⚙️ Chapter 25: Mechanical Ventilation — Modes of Ventilation

Control variables (pressure, volume, flow) · Phase variables (trigger, limit, cycle) · Pressure Control (PC) · Volume Control (VC) · SIMV · PRVC · Pressure Support (PS) · CPAP · Automode · ATC · Flow patterns · Clinical applications

🔍 Core Concepts: Modes of Mechanical Ventilation

📊 Control Variables
Pressure, Volume, Flow. Determine how gas is delivered. Pressure control: decelerating flow, limits barotrauma. Volume control: guaranteed tidal volume, constant flow.
⚙️ Phase Variables
Trigger (time, pressure, flow) → initiates breath. Limit (pressure/volume) → controls breath. Cycle (time, flow, volume) → switches to expiration.
🫁 Controlled Modes
Pressure Control (PC): pressure-limited, time-cycled, decelerating flow. Volume Control (VC): volume-limited, time-cycled, constant flow. SIMV: synchronised mandatory breaths + spontaneous.
🔄 Hybrid/Dual Modes
PRVC: pressure-regulated volume control — delivers set volume at lowest possible pressure. VAPS: volume-assured pressure support — switches from pressure to volume within a breath.
💨 Support Modes
Pressure Support (PS): pressure-targeted, flow-cycled, patient-triggered. CPAP: continuous positive airway pressure, spontaneous breathing. Volume Support: smart PS targeting volume.
📈 Flow Patterns
Constant (square) — volume control. Decelerating — pressure control (safer, better distribution). Sinusoidal — spontaneous breathing.

🩺 Stepwise Approach: Choosing & Using Ventilator Modes

1
Assess underlying pathology & physiology
Determine time constant (TC = compliance × resistance). Restrictive (ARDS) → short TC, need faster rate, decelerating flow. Obstructive (asthma) → long TC, need slow rate, long expiratory time.
2
Initial mode selection: controlled vs. assisted
Paralysed or no respiratory effort: pressure control (PC) or volume control (VC). Spontaneous breathing: SIMV + PS or PRVC. ARDS: low tidal volume (6 mL/kg) in VC or PC, monitor plateau pressure.
3
Pressure Control (PC) indications
ARDS, heterogeneous lung disease, risk of barotrauma. Decelerating flow improves distribution. Monitor tidal volume (may vary).
4
Volume Control (VC) indications
Guaranteed minute ventilation (asthma, neuromuscular disease). Measure plateau pressure. Risk: high pressure if compliance drops.
5
Weaning modes
SIMV + Pressure Support (PS) or PRVC. PS reduces work of breathing, flow-cycled. CPAP trial for extubation readiness. Automode can transition automatically.
6
Troubleshooting & optimisation
Use ventilator graphics (flow, pressure, volume curves). Adjust trigger sensitivity (avoid auto-triggering or missed efforts). Set appropriate PEEP, I:E ratio, and flow pattern.