The pressure or flow threshold required to trigger a breath is adjustable and termed the trigger sensitivity. The ventilator senses effort through changes in airway pressure (pressure triggered) or inspiratory flow (flow triggered). Breaths initiated by a patient’s effort are patient triggered. “Trigger” refers to the stimulus that initiates a breath. To understand differences among available modes, it is important to be familiar with three terms: trigger, target, and cycle. A discussion of advanced modes of ventilation including airway pressure release ventilation and BiLevel are beyond the scope of this article, but have been reviewed elsewhere. In PC, we discuss AC, pressure support (PS), SIMV, and pressure-regulated volume controlled (PRVC). Within VC there are two common strategies of breath sequencing: assist control (AC) and synchronized intermittent mandatory ventilation (SIMV). Conversely, if a pressure is set and the tidal volume is not, then the patient is receiving pressure-controlled (PC) ventilation. If a tidal volume is set and the pressure that results from delivering that volume is not, then the patient is receiving volume-controlled (VC) ventilation. How the clinician sets these variables determines the mode. Ventilators are inspiratory assist devices that integrate volume, pressure, time, and flow (each as dependent or independent variables) to deliver a tidal breath under positive pressure.
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