A 35-year-old man with known asthma presents with an acute exacerbation. Baseline spirometry (FEV~1~ 65% predicted) shows FEV~1~ 45% predicted during the attack. After 2 hours of standard therapy (systemic corticosteroids, oxygen, nebulized salbutamol), FEV~1~ is 52% predicted with persistent symptoms. Which investigation is most appropriate to assess severity and guide further management?

Explanation

## Assessment of Acute Asthma Exacerbation Severity **Key Point:** Arterial blood gas (ABG) analysis is the most appropriate investigation to assess the severity of an acute asthma exacerbation and guide escalation of therapy when initial treatment response is inadequate. ### Why ABG in Acute Exacerbation? ABG reveals the **ventilatory status and oxygenation** in severe exacerbation: **High-Yield:** ABG findings that indicate severe/life-threatening exacerbation: - **Hypoxemia:** PaO~2~ <60 mmHg (on room air) or <80 mmHg (on supplemental O~2~) - **Hypercapnia:** PaCO~2~ >45 mmHg (sign of respiratory fatigue, impending respiratory failure) - **Respiratory alkalosis:** Early exacerbation (hyperventilation) - **Respiratory acidosis:** Late/severe exacerbation (CO~2~ retention = respiratory muscle fatigue) **Clinical Pearl:** A **normal or rising PaCO~2~** in an acutely dyspneic asthmatic is an **ominous sign** indicating respiratory muscle fatigue and impending respiratory failure—requires ICU admission and mechanical ventilation. ### Severity Stratification by ABG | ABG Pattern | Severity | Clinical Implication | | --- | --- | --- | | PaO~2~ >80, PaCO~2~ 35–45, pH >7.35 | Mild–moderate | Respond to standard therapy | | PaO~2~ 60–80, PaCO~2~ 35–45, pH >7.35 | Moderate–severe | May need escalation (IV magnesium, aminophylline) | | PaO~2~ <60, PaCO~2~ >45, pH <7.35 | Life-threatening | ICU, mechanical ventilation | **Mnemonic: ABCDE of Severe Asthma** — **A**ltered mental status, **B**radycardia, **C**yanosis, **D**iaphoresis, **E**xcessive use of accessory muscles; ABG confirms with hypoxia + hypercapnia. ### Management Algorithm Based on ABG ```mermaid flowchart TD A[Acute asthma exacerbation]:::outcome --> B[Perform ABG]:::action B --> C{PaO2 and PaCO2 levels?}:::decision C -->|PaO2 >80, PaCO2 <45| D[Mild-moderate exacerbation]:::outcome C -->|PaO2 60-80, PaCO2 35-45| E[Moderate-severe exacerbation]:::outcome C -->|PaO2 <60, PaCO2 >45| F[Life-threatening exacerbation]:::urgent D --> G[Continue standard therapy]:::action E --> H[Add IV magnesium, consider aminophylline]:::action F --> I[ICU admission, mechanical ventilation]:::urgent ``` **Warning:** Do NOT rely on spirometry alone in acute exacerbation—patient may be too dyspneic to perform adequate maneuver. ABG is more reliable for assessing severity and guiding escalation. [cite:Harrison 21e Ch 297]