SIMV stands for synchronized intermittent mandatory ventilation. A SIMV ventilator is designed for patients who are able to do some, but not enough, breathing on their own. Mechanical ventilation ensures that a respiratory-compromised patient will inhale and exhale by supplying the oxygen and providing for the exhalation of carbon dioxide. A SIMV ventilator is used when a patient makes some spontaneous respiratory effort independently, such as when he has recovered sufficiently for doctors to consider removing ventilation at some point.

SIMV ventilator mode is used to support a patient who is taking some breaths alone. This is done by setting the vent to give a certain amount of pressure along with the spontaneous breath. This mode also prevents too many breaths — the patient's natural breaths and the ventilator's mechanical ones — from being stacked back to back and leaving the patient in more distress because of air pressure building up in the lungs. The pressure amount ordered with SIMV ventilators allows each breath the patient takes to be a full and adequate breath.

SIMV mode is most often successful for those clients who are somewhat awake, are taking at least minimal breaths per minute independently, and for those patients who are preparing to be extubated and weaned from the ventilator all together in the near future. SIMV is the most common form of ventilation and the least damaging to lung tissue, but it also requires the patient to do most of the work. This mode of ventilation is tolerated the best in most instances. It does, however, increase the patient's breathing workload and can increase respiratory muscle fatigue.

The choice of which mode of ventilation to use is determined by the condition of the patient, the illness or injury, and the preference of and orders from the prescribing physician. SIMV ventilator mode is the one most often used in children. Adults tolerate SIMV unless heavily sedated or the injury to the lungs does not allow for spontaneous respirations. It is the physician who makes the determination about what mode of ventilation to use based on clinical assessments and blood gas analysis. Patients may tire on SIMV and need another mode for a day or two before being placed back on SIMV before being weaned from ventilation all together.