Phase Variables: Triggering, Limits, Cycling and PEEP

The phase variables control the different phases of a mechanical breath.

INITIATION AND TRIGGERING: who decides when to take a breath?

When the trigger variable is met, the ventilator cycles to inspiration. Multiple trigger variable can be active at the same time.

For example, SIMV is a mode which is time triggered as well as flow-triggered.

  • TIME-TRIGGERED: “MANDATORY”, Machine decides when you breathe. The timer triggers.
  • FLOW-TRIGGERED: Patient triggers  by sucking a small amount of flow away from the circuit
  • PRESSURE-TRIGGERED: Patient triggers by generating a small amount of negative pressure
  • VOLUME-TRIGGERED:  Patient triggers by inhaling a certain set volume of gas

 LIMIT:  what decides when inspiration stops?

A “limit” is reached before the end of inspiration.  It doesn’t necessarily end the inspiration; it just restricts that variable so that it doesn’t increase beyond that  limit. All of the limits can be set all at once.

  • FLOW-LIMITED: There is a set flow, and the ventilator will not allow any greater rate of flow during inspiration
  • PRESSURE-LIMITED: the ventilator will not exceed the set  pressure during inspiration
  • VOLUME-LIMITED:   the ventilator will not exceed the set volume during inspiration  (so if the volume is reached before the inspiration is over, there is an inspiratory pause)

CYCLING: what decides when to switch from inspiration to expiration?

When the cycling variable is met, it causes the ventilator to open its expiration valve, and the patient exhales. There  can be only one cycling variable.

  • TIME-CYCLED: the clock decides. The inspiratory pause is timed.
  • FLOW-CYCLED: once the inspiratory flow rate falls to a certain set rate, the ventilator allows you to exhale.
  • PRESSURE-CYCLED: once the set peak inspiratory pressure is reached, the ventilator allows you to exhale.
  • VOLUME-CYCLED: once you reach the target volume, the ventilator allows you to exhale.

PEEP is the fourth variable

PEEP has been discussed elsewhere; it is the last variable: the pressure at end-expiration.


Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.