A 28-year-old woman with a history of asthma presents with episodic wheezing and dyspnea. Her baseline spirometry shows FEV₁/FVC ratio of 0.82 with normal absolute values. During an acute exacerbation, she reports worsening dyspnea. Which investigation is most appropriate to assess the degree of airway obstruction during the acute episode?

Explanation

## Investigation of Choice: Peak Expiratory Flow (PEF) Measurement **Key Point:** Peak expiratory flow (PEF) is the most practical, rapid, and non-invasive bedside test to assess severity of airway obstruction during acute asthma exacerbations. It can be performed repeatedly and is ideal for monitoring response to bronchodilators. **High-Yield:** PEF characteristics in acute asthma: - Normal PEF: 400–700 L/min (varies by age, sex, height) - Mild exacerbation: PEF 70–90% predicted - Moderate exacerbation: PEF 40–70% predicted - Severe exacerbation: PEF < 40% predicted - Rapid improvement with bronchodilators suggests reversible airway obstruction **Clinical Pearl:** PEF can be measured at the bedside using a simple peak flow meter in seconds, making it ideal for acute settings. Serial PEF measurements track bronchodilator response better than a single spirometry. **Mnemonic: PEF in Acute Asthma** — **QUICK** - **Q**uick bedside test - **U**seful for serial monitoring - **I**mmediate bronchodilator response assessment - **C**linically guides therapy escalation - **K**ey for discharge criteria ### Why PEF is Optimal in Acute Settings 1. **Speed:** Results in < 1 minute vs. 5–10 minutes for full spirometry 2. **Repeatability:** Can be measured every 15–30 minutes to track response 3. **Effort-dependent:** Less demanding than FEV₁ in acutely dyspneic patients 4. **Prognostic value:** PEF < 40% predicted indicates severe exacerbation requiring hospitalization ### Comparison of Spirometric Investigations in Asthma | Investigation | Acute Exacerbation | Baseline Assessment | Bronchial Hyperresponsiveness | |---|---|---|---| | PEF | **FIRST-LINE** | Supplementary | Not suitable | | Full Spirometry | Secondary (if stable enough) | Diagnostic | Not for diagnosis | | Bronchodilator challenge | Not in acute phase | Confirm reversibility | Not for diagnosis | | Methacholine challenge | **CONTRAINDICATED** (risk of severe bronchospasm) | Diagnostic (if FEV₁ normal) | **GOLD STANDARD** | | Body plethysmography | Not acute assessment | Measure RV, TLC | Not for acute exacerbation | **Warning:** Methacholine challenge is **absolutely contraindicated** during acute asthma exacerbation because it can provoke severe, life-threatening bronchospasm. It is only performed during stable baseline periods when FEV₁ is normal or near-normal. [cite:Harrison 21e Ch 309; GINA Guidelines 2023]