In pressure-controlled ventilation (PCV), which parameter is directly controlled by the ventilator while tidal volume becomes a dependent variable?

Explanation

## Pressure-Controlled Ventilation (PCV): Fundamental Principle **Key Point:** In PCV, the ventilator delivers a preset inspiratory pressure for a fixed inspiratory time. Tidal volume is NOT preset — it is a consequence of the set pressure, patient compliance, and airway resistance. ### What the Ventilator Controls 1. **Inspiratory pressure** — the target pressure above PEEP 2. **Inspiratory time (T~i~)** — duration of the pressure delivery phase 3. **PEEP** — positive end-expiratory pressure 4. **Respiratory rate** — number of breaths per minute ### What Becomes Dependent - **Tidal volume (V~t~)** — determined by: $V_t = (P_{insp} - PEEP) \times C_{dyn}$ - Where C~dyn~ = dynamic compliance - Varies with patient lung mechanics ### Clinical Implications | Feature | PCV | VCV | |---------|-----|-----| | **Set parameter** | Inspiratory pressure & time | Tidal volume | | **Variable parameter** | Tidal volume | Inspiratory pressure | | **Advantage** | Reduced peak pressure, better for ARDS | Guaranteed minute ventilation | | **Disadvantage** | V~t~ unpredictable if compliance changes | Risk of barotrauma if compliance ↓ | **High-Yield:** PCV is preferred in ARDS and acute lung injury because it limits peak inspiratory pressure, reducing ventilator-induced lung injury (VILI). However, clinicians must monitor V~t~ continuously — if compliance worsens, V~t~ drops and hypoventilation may occur. **Clinical Pearl:** In PCV, if a patient's compliance decreases (e.g., pulmonary edema develops), the same inspiratory pressure will deliver a smaller tidal volume. This is why minute ventilation monitoring is essential.