A 42-year-old woman with aspiration pneumonia develops ARDS (PaO₂/FiO₂ = 150). She is on mechanical ventilation with PEEP 10 cm H₂O. To assess lung recruitability and predict response to prone positioning, which investigation is most appropriate?

Explanation

## Assessment of Lung Recruitability in ARDS **Key Point:** **Quantitative CT scan with recruitment maneuver** is the gold standard investigation to assess lung recruitability and predict response to prone positioning and other recruitment strategies in ARDS. ## Lung Recruitability and Its Clinical Significance **High-Yield:** ARDS lungs are heterogeneous—some regions are recruitable (collapsed but capable of re-expansion), while others are consolidated and non-recruitable. Patients with high recruitability benefit from: - Prone positioning - Higher PEEP - Recruitment maneuvers Patients with low recruitability may suffer ventilator-induced lung injury (VILI) from excessive PEEP and should be managed conservatively. ## Quantitative CT with Recruitment Maneuver | Feature | Details | | --- | --- | | **Method** | CT imaging at baseline PEEP, then after recruitment maneuver (sustained inflation to 40 cm H₂O × 40 sec), then at higher PEEP | | **Measurement** | Quantifies recruitable lung volume (mL) and recruitability index (%) | | **Cutoff for benefit** | Recruitability >15% predicts favorable response to prone positioning | | **Sensitivity/Specificity** | Gold standard; directly visualizes collapsed vs. recruitable lung | | **Limitation** | Requires transport to CT scanner; not bedside; radiation exposure | **Clinical Pearl:** A **high recruitability index (>15%)** on quantitative CT predicts: - Better oxygenation response to prone positioning - Benefit from higher PEEP - Lower mortality with lung-protective ventilation + recruitment strategies ## Why Quantitative CT Is Superior to Alternatives ```mermaid flowchart TD A[ARDS patient on mechanical ventilation]:::outcome --> B{Assess lung recruitability?}:::decision B -->|Gold standard| C[Quantitative CT + recruitment maneuver]:::action C --> D[Measure recruitable volume & recruitability index]:::action D --> E{Recruitability >15%?}:::decision E -->|Yes| F[Prone positioning likely beneficial]:::action E -->|No| G[Conservative ventilation strategy]:::action B -->|Bedside alternative| H[EIT - real-time monitoring]:::action B -->|Esophageal pressure| I[Assess transpulmonary pressure]:::action ``` **Mnemonic: RECRUIT** — **R**ecruitment maneuver, **E**xamine CT, **C**ompute recruitable volume, **R**isk stratify, **U**se prone if high recruitability, **I**mprove oxygenation, **T**ype of ventilation strategy. [cite:Harrison 21e Ch 295]