Guide — Mechanical Ventilation
Ventilator Modes Explained
A ventilator mode is just a set of rules for who starts each breath, what the machine guarantees, and what it leaves up to the patient. This guide builds the trigger-limit-cycle framework first, then walks every common mode — AC/VC, AC/PC, SIMV, PSV, PRVC, APRV, and CPAP — through the same handful of questions.
12 min read · Mechanical Ventilation
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
A mode decides what starts each breath, what the machine controls during the breath, and what it leaves up to the patient. Learn those rules and the alphabet soup — AC/VC, AC/PC, SIMV, PSV, PRVC, APRV, CPAP — collapses into a small set of predictable behaviors. The single most useful habit is to ask, for any mode, which variable is guaranteed and which one is allowed to float.
This guide builds the trigger-limit-cycle framework first, then walks each mode through the same five questions: what you set, what varies, who initiates the breaths, when it is used, and what to watch. Get the framework and every new mode name becomes a small variation on something you already understand.
Key Concepts — Trigger, Limit, Cycle
Every breath a ventilator delivers is described by three events. Naming all three for a given mode tells you almost everything about how it behaves at the bedside.
| Phase | Question It Answers | Options |
|---|---|---|
| Trigger | What starts the breath? | Patient (a pressure or flow change from inspiratory effort) or the machine (time, from the set rate) |
| Limit | What is held constant during the breath? | Pressure (capped at a set value) or flow (delivered at a set pattern) |
| Cycle | What ends inspiration? | Volume delivered, time elapsed, or inspiratory flow decelerating to a threshold |
Patient trigger versus time trigger decides whether the patient or the clock begins the breath; pressure limit versus flow limit decides what the machine holds constant; and volume, time, or flow cycling decides what ends inspiration. Volume-targeted breaths cycle on volume, pressure-control breaths cycle on time, and pressure-support breaths cycle on flow.
Assessment & Findings — The Modes
Each mode below is read the same way: what you set, what varies, who initiates the breaths, the typical use, and the watch-outs. The guaranteed variable and the floating variable are the heart of every row.
AC/VC
Assist-Control, Volume Control
- Set by you
- Tidal volume and respiratory rate (plus PEEP, FiO₂, flow)
- What varies
- Airway pressure — rises as compliance falls
- Who breathes
- Every breath, patient- or time-triggered, delivers the full set tidal volume
- Typical use
- Full support for apneic, sedated, or unstable patients; guarantees a minimum minute ventilation
- Watch-outs
- Respiratory alkalosis if the patient is tachypneic; breath stacking and auto-PEEP; high plateau pressures as the lungs stiffen
AC/PC
Assist-Control, Pressure Control
- Set by you
- Inspiratory pressure, inspiratory time, and rate
- What varies
- Tidal volume — falls as compliance or resistance worsens
- Who breathes
- Every breath, patient- or time-triggered, delivers the set inspiratory pressure
- Typical use
- Tighter pressure control for injured or stiff lungs, such as ARDS strategies
- Watch-outs
- A falling tidal volume is the alarm sign of worsening mechanics; minute ventilation is not guaranteed
SIMV
Synchronized Intermittent Mandatory Ventilation (± PS)
- Set by you
- Mandatory breath count and type, usually with pressure support added to spontaneous breaths
- What varies
- Spontaneous tidal volumes taken between mandatory breaths
- Who breathes
- Mandatory breaths synchronize to patient effort; spontaneous breaths fall in between
- Typical use
- Historically a weaning mode; mixes machine and patient breaths
- Watch-outs
- Higher work of breathing than AC at low mandatory rates; largely supplanted by PSV for weaning
PSV
Pressure Support Ventilation
- Set by you
- Inspiratory pressure support and PEEP; no set rate
- What varies
- Rate, inspiratory time, and tidal volume — all patient-controlled
- Who breathes
- Purely spontaneous: the patient triggers every breath, gets a pressure boost, and the breath is flow-cycled
- Typical use
- The standard spontaneous breathing trial mode (PS 5–8 cm H₂O); comfortable for stable, awake patients
- Watch-outs
- No guaranteed rate, so an apnea backup is essential; apnea or oversedation can stop ventilation
PRVC
Pressure-Regulated Volume Control (dual control)
- Set by you
- Target tidal volume and rate; the ventilator selects the pressure
- What varies
- Inspiratory pressure — adapted breath to breath to hit the target volume at the lowest pressure
- Who breathes
- Every breath targets the set volume using a decelerating-flow pressure breath
- Typical use
- Pairs a volume guarantee with pressure-limited delivery
- Watch-outs
- Rising pressures signal worsening mechanics; the mode can mask deterioration by quietly raising pressure
APRV
Airway Pressure Release Ventilation
- Set by you
- High pressure (P-high), low pressure (P-low), and their times (T-high, T-low)
- What varies
- Spontaneous tidal volumes layered on P-high, plus the release volumes
- Who breathes
- Sustained high continuous pressure with brief timed releases; the patient breathes spontaneously throughout
- Typical use
- Refractory hypoxemia and some ARDS strategies, per institutional protocol
- Watch-outs
- Depends on spontaneous effort to work well; complex to set and wean; institution-specific
CPAP
Continuous Positive Airway Pressure
- Set by you
- A single continuous pressure and FiO₂
- What varies
- Everything else — the patient does all the ventilatory work
- Who breathes
- No machine breaths at all; constant pressure splints the airway open
- Typical use
- Oxygenation support and SBTs in spontaneously breathing patients; obstructive sleep apnea
- Watch-outs
- Provides no ventilatory assistance, so it cannot correct a high CO₂ from hypoventilation
RT Priorities & Interventions
- Match the mode to the patient. Full control modes (AC/VC, AC/PC) suit absent or unreliable drive; pressure-targeted modes protect injured lungs; spontaneous modes (PSV, CPAP) belong to awake patients who are weaning.
