A 68-year-old man with COPD is admitted to ICU with acute exacerbation and respiratory failure. He is initiated on mechanical ventilation. After 48 hours, the physician suspects ventilator-associated lung injury (VALI) due to high plateau pressures. Which investigation is most appropriate to confirm the diagnosis and guide further ventilatory management?

Explanation

## Investigation of Choice for VALI Detection **Key Point:** Static compliance (C~static~) and plateau pressure (P~plat~) measurement is the gold standard bedside investigation to detect and quantify ventilator-induced lung injury (VILI) and guide lung-protective ventilation strategies. ### Why Static Compliance & Plateau Pressure? **High-Yield:** Plateau pressure reflects alveolar pressure at end-inspiration and directly correlates with barotrauma risk. Measurement requires: 1. Inspiratory hold maneuver (0.5–1 second pause at end-inspiration) 2. Reading pressure from ventilator display 3. Calculation: C~static~ = $\frac{V_T}{P_{plat} - PEEP}$ **Clinical Pearl:** - P~plat~ > 30 cm H~2~O indicates high VILI risk - Falling static compliance over time suggests worsening lung injury - This is a **real-time, bedside measurement** — no transport or imaging delay ### Comparison with Other Investigations | Investigation | Role | Limitation | |---|---|---| | **Plateau pressure & C~static~** | Direct assessment of alveolar mechanics | Requires sedation/paralysis for accuracy | | Chest X-ray + Horowitz ratio | Assesses oxygenation impairment (ARDS severity) | Does not directly measure lung mechanics or VILI | | ABG alone | Assesses gas exchange | No information on mechanical lung injury | | HRCT chest | Detects structural lung changes | Requires transport; not real-time; not routine in acute VILI | **Mnemonic: PEEP-PLAT** — **P**ositive **E**nd-**E**xpiratory **P**ressure and **P**lateau **P**ressure Assessment Tool — the bedside duo for lung mechanics. ### Clinical Algorithm for VILI Suspicion ```mermaid flowchart TD A[High FiO2 requirement or rising pressures on ventilator]:::outcome --> B{Measure plateau pressure}:::decision B -->|P_plat > 30 cm H2O| C[Reduce TV to 6 mL/kg IBW]:::action B -->|P_plat 25-30| D[Monitor C_static trend]:::action B -->|P_plat < 25| E[Continue current settings]:::action C --> F[Recheck P_plat in 1 hour]:::action D --> G[If C_static falls: reduce TV]:::action E --> H[Reassess if clinical change]:::action ``` **Key Point:** Plateau pressure measurement is **non-invasive, real-time, and actionable** — making it the investigation of choice in suspected VALI.