triggers, targets, cycles of ventilators
- related: ICU intensive care unit, basic understanding of ventilators
- tags: #permanent
- source: design features of modern mechanical ventilators, Murray and Nadel Ch 135 Mechanical Ventilation
Trigger
A trigger is what initiates each breath. Assisted or supported breaths are patient triggered. Controlled breaths are machine triggered. For assisted or supported breaths, the trigger is either inspiratory flow or negative pressure. For controlled breaths, the trigger is time.
Target
The target of the breath is the independent variable that you set. This could be either pressure or flow. For pressure targeted breaths (pressure control or support), you set the pressure (e.g. pressure support), and the machine will change the flow (tidal volume) to achieve that pressure. For flow targeted breaths, you set the volume (and thus flow because flow is volume in time, e.g. tidal volume 300 ml with 16 breaths per min), and the machine will change the pressure to achieve that volume.
Cycle
This determines when the breath is terminated. This could be set to either time, flow, or volume. When set to time, the breath will end after certain amount of time (e.g. 15 breaths in 1 minute = 4 seconds per breath for controlled ventilation). When set to flow, the breath will end after there is decrease in patient flow (e.g. cycle ends when inspiratory flow is 40% of peak inspiratory flow in pressure support ventilation).