- Confirm the guaranteed and floating variables. In volume modes, watch the pressures; in pressure-targeted modes, watch the tidal volume. Set the alarm on whichever variable is allowed to drift.
- Recognize asynchrony. Trigger asynchrony (missed or auto-triggered breaths), flow asynchrony (flow starvation and air hunger in volume modes), and cycle asynchrony (premature or delayed termination) are fixed by adjusting trigger sensitivity, flow or rise time, and cycle settings — not by reaching first for sedation.
- Guarantee a backup. Every purely spontaneous mode (PSV, CPAP) must have an apnea backup armed before you leave the bedside.
- Recheck after every change. After switching modes or adjusting settings, reassess a blood gas, the waveforms, and the alarms before signing off on the change.
Common Pitfalls
- Confusing the guaranteed and variable parameters — chasing a low-tidal-volume alarm in a pressure mode when the real story is falling compliance.
- Leaving no apnea backup armed in PSV or CPAP, so a sedated patient who stops triggering simply stops breathing.
- Treating SIMV at a low rate as a “gentle wean” while unknowingly multiplying the patient’s work of breathing.
- Ignoring auto-PEEP from breath stacking when AC is paired with a high spontaneous rate.
- Assuming PRVC’s steady tidal volumes mean steady lungs — the rising pressure is the warning you are meant to read.
Board Exam Pearls
- Volume control guarantees volume and lets pressure vary; pressure control guarantees pressure and lets volume vary. Memorize which variable each one alarms on.
- PSV is flow-cycled with no set rate, so an apnea backup is mandatory; it is the classic spontaneous breathing trial mode at PS 5–8 cm H₂O.
- In assist-control every triggered breath is a full breath, so tachypnea drives respiratory alkalosis and auto-PEEP.
- PRVC is dual control: it targets a set volume at the lowest pressure, so a rising pressure means worsening compliance.
- APRV is high continuous pressure with brief timed releases and spontaneous breathing throughout, used for refractory hypoxemia.
FAQ
What is the difference between volume control and pressure control?
In volume control (AC/VC) you set the tidal volume and the ventilator delivers it on every breath, so volume is guaranteed but airway pressure rises and falls with the patient's lung mechanics. In pressure control (AC/PC) you set the inspiratory pressure and inspiratory time, so pressure is guaranteed but the tidal volume varies — a falling volume is the warning sign that compliance is worsening.
Which mode is used for a spontaneous breathing trial?
Pressure support ventilation (PSV) is the standard SBT mode. The patient triggers every breath and controls the rate and timing, while a low pressure boost of about 5 to 8 cm H₂O offsets the resistance of the circuit and tube. Because there is no set rate, an apnea backup must always be enabled.
Why does the tidal volume change in pressure control and PRVC?
Both modes deliver a set or targeted pressure rather than a fixed volume, so the resulting tidal volume depends on the patient's compliance and resistance. In straight pressure control the volume simply rises and falls with mechanics. In PRVC the ventilator adjusts its pressure breath to breath to hit a target volume, so rising pressures — not a falling volume — are the clue that mechanics are deteriorating.
What makes SIMV different from assist-control?
In assist-control every breath, whether triggered by the patient or the timer, receives the full set support. In SIMV only the set number of mandatory breaths are fully supported and synchronized to the patient's effort; any breaths above that rate are spontaneous and usually pressure-supported. At low mandatory rates SIMV imposes more work of breathing than assist-control.
Test yourself
Drill your recognition against board-style stems in the free practice quizzes — each with a rationale on every option.
Open the practice quizzes →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Mechanical ventilation chapters.
- Hess DR, Kacmarek RM. Essentials of Mechanical Ventilation. 4th ed. McGraw-Hill Education; 2019.
- Chatburn RL, El-Khatib M, Mireles-Cabodevila E. A taxonomy for mechanical ventilation: 10 fundamental maxims. Respir Care. 2014;59(11):1747-1763.