The first breath in this cycle is one initiated by the ventilator. The second breath is the patient triggered breath. There is a slight dip at the beginning of the breath, which is the patient creating a negative pressure. The ventilator senses this and delivers another breath. The user can set what the trigger is, in litres per minute of flow for example, to make it easier or harder for the patient to trigger the breath.
The ventilator will then deliver that breath to the set volume or pressure depending on the parameters set by the user i.e. is it pressure controlled or volume controlled ventilation.
So the ventilator assists the patient by controlling the amount of volume the patient receives.
If the patient is weak, they will always receive a full volume/pressure breath, even when they initiate it themselves.
Because the patient will always receive the same pressure/volume they may be at risk of hyperventilation if they do not require such big breaths.
Just as in AC mode, if the patient does not trigger a breath, the patient will receive a set volume/pressure breath, as in the first breath here.
However in SIMV when a triggered breath is initiated the patient determines the volume, which may be smaller than the non triggered breath.
So if the patient wants a lower volume during their spontaneous efforts they will receive a lower volume.
If the patient is taking good volumes during their spontaneous breaths, this may indicate that weaning might be possible.
If the spontaneous breaths are too small, then pressure support can be added to each breath to help the patient.