Pressure Assist-Control (PAC) and Pressure Control (PC) Are Essentially the Same Mode
Pressure assist-control ventilation and pressure control ventilation refer to the same fundamental mode of mechanical ventilation—they are simply different names for the same thing. The terminology varies by manufacturer and region, but the operational characteristics are identical 1.
Understanding the Terminology
Both PAC and PC are pressure-targeted modes where:
- The clinician sets a fixed inspiratory pressure level
- The ventilator delivers breaths at a predetermined rate (the "control" component)
- The patient can trigger additional breaths above this set rate (the "assist" component)
- Tidal volume varies based on lung compliance and resistance, rather than being fixed 1
The 2023 American College of Chest Physicians guideline explicitly lists "PAC = pressure assisted control; PC = pressure" as equivalent terms in their abbreviation key 2, confirming these are interchangeable designations for the same mode.
Key Distinction from Pressure Support
The critical difference is not between PAC and PC (which are the same), but rather between these modes and pressure support ventilation (PSV):
- PC/PAC: Uses a preset mechanical inspiratory time (Ti) to cycle from inspiration to expiration 1, 3
- PSV: Cycles to expiration when inspiratory flow decreases to a predetermined threshold (typically 25% of peak flow), giving the patient control over inspiratory duration 1, 3
Research directly comparing these modes found that "in a given ventilator little difference exists in gas delivery and response variables between PS and P A/C" except for the expiratory transition mechanism 3.
Clinical Implications
When selecting between these modes:
- Use PC/PAC when you need guaranteed minute ventilation with a backup rate, particularly in patients with unreliable respiratory drive
- Use PSV when the patient has adequate respiratory drive and you want to allow more patient control over breathing pattern
- Both modes compensate well for air leaks (important for non-invasive ventilation) and provide constant pressure delivery 1
Common Pitfall
Don't be confused by manufacturer-specific terminology—some ventilators label this mode as "pressure control," others as "pressure assist-control," and still others as "timed mode" or "control mode" 1. They all describe the same pressure-targeted, time-cycled mode with a backup rate.