daily throughout hospital stay 1, 2
• Multiple PEF measurements over time correlate better with airway function (r=0.77) than single measurements (r=0.49) 5

Response Assessment

• If improving: Continue treatment and monitor PEF every 4 hours 1, 2
• If NOT improving after 15-30 minutes: Increase nebulized β-agonist frequency to every 30 minutes and add ipratropium 1, 2
• Deteriorating PEF despite treatment mandates ICU transfer 1, 2

Discharge Criteria

Before discharge, the child must achieve PEF >75% of predicted or personal best with diurnal variability <25% (unless agreed otherwise with respiratory physician) 1, 2

• Child must have been on discharge medication for 24 hours with documented proper inhaler technique 1, 2
• Provide written self-management plan and arrange GP follow-up within 1 week and respiratory clinic follow-up within 4 weeks 1, 2

Key Safety Points

• Never administer sedatives in acute asthma—they are absolutely contraindicated and can precipitate respiratory arrest 1, 2, 3, 8
• Oxygen does not aggravate CO₂ retention in asthma; give high-flow oxygen without hesitation 1, 2
• Maintain SpO₂ >92% continuously with oximetry monitoring 1, 2
• The presence of ANY life-threatening feature (PEF <33%, silent chest, cyanosis, confusion, exhaustion) should trigger immediate maximum therapy and ICU consultation 1, 2, 3