AC vent triggered breath1_opt


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.


SIMV vent triggered breath_opt

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2 Responses to AC versus SIMV mode.

  1. Dave Cook says:

    I work pre hospital and my system uses the IMPACT vent. We have the ability to utilize CMV, A/C and non invasive modes. Our controls are volume or pressure and assist with PEEP and pressure support. We have very open protocols that allow you to provide what the pt needs based on critical thinking rather than a strict protocol that says “you must do this and use these settings”. I have heard from different sources that SIMV should not be utilized in the pre hospital setting for someone experiencing an acute problem, whether trauma or medical. What is never made clear is if using pressure support in SIMV negates the problem of lower volumes and making the pt work harder to breathe. If you have any insight or clarification this would be much appreciated. A quick example is a pt who is unconscious, post cardiac arrest with a history of CHF. The pt is breathing on their own at a rate of 26. The question posed is, is it better to avoid breath stacking by using SIMV with PS, or to increase sedation and utilize A/C. Meds for post intubation management are Ketamine, Fentanyl and Versed. Ketamine is to be avoided in pt’s with CHF, Fentanyl is a good drug that I have yet to see cause any hemodynamic problems, and everyone knows Versed isn’t the greatest drug. The last alternative would be to provide sedation along with paralyzing the pt, however we try to avoid post intubation paralytics if at all possible. Sorry for the long question there. Thanks for your time.

    • Gavin says:

      1. A.C is very much a US thing based on my reading.
      2. Pressure or volume control SIMV is a fairly standard UK ITU mode. If the patient isn’t breathing the mode is largely irrelevant, use what you understand.
      3. If the patient is breathing spontaneously, it depends on their FIO2/WOB/haemodynamics stability as to what is appropriate.
      – if haemodynamics unstable, deepening sedation to control ventilation might worsen haemodynamics.
      – if Fio2 and work of breathing stable, let them breath.
      -also depends on etco2, as if post ROSC you’ll want to target the Co2

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