Ventilator breaths are primarily characterized by the variable that is set to produce air movement into the patient when the inspiratory valve opens (controlling variable) and the variable that directs  the ventilator computer to end the inspiration by closing the inspiratory valve and opening the expiratory valve (cycling variable). The most commonly used ventilator breaths include “volume control” (flow controlled, volume cycled), “pressure control” (pressure controlled, time cycled) and “pressure support” (pressure controlled, flow cycled). Note that only one of these three breaths is named after its controlling variable. The traditional advantage of a flow controlled volume cycled breath is a guaranteed volume delivery unless peak pressure alarm is exceeded.  In contrast, the volume of a pressure controlled breath increases or decreases based on lung compliance, variations in patient inspiratory effort or endotracheal tube or tracheobronchial resistance. In patients that are not heavily sedated, fluctuation of inspiratory effort is the most common reason for variations in tidal volume with pressure control breaths. Increased inspiratory effort (more negative downstream pressure) will lead to an increase in inspiratory flow with a set upstream pressure and fixed inspiratory time. 

Pressure regulated volume control (PRVC) (also “pressure controlled volume guaranteed” or PCVG) is actually a pressure control breath (pressure controlled, time cycled), not a volume control breath. With a PRVC (or PCVG) breath, the ventilator computer is allowed to adjust applied pressure up or down based on whether the exhaled tidal volume is above target (applied pressure for next breath is decreased) or below target (applied pressure for next breath is increased) in an attempt to hold tidal volume constant. Once the ventilator computer decides on the pressure to be applied, the breath delivered is a true pressure controlled, time cycled breath. 

Figure A shows baseline graphics (flow, volume and pressure over time) in a patient receiving unassisted breathing with PRVC (no patient participation in breathing). Figure B demonstrates what happens with PRVC breaths when there is an acute increase in resistance or decrease in lung compliance (increased impediment to air flow) in this patient. There is a drop in exhaled tidal volume on the next breath and on subsequent breaths there is a ratcheting up of the applied pressure occurs to return tidal volume to the previous value, but now at a higher applied pressure.  This may take several breaths to accomplish. Also note in the figure that the initial flow over time decreased with the increase in impediment to flow and then returned to baseline levels as applied pressure was increased. This is a reminder that tidal volume is always determined by the combination of flow and time.  What happens in a patient receiving PRVC and having variations in inspiratory effort? If a breath is initiated with a greater inspiratory effort, flow will increase, producing a larger tidal volume. The computer reacts to the increase in exhaled tidal volume and reduces the applied pressure. However, if in the next breath, the patient has a meager inspiratory effort  resulting in a large reduction in tidal volume, the ventilator computer will raise the applied pressure to even higher than baseline when the mode was initially set. Thus, in patients with marked variability in inspiratory effort, the applied pressure will have large swings up and down resulting in tidal volumes both much greater and much less than desired. In this circumstance, PRVC is not an ideal mode. Either switching to pressure control, where variability will still exist but will not be compounded by ventilator adjustments in applied pressure, or switching to volume control would be a better alternative. Regardless, when a breath in PRVC mode is triggered, it is predetermined to be pressure controlled and time cycled and therefore is a pressure control breath. 

Phil Dellinger, MD