Question 11.1

In relation to describing the mode of mechanical ventilation, define the following terms:

a) Flow triggered.

b) Pressure limited.

c) Time cycled

(30% marks)

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College answer

a)    Flow Triggered:                                          
•    The signal to start inspiration is a change in flow, i.e. a change in flow results in opening of the inspiratory valve 
b)    Pressure Limited: 
•    Pressure is factor that limits the way gas flows into the lung during inspiration. The pressure within the lung cannot exceed the set limit.  
c)    Time Cycled: 
•    Time is the signal that stops inspiration i.e. the inspiratory valve closes and the expiratory valve opens OR Inspiration switches to expiration once the set inspiratory time elapses. 


Triggering, limits and cycling are what is called "phase variables" of mechanical ventilation. "Egan's Fundamentals of Respiratory Care" is what I used for these definitions, it being the bible for respiratory therapists in America. Locally we have no such bible, but Oh's Manual contains within it the definitive "Mechanical Ventilation" chapter by Andrew Bersten which does not define these variables.

Anyway: Egan's defines these variables quite well around pages 1012-1014 of the 11th edition.  The definitions offered there are similar to, and possibly non-inferior to, the canonic college definitions.

In short:

Flow triggering is where the difference in flow between the expiratory and inspiratory limb of the ventilator is used as the trigger variable to initiate a mechanical breath

Pressure limit is the pressure at which the ventilator opens the expiratory valve regardless of which phase of ventilation is occuring at the time. Pressure limited ventilation 

Time cycling means the expiratory flow valve is opened after a preset time interval has elapsed, and the inspiratory valve is closed.


Travers, Colm P., et al. "Classification of Mechanical Ventilation Devices." Manual of Neonatal Respiratory Care. Springer International Publishing, 2017. 95-101.

Heuer, Albert J., James K. Stoller, and Robert M. Kacmarek. "Egan's Fundamentals of Respiratory Care." (2016).