(Synchronized) Intermittent Mandatory Ventilation




Intermittent mandatory ventilation was developed as a method of partial ventilatory support to facilitate liberation from mechanical ventilation. A demand valve was placed in the breathing system, through which the patient could take a spontaneous breath (the gas for this breath is derived from a reservoir bag), without having to breathe through the various valves and apparatus of the ventilator. The patient could breathe spontaneously while also receiving mandatory breaths. As the patient’s respiratory function improved, the number of spontaneous breaths was decreased, until the patient was breathing unassisted on CPAP.

Unfortunately, there were two problems with this system: 1) it was possible for the patient and the ventilator to inspire in series, thus “stacking” one breath on top of another, leading to high airway pressures. 2) the workload of  spontaneous breaths remained quite high – remember that the patient still has to inspire without assistance through an endotracheal tube and open a demand valve – a difficult prospect with normal lungs, a serious burden with an acute lung injury.

Similar to the anesthesia ventilator, the patient can receive either a mandatory breath from the ventilator or take a spontaneous breath from the reservoir bag.

In SIMV if the patient takes a spontaneous breath within the control breath window, the two are synchronized.



The first problem was solved with the development of micro-processor technology: the ventilator was fitted with a sensor that synchronized the patient’s spontaneous breaths (up to the mandatory rate) in a manner similar to assist-control. The problem of the excessive effort of the spontaneous efforts was solved by introducing an assisted spontaneous breathing mode – “pressure support” ventilation.


SIMV plus Pressure Support plus CPAP

Note the two different types of breath: the characteristic flat topped pressure limited breath (center) delivers a similar volume to the volume breaths, but in a shorter time, as determined by the patient.

In pressure support, the patient triggers the ventilator and a pressure-limited breath is delivered: the patient determines the rate, the duration of inspiration and the tidal volume. The physician can determine how much work the ventilator can take from the patient, by altering the pressure limit.

SO When SIMV  is used, the patient receives three different types of breath:

1. The controlled (Mandatory) breath.

2. Assisted (synchronized) breaths.

3. Spontaneous breaths, which can be pressure supported.





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