## Distinguishing Feature: Tidal Volume Guarantee **Key Point:** The fundamental discriminator between VCV and PCV is the PRIMARY CONTROLLED VARIABLE. ### Volume-Controlled Ventilation (VCV) - **Preset variable:** Tidal volume (V~T~) is set and guaranteed - **Consequence:** Peak inspiratory pressure (PIP) varies with lung compliance and airway resistance - **Flow pattern:** Square wave (constant flow rate) - **Advantage:** Consistent minute ventilation regardless of compliance changes ### Pressure-Controlled Ventilation (PCV) - **Preset variable:** Inspiratory pressure is set and guaranteed - **Consequence:** Tidal volume varies with lung compliance and airway resistance - **Flow pattern:** Decelerating (exponential decay) - **Advantage:** Lower peak pressures, reduced barotrauma risk ### Comparison Table | Feature | VCV | PCV | | --- | --- | --- | | **Controlled Variable** | Tidal Volume (fixed) | Inspiratory Pressure (fixed) | | **Peak Inspiratory Pressure** | Varies | Fixed | | **Tidal Volume** | Fixed | Varies | | **Flow Pattern** | Constant (square wave) | Decelerating (exponential) | | **Minute Ventilation** | Guaranteed | Depends on compliance | | **Barotrauma Risk** | Higher (if PIP rises) | Lower | | **Ideal For** | Normal lungs, predictable mechanics | ARDS, low compliance, high airway resistance | **Clinical Pearl:** In VCV, if a patient develops decreased lung compliance (e.g., pulmonary edema, ARDS), the ventilator will maintain the set V~T~, but PIP will RISE — potentially causing barotrauma. In PCV, the same compliance change causes V~T~ to FALL, protecting against high pressures but risking hypoventilation. **High-Yield:** The exam question "What changes when compliance decreases?" is the classic discriminator: - VCV → PIP rises (V~T~ protected) - PCV → V~T~ falls (PIP protected) [cite:Gupta & Sharma Critical Care Medicine, Ventilation Modes Ch 8]
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