Ventilator Modes Explained: AC, SIMV, Pressure Support, and PRVC Without the Jargon

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Updated July 2026  |  More ICU nursing guides →

Ventilator mode names are one of the most intimidating alphabet soups in the ICU — AC, SIMV, PSV, PRVC — and they are made worse by the fact that different manufacturers use different names for nearly identical things. But underneath the acronyms, every mode is answering just two questions: who triggers the breath, and what does the machine guarantee — a set volume, a set pressure, or nothing at all. Hold those two questions in your head and the modes stop being memorization and start making sense.

The two questions that define every mode

Before the names, anchor on these:

• Who triggers? The machine on a timer (mandatory breaths), the patient (spontaneous breaths), or a mix of both.
• What's controlled? Volume (you set the tidal volume, pressure varies to deliver it) or pressure (you set the pressure, volume varies with lung stiffness). This is the volume-vs-pressure choice that shows up inside several modes.

Volume control guarantees the breath size but lets pressure rise — good for controlling minute ventilation, but you must watch the pressures. Pressure control guarantees the pressure (protecting against barotrauma) but lets the tidal volume swing if the lungs get stiffer — so you watch the delivered volumes. Neither is universally "better"; they suit different situations.

Assist-Control (AC): full support for every breath

AC is the workhorse of the freshly intubated, sick, or heavily sedated patient. In AC, every breath gets full support — whether the patient triggers it or the machine delivers it on the set rate. Trigger ten breaths above the set rate and all ten are fully supported. This is the mode you want when the goal is to do the work for the patient. AC comes in a volume-control flavor (set tidal volume) and a pressure-control flavor (set pressure). The main caution: because every triggered breath is fully supported, a patient breathing fast can over-ventilate and, in volume control, can stack breaths — one reason to watch for dyssynchrony.

SIMV: mandatory breaths plus spontaneous ones

Synchronized Intermittent Mandatory Ventilation delivers a set number of fully supported (mandatory) breaths, synchronized to the patient's effort, but any breaths the patient takes above that set rate are spontaneous — supported only by whatever pressure support is added, or not at all. Historically SIMV was used to "wean" by dialing down the mandatory rate and letting the patient do more. In practice, contemporary evidence has not shown SIMV to be a superior weaning mode, and many units favor other approaches, but you will still encounter it and should know that its spontaneous breaths are not fully supported the way AC breaths are.

Pressure Support (PSV): the patient drives, the vent assists

Pressure Support Ventilation is a spontaneous mode: there is no set rate. The patient triggers every breath, and the ventilator responds with a set level of pressure to make each breath easier — but the patient controls the timing and, largely, the size. Because the patient must initiate every breath, PSV requires an awake enough, strong enough patient with a reliable respiratory drive. It is the mode most associated with weaning and spontaneous breathing trials: turn the support down and see if the patient can carry the load. See ventilator weaning and spontaneous breathing trials.

PRVC: the machine that adjusts itself

Pressure-Regulated Volume Control is a hybrid that tries to give you the best of both worlds: you set a target tidal volume, and the ventilator delivers it using a pressure-controlled breath, automatically adjusting the pressure breath-to-breath to hit the volume at the lowest pressure it can. In effect it guarantees the volume like volume control while capping and minimizing pressure like pressure control. Brand names vary (AutoFlow, VC+, and others are close cousins). It is popular because it is forgiving, but the nurse should still watch that the delivered pressures and volumes both look reasonable.

What the mode change is telling you

For the bedside nurse, a mode change is a story about the patient's trajectory. Moving from AC toward pressure support usually means the team thinks the patient is waking up and getting stronger — heading toward extubation. Moving the other way — deepening support, adding sedation — means the patient is getting sicker or needs the work taken off them. When you see the respiratory therapist change the mode, ask which direction the plan is going; it frames everything else you watch that shift.

The bottom line

You do not need to program the ventilator to be excellent at the bedside — you need to know what the current mode guarantees and who is doing the breathing, so you can tell when the patient's needs and the machine's behavior stop matching. Every mode reduces to the same two questions. Answer them, and the acronyms lose their power.

Pair this with initial ventilator settings after intubation, protect the lungs with lung-protective ventilation, and troubleshoot the fighting patient with ventilator dyssynchrony. For the foundations, start at mechanical ventilation basics.