Intermittent mandatory ventilation (IMV) was an earlier version of the more advanced SIMV. In this mode of ventilation, a preset respiratory rate is delivered at a specified time interval. For a patient receiving 10 breaths per minute, a breath is delivered every six seconds regardless of the patient's efforts. The theoretical disadvantage of this form of ventilation is that the patient may take a spontaneous breath and could receive a machine delivered breath at the same time or during expiration, causing hyperinflation and high peak airway pressures. SIMV is said to avoid this problem by monitoring the patient's respiratory efforts and delivering breaths in response to the patient's inspiratory efforts. The patient can breathe spontaneously in between the mandatory breaths and those breaths can be pressure supported.
SIMV is similar to IMV in that it will still deliver a minimum number of breaths, despite the potential lack of inspiratory effort from the patient. If the ventilator is set to deliver 10 bpm the patient will receive these breaths whether he is breathing or not. SIMV utilizes a window of time in which the circuit is open for the patient and can breathe spontaneously. During this window, any spontaneous breath can be supported with pressure support (triggered window for supported breaths). In addition, SIMV utilizes another window in which a mandatory breath is due and will look to deliver this breath within a specified time frame, if the patient makes a sufficient inspiratory effort (governed by sensitivity) the machine will sense this effort and give the patient the breath during this time, synchronized to his own effort (triggered window for synchronized breaths)