Ventilator Modes 

 I. Foundations: How Ventilator Modes Are Classified

Every ventilator breath is defined by:

1️⃣ Trigger – What starts the breath?

  • Patient trigger (flow or pressure)
  • Time trigger (machine initiated)

2️⃣ Limit – What variable is limited during inspiration?

  • Pressure-limited
  • Flow-limited
  • Volume-limited

3️⃣ Cycle – What ends inspiration?

  • Volume-cycled
  • Time-cycled
  • Flow-cycled
  • Pressure-cycled

4️⃣ Control Variable

  • Volume Control
  • Pressure Control


Mode

Trigger

Limit Variable

Cycle Variable

AC-VC

Patient or Time

Flow

Volume

SIMV-VC

Time (mandatory) + Patient (spontaneous)

Flow (mandatory)

Volume (mandatory)

PCV (AC-PC)

Patient or Time

Pressure

Time

SIMV-PC

Time + Patient

Pressure

Time

PSV

Patient only

Pressure

Flow

CPAP

Patient only

Pressure (PEEP)

Patient ends breath

PRVC

Patient or Time

Pressure (auto-adjusted)

Time

APRV

Time + spontaneous allowed

Pressure

Time (release)



 II. Volume-Controlled Modes (VCV)

In volume modes, tidal volume (VT) is fixed, pressure varies depending on compliance and resistance.


1️⃣ Assist-Control Ventilation (AC-VC)/VCV mode 

🔹 What Happens?

  • Set VT, RR, flow
  • Every breath (patient or machine triggered) delivers full preset VT

🔹 Settings

  • VT(How VT is delivered → depends on flow waveform)
  • RR
  • PEEP
  • FiO₂
  • Flow pattern (square/decelerating)

🔹 Advantages

✔ Full ventilatory support
✔ Easy CO₂ control
✔ Good for deep sedation/paralysis

🔹 Disadvantages

 High risk of barotrauma
 Patient-ventilator dyssynchrony
 Auto-PEEP risk


2️⃣ SIMV (Synchronized Intermittent Mandatory Ventilation)

  • A set number of mandatory breaths are delivered
  • These breaths are synchronized with patient effort
  • Mandatory Breath Window—Ventilator waits for scheduled time
  • If patient initiates within synchronization window → breath delivered,If not → machine delivers time-triggered breath
  • Between mandatory breaths → patient can breathe spontaneously
  • Spontaneous breaths may be:
    • Unsupported
    • Or supported with Pressure Support (SIMV + PS)

🔹 Reality (Modern ICU)

⚠ Not preferred for weaning (increases work of breathing)

🔹 Historical Use

  • Gradual weaning (now largely replaced by PSV)



 III. Pressure-Controlled Modes (PCV)

In pressure modes:

  • Pressure is fixed
  • Volume varies depending on compliance


3️⃣ Pressure Control Ventilation (PCV)

🔹 What Is Set?

  • Inspiratory pressure
  • RR
  • I:E ratio
  • PEEP
  • FiO₂

🔹 What Varies?

  • Tidal volume

🔹 Advantages

✔ Lower peak airway pressure
✔ Better alveolar recruitment
✔ Preferred in ARDS (lung protective strategy)

🔹 Risks

 Hypoventilation if compliance drops
 Need continuous VT monitoring



 IV. Spontaneous Modes


4️⃣ Pressure Support Ventilation (PSV)

4

🔹 Key Feature

  • Patient initiates all breaths
  • Set inspiratory pressure support
  • Breath ends when flow drops (flow-cycled)

🔹 Used For

✔ Weaning
✔ SBT (Spontaneous Breathing Trial)
✔ Partial ventilatory support


5️⃣ CPAP (Continuous Positive Airway Pressure)

  • Provides constant PEEP
  • No mandatory breaths
  • Used in:
    • Weaning
    • Cardiogenic pulmonary edema
    • OSA