Synchronized Intermittend Mandatoryl Ventilation (SIMV) describes a method of providing mechanical breaths to a patient. An "SIMV of 12" means that the patient is getting 12 guaranteed (mandatory) breaths per minute from the ventilator. In the SIMV mode, the patient is allowed to take additional breaths in between the mechanical breaths. The patient's own breaths are called "spontaneous breaths". The size of these breaths may be large or small, depending upon the patient's abiltiy.

The ventilator detects the patients spontaneous breathing, and waits until the patient exhales before delivering another mechanical breath. This "synchronizes" the ventilator to the spontaneous breathing.

A patient on an SIMV of 12 with a total breathing rate of 20 breaths per minute is receiving 12 breaths from the ventilator and taking 8 spontaneous breaths.