In pressure-controlled ventilation (PCV), which parameter is directly controlled by the ventilator?

Explanation

## Pressure-Controlled Ventilation (PCV): Core Mechanism **Key Point:** In PCV, the ventilator delivers a preset inspiratory pressure, not a preset volume. The tidal volume delivered depends on lung compliance and airway resistance. ### What the Ventilator Controls The ventilator: - Sets a target **airway pressure** (e.g., 20 cm H₂O) - Delivers gas flow until that pressure is reached - Maintains that pressure for a set inspiratory time - Allows passive exhalation ### Consequence: Variable Tidal Volume Because tidal volume is **not** preset: - If compliance decreases (e.g., ARDS, pulmonary edema) → tidal volume **falls** - If compliance improves (e.g., weaning) → tidal volume **rises** - Clinician must monitor exhaled V~T~ continuously ### Comparison with Volume-Controlled Ventilation (VCV) | Parameter | PCV | VCV | |-----------|-----|-----| | **Controlled variable** | Airway pressure | Tidal volume | | **Tidal volume** | Variable (depends on compliance) | Fixed/preset | | **Peak airway pressure** | Fixed/preset | Variable (depends on compliance) | | **Risk if compliance ↓** | V~T~ ↓ → hypoventilation | P~peak~ ↑ → barotrauma | | **Risk if compliance ↑** | V~T~ ↑ → hyperventilation | P~peak~ ↓ → adequate ventilation | **High-Yield:** PCV is preferred in **ARDS** and **stiff lungs** because it limits peak pressures and reduces ventilator-induced lung injury (VILI), even though V~T~ becomes unpredictable. **Clinical Pearl:** In PCV, always set a **backup respiratory rate** to ensure minimum minute ventilation if the patient becomes apneic or sedated.