SIMV (synchronized intermittent mandatory ventilation)
With SIMV, we set the rate, set either volume or pressure, and add in pressure support (although this is not required). If the patient is not making any spontaneous effort, this mode functions the same as A/C.
To understand SIMV, we first have to review breath cycles. A breath cycle is simply the time period of one inhalation and exhalation. We can calculate it by dividing 60 by the rate. So if we have a set rate of 12, our breath cycle will be 5 seconds. If we have a rate of 20, our breath cycle will be 3 seconds.
The first time in a breath cycle when patient triggers a breath, the patient receives a full assist breath (remember that an assist breath is the full tidal volume or pressure that we have set). For each breath after that, until the next breath cycle, the volume is whatever patient draws themselves along with the set pressure support. The pressure support is used to help overcome the dead air space in the ET tube and the vent circuit. Pressure support should be set 5-10 cmH2O above the set PEEP (10 is a good starting point).
If the patient takes a breath in the current breath cycle, the vent will not give a control breath in the next cycle unless the apnea backup limit is reached. So it is possible that the patient will go an entire breath cycle without receiving any breath. But once that next breath cycle starts, a machine breath will be given.
Also note that unlike A/C mode, the breath period does not reset. After a patient triggered breath in A/C mode, the next breath is timed in relation to that last breath and the breath cycle starts over. In SIMV mode, the breath cycle stays constant unless a manual breath is given.
With SIMV, the patient still receives a set minimum number of breaths (the set rate) and any other breaths are determined by the patient. Compared to A/C mode, SIMV mode helps to prevent breath stacking and Auto PEEP.
