Cycling from Inspiration to Expiration

Cycling variables

A "cycling" variable determines when the ventilator "cycles", which just means "changes over from inspiration to expiration". When this variable is reached, the expiration is triggered.

Time-cycled: its the machine's choice

time cycled

The machine has a set respiratory cycle time, and a set respiratory rate.

The ventilation pattern is set: for example, 1 second for inspiration, 1 second for inspiratory pause, 3 seconds for expiration.

Once the timer counts down to 1 second, the machine cycles to inspiratory pause, and then to expiration.

The decision to start expiration depends on what time it is, not on what the patient is doing, or what the other variables are. The patient is not involved in the triggering process, does not control their respiratory rate, and does not get to decide when to stop holding their breath.

Flow cycled

flow cycled

Once the flow rate reaches a certain minimum, the machine cycles to expiration or inspiratory pause.

Here it is demonstrated in Pressure Support Ventilation, which is where you would normally find this sort of cycling.

The flow rate starts high to reach the designated pressure limit; it then slows to maintain a steady regulated pressure.

If the flow rate slows down to the cycling threshold, the ventilator assumes that the patient is finished with that breath, and it cycles to exhalation.

Obviously, in lungs with poor compliance, the cycling threshold will be reached more quickly and the exhalation starts early, resulting in a smaller tidal volume.

Pressure cycled

pressure cycled

Once the flow rate reaches a certain minimum, the machine cycles to expiration or inspiratory pause.

The tidal volume depends on how long it takes to reach that threshold.

This givers you good volumes in nice young compliant lungs, and useless little volumes in stiff non-compliant lungs.

Most ventilators will have a "backup cycling limit" for pressure – to prevent overinflating the patient. They will alarm if the backup limit is reached, and the ventilator will cycle to expiration. This typically happens at 40cmH2O

Volume cycled

volume cycled

Once a set volume has been achieved, the machine cycles to expiration or inspiratory pause.

While flow-cycling is used in spontaneous modes, volume-cycling is used more in mandatory modes.


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.