A/C (Assist/Control)
With A/C mode, we set the rate and either volume or pressure. When the patient is sedated and makes no respiratory effort, they will only get the control breaths at the set rate.
When the patient triggers a breath, they receive an assist breath. The assist breath will be whatever tidal volume or pressure we have set. So if the patient triggers a breath when we have the vent set to a volume breath type at 500 mL, the vent will give 500 mL when the patient triggers the vent.
The vent will then time the next machine breath in relation to that assist breath. So if we have a rate of 12 and the patient just triggered a breath, the next machine breath will be in 5 seconds, unless they trigger another breath. This means that the vent will not stack control breaths, but the patient can start breath stacking if they keep on triggering the vent.
Note that for the patient to trigger a breath, the vent must be in exhalation and it must have been in exhalation for the minimum exhalation time of 0.346 seconds. The patient cannot trigger a breath during this time.
When using A/C mode and the patient is not fully sedated, pay attention to how fast the patient is triggering breaths, if at all. Patients triggering too many breaths can lead to breath stacking, Auto-PEEP, increased discomfort, poor gas exchange, and hypotension due to decreased venous return. It may be ideal to have these patients fully sedated if using A/C mode.
