Study Notes – Basic Ventilator Modes

Ventilators used today are positive pressure ventilators.

• Modes fall into two main categories:

  1. Volume Control (VC) → set volume is constant, pressure varies

  2. Pressure Control (PC) → set pressure is constant, volume varies

Key concept:

• VC → Volume fixed, Pressure variable

• PC → Pressure fixed, Volume variable

This distinction is critical when evaluating patient-ventilator interactions.

Key Ventilator Settings

1. Tidal Volume (VT)

• Volume of air delivered with each breath.

• Usually based on ideal body weight (6–8 mL/kg IBW for lung protection).

2. Frequency / Respiratory Rate (RR or f)

• Breaths delivered per minute.

• Determines spacing of breaths (60 ÷ RR = seconds between breaths).

3. Fraction of Inspired Oxygen (FiO₂)

• Expressed as a decimal fraction (e.g., 0.5 = 50%).

• Range: 0.35–1.0 (never below 0.35 on a vent).

4. Positive End-Expiratory Pressure (PEEP)

• Pressure maintained at end expiration.

• Prevents alveolar collapse, improves oxygenation.

• Normal physiologic PEEP = 5 cmH₂O.

• Range: 5–20 cmH₂O.

5. Pressure Support (PS)

• Extra inspiratory pressure given to assist spontaneous breaths.

• Analogy: a “spotter” at the gym.

• Typical: 8–10 cmH₂O to overcome ET tube resistance.

• Range: 5–20 cmH₂O.

Key Monitoring Parameters

1. Minute Ventilation (VE)

   • Formula: VE = VT × RR.

   • Normal: 5–10 L/min.

   • ↑VE → ↑CO₂ clearance (↓PaCO₂).

   • ↓VE → ↓CO₂ clearance (↑PaCO₂).

2. Peak Inspiratory Pressure (PIP)

   • Max pressure reached during inspiration.

   • Goal: < 35 cmH₂O to prevent barotrauma.

3. Plateau Pressure (Pplat)

   • Measured at end-inspiration with pause maneuver.

   • Reflects lung compliance.

   • Goal: < 30 cmH₂O.

   • ↑Pplat = ↓compliance (e.g., ARDS, fibrosis).

Ventilator Modes (Volume Control Focus)

1. Assist-Control (AC or VC)

• Settings: VT, RR, FiO₂, PEEP.

• No PS (vent delivers full set breath if triggered).

• Vent gives:

  • Mandatory breaths (at set rate/VT).

  • Any spontaneous breath → still receives full set VT.

Pros: Full control, useful post-intubation.
Cons: Risk of respiratory alkalosis if tachypneic (e.g., anxiety, pain, CNS causes).

2. Synchronized Intermittent Mandatory Ventilation (SIMV)

• Settings: VT, RR, FiO₂, PEEP, PS.

• Vent ensures a minimum set number of breaths at set VT.

• Spontaneous breaths: patient determines VT, but assisted with PS.

Pros:

• Supports spontaneous breathing.

• Used for weaning from vent.

Cons:

• If patient effort is poor → hypoventilation, CO₂ retention → respiratory acidosis.

• If excessive effort → tachypnea, hyperventilation → respiratory alkalosis.

Key Point:

• If patient paralyzed (no spontaneous effort), SIMV = AC (no difference).

3. Pressure Support Ventilation (PSV)

• Purely spontaneous mode (not strictly volume control).

• Settings: FiO₂, PEEP, PS.

• No set RR or VT → patient effort + PS determines volume.

• Safety: backup rate if apnea occurs.

Uses:

• Final step before extubation.

• Spontaneous Breathing Trial (SBT): often run at 10/5 (PS/PEEP) or 5/5.

Support Continuum (Work of Breathing)

• Most supportive → Least supportive:

  • AC (vent does all work, full control)

  • SIMV (shared work: vent + patient)

  • PSV (patient does most work, vent assists only)