Ventilator Classification & Modes of Ventilation

Objectives — Identify the design features and classification of mechanical ventilators and apply classification to modes of ventilation.

---

The Equation of Motion

All ventilator function is governed by Newton's equation of motion:

Pressure = Raw × Flow + Volume / Compliance

• Raw = airway resistance
• Flow = gas flow rate
• Volume = tidal volume delivered
• Compliance = lung and chest wall distensibility

Every ventilator setting and every monitored value relates back to this equation.

---

Classification of Ventilators

Ventilators are classified based on five components.

1. Input Power

2. Power Conversion & Transmission

How the ventilator converts input power into pressurized gas flow delivered to the patient.

Drive Mechanisms:

• Bellows
• Piston
• Turbine

Output Control Valve (Demand Valve / Inspiratory Valve):

• Controls gas delivery to the patient
• Can "shape" the flow waveform (square, sine, descending ramp)

3. Control Circuit

The system that governs ventilator operation:

• Mechanical
• Pneumatic
• Fluidic
• Electric
• Electronic (most modern ICU vents)

4. Control Variables

The primary variable the ventilator controls during inspiration. Only one variable can be controlled at a time.

Pressure Control

• Inspiration ends when preset pressure is reached
• Pressure is constant; volume is variable
• Volume decreases with decreased compliance or increased resistance
• Volume increases with increased compliance or decreased resistance

Volume Control

• Inspiration ends when preset volume is reached
• Volume is constant; pressure is variable
• Pressure increases with decreased compliance or increased resistance
• Pressure decreases with increased compliance or decreased resistance

5. Phase Variables

Phase variables define what happens at each phase of the respiratory cycle.

Trigger

• Time trigger: Breath begins based on set respiratory rate (e.g., RR = 10 = one breath every 6 sec)
• Pressure trigger: Ventilator senses a drop in baseline pressure (patient effort) — measured in cmH₂O
• Flow trigger: Ventilator senses a drop in bias flow — measured in L/min; requires less patient effort than pressure trigger

Target

The variable controlled during inspiration — also called a "limit" if an alarm condition is met.

Cycle

The variable that causes inspiration to end:

• Volume (most common)
• Pressure (2nd most common)
• Flow (PSV — flow cycled at 25% of peak flow)
• Time (neonatal ventilation)

Baseline

The period from the start of exhalation to the start of the next breath. Pressure is the most common baseline variable — set as PEEP.

---

Breath Types

The combination of phase variables defines the breath type.

Spontaneous unsupported breaths show a negative deflection on inspiration and positive on expiration on the pressure-time scalar — the opposite of mandatory breaths.

---

Modes of Ventilation

A mode is the method of inspiratory support and breath delivery. Modes combine breath types and timing algorithms to distribute WOB between the patient and the ventilator.

CMV — Continuous Mandatory Ventilation (A/C)

Also called Assist/Control (A/C).

• Trigger: Patient or time (whichever comes first)
• Target: Pressure or Volume
• Cycle: Ventilator
• All breaths are mandatory or assisted — every breath is ventilator-cycled
• The initial "rest" mode — ventilator assumes all or most WOB
• Available as Volume A/C or Pressure A/C

SIMV — Synchronized Intermittent Mandatory Ventilation

• Trigger: Patient or ventilator
• Target: Volume, Pressure, Time, or Flow
• Cycle: Volume, Pressure, Time, or Flow
• Breath types: Controlled, assisted, and spontaneous
• Demand valves sense patient effort; timing algorithm synchronizes mandatory breaths with patient effort
• Used for both resting and weaning
• SIMV + PS: Spontaneous breaths are augmented with pressure support
  • Overcomes RAW from the ET tube
  • Improves patient-ventilator synchrony
  • Prevents respiratory muscle atrophy

CSV — Continuous Spontaneous Ventilation (CPAP)

Also called CPAP in the ventilator context.

• Spontaneous breathing at or above atmospheric pressure
• Trigger: Patient
• Limit: Pressure alarm
• Cycle: Patient
• Used to evaluate patients for weaning and ventilator discontinuation

PSV — Pressure Support Ventilation (CPAP + PS)

• Trigger: Patient
• Target: Pressure (above baseline)
• Cycle: Flow — inspiration ends when inspiratory flow decelerates to 25% of peak flow
• Reduces WOB, increases spontaneous Vt, improves synchrony
• Used as a weaning mode or combined with SIMV

APRV — Airway Pressure Release Ventilation

Also known as: BiLevel, BiVent, BiPhasic, PCV+, DuoPAP.

• Time-cycled, pressure-controlled (dual)
• Patient can breathe spontaneously throughout the cycle
• Uses two pressure levels:
  • P-High: elevated baseline pressure (like high CPAP)
  • P-Low: brief pressure release to allow exhalation
• Inverse ratio: Time-High is much longer than Time-Low (e.g., 4–5 sec : 0.4–0.8 sec)
• Used to treat refractory hypoxemia (ARDS)

---

Additional Modes (Overview)

These advanced modes are covered in Phase II but worth recognizing by name:

• PRVC — Pressure-Regulated Volume Control
• PCIRV — Pressure-Controlled Inverse Ratio Ventilation
• NAVA — Neurally Adjusted Ventilatory Assist
• PAV — Proportional Assist Ventilation
• ASV — Adaptive Support Ventilation
• HFOV — High-Frequency Oscillatory Ventilation
• HFV-A / HFV-P — High-Frequency Ventilation variants
• Volume Guarantee