Phase Variables: Trigger, Control and Cycling Variables

Phase variables are the variables that control the phases of the breathing cycle of a patient under a mechanical ventilator.

Types of Phase Variables

1) Trigger Variables

Trigger variables define how the ventilator determines when to indicate the machine-driven breath. In other words, they control the initiation of inspiration.

  • Time Triggered:- It is a mandatory mode. It triggers when a certain set time has elapsed.
  • Pressure Triggered:-┬áIt is a spontaneous mode. It triggers when the value of pressure gets lower than the set threshold value.
  • Flow Triggered:- It is a spontaneous mode. It triggers when the value of flowrate gets lower than the set threshold value.
  • Electrical Impedance of the Chest Wall:- Spontaneous mode
  • Volume Triggered:- Spontaneous mode
  • Manual:- In control of the operator

2) Control Variables

Control variables define what aspect of inspiration is the primary variable in control of the ventilator during inspiration.

  • Flow (volume) Controlled:- If you set the flow or volume to be constant, the variation in the resistance of the respiratory tract or the lung compliance will alter the airway pressure.
  • Pressure Controlled:- If you fix the airway pressure, there is a possibility of variation in the airflow or tidal volume.
  • Time Controlled:- There can be variation in volume, flow, and pressure.

3) Cycling Variables

Cycling variables define what signals the ventilator to stop the inspiration. In other words, controls the initiation of expiration.

  • Volume Cycled:- After the preset volume of air has been supplied.
  • Flow Cycled:- After airflow drops below the preset threshold value.
  • Time Cycled:- (mandatory mode) After the preset time has been reached.


Variables Common Options General Settings
Trigger Time



To deliver RR of 14 to 20

2-3 Liters/min

0.5 to 3 cm H2O

Control Flow (volume)



By setting RR, Vt, and I: E value

To provide Vt of 6-8 ml/kg

Cycling Flow



Less than 25% of peak flow

To provide Vt of 6-10 ml/kg

To provide RR of 14-20

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