Expiration occurs passively because of the elastic recoil of the lung.
The only feature controlled in expiration is the baseline pressure or positive end expiratory pressure (PEEP).
I:E is the ratio of the set inspiratory time (Ti) to the expiratory time (TE).
o Not truly a controlled feature of expiration since it is the Ti that is set
§ Ti is set on the ventilator in seconds
§ The rest of the breath is therefore expiration
§ Example 10 breaths per minute gives 6 seconds per breath
§ Ti set at 1.5s
§ Expiratory time is 4.5s
§ I : E = 1 : 3
o This can be altered for specific conditions for example
§ Longer Ti is used in hypoxia
§ Longer TE is used in obstructive lung disease to prevent gas trapping