A/C (assist/control): Set rate, set tidal volume or pressure (except with PRVC breath type). When the patient triggers a breath, they get the full set tidal volume breath (or pressure) with each breath that they take. This can lead to air trapping and high pressures. When the vent delivers a patient-triggered breath, the next breath is timed in relation to this.
SIMV (synchronized intermittent mandatory ventilation): Set rate, set tidal volume, set pressure support. If the patient is not making any spontaneous effort, this mode functions the same as A/C. The first time in a breath cycle when patient triggers a breath, the patient receives a full assist breath. For each breath after that until the next breath cycle, the volume is whatever patient draws themselves along with pressure support. The patient still receives a set minimum number of breaths (the set rate) and any other breaths are determined by the patient. SIMV mode helps to prevent breath stacking and Auto PEEP. Pressure support should be set 5-10 cmH2O above PEEP (10 is a good starting point).
CPAP+PS: Delivers constant PEEP (like a CPAP mask) but will also provide pressure support breaths for spontaneous breaths taken by the patient. This mode will default to giving mandatory breaths based on apnea backup settings.
Breath terminology explained
Mandatory: Breaths are initiated by the ventilator and the ventilator performs the work of inspiration during those breaths.
Assisted: Breaths are initiated by the patient, but the ventilator performs at least some of the work of inspiration for those patient initiated breaths.
Spontaneous: Breaths are initiated by the patient and the patient performs the entire work of inspiration for those patient initiated breaths.