CONCEPTS OF THE VENTILATOR
Like many talks on ventilators, the easiest way to approach the topic is to split off the concepts of the vent:
Ventilators have a variety of settings. The important thing to remember is that you can break it down to two strategies. 1) The lung protection strategy 2) The lung obstruction strategy. From here, conceptualizing the different settings is easier as you can pigeon-hole them into either of the two strategies. Below are a handful of vent settings and the corresponding concepts:
– Ventilation –> RR (respiratory rate)
– Oxygenation –> PEEP (positive end expiratory pressure) / FiO2
– Protection –> Vt (tidal volume)
– Patient comfort –> Inspiratory flow rate
An easy way to think this is to imagine your goals for a patient. For example, an ARDS pt would have damaged lungs. To prevent increased trauma to the alveoli, you would make sure to use small tidal volumes. Therefore, Vt is conceptualized as lung protection.
(i.e. a set volume is targeted, so pressure is variable)
(i.e. a specific pressure is targeted, so volume is variable)
Assist control –> Most common mode used. Easiest to think of this setting as the “being nice to your patient” setting. Vent will deliver breath with pt attempt to breathe (“pt-triggered”) + vent delivers breath at set frequency (“time-triggered”). If pt doesn’t trigger, vent will deliver breath. If pt wants / triggers a breath, vent also delivers a breath.
Synchronized intermittent mandatory ventilation –> A mode to wean a pt off the vent. Like assist control, this mode is vent-triggered and patient-triggered.
Pressure support ventilation
Time –> Vent cycles at a set frequency
Pressure –> Vent senses pressure change due to pt inspiratory effort. Pt effort causes negative pressure to hit preset value, triggering vent to deliver breath.
Flow –> Vent delivers constant flow throughout respiratory cycle. Disruption in this constant flow from pt inspiratory effort triggers vent.