One table, four modes, every question that matters: what does the ventilator control, what does the patient get to do, and what goes wrong? Each mode column links to the live simulator so you can see the waveforms in motion.
Four modes, one question each row answers: what does the ventilator control, and what does the patient get to do? Every mode is live in the simulator — tap “See it live” to open it with that mode loaded. Scroll sideways on mobile; the row labels stay pinned.
| MODE → | AC (Volume)▸ See it live | SIMV (+PS)▸ See it live | PRVC▸ See it live | APRV▸ See it live |
|---|---|---|---|---|
| Full name | Assist-Control, volume-targeted | Synchronized Intermittent Mandatory Ventilation, usually with Pressure Support | Pressure-Regulated Volume Control (adaptive pressure control) | Airway Pressure Release Ventilation |
| Control variable | Volume (set VT, set flow) | Volume for mandatory breaths; pressure for supported breaths | Pressure, auto-adjusted breath-to-breath to hit a target VT | Pressure — long high level (P-high) with brief timed releases (P-low) |
| What a patient effort gets | Every effort triggers a full machine breath at set VT | Efforts near a scheduled breath get a synchronized mandatory breath; extra efforts get PS only | Every effort triggers a full breath at whatever pressure hits the target VT | Unrestricted spontaneous breathing at both pressure levels — its defining feature |
| Guaranteed VT? | Yes, every breath | Mandatory breaths only | Targeted on average — actual VT varies breath to breath | No — VT depends on compliance, release time, and patient effort |
| PIP behavior | Varies with compliance and resistance — rises as lungs stiffen | Varies on mandatory breaths | Capped: vent titrates pressure between PEEP and Pmax | Capped at P-high by definition |
| Work of breathing | Low — machine does nearly all the work | Can be high if PS is set too low for the spontaneous breaths | Low, but support falls as patient effort rises (see pitfall) | Patient-dependent; spontaneous effort is expected and encouraged |
| Main pitfalls | Respiratory alkalosis with high drive; breath stacking; fixed flow can cause flow starvation | Patient–vent asynchrony; higher WOB; weaning by rate reduction is slower than PS/SBT in trials | 'Support runaway': vigorous effort makes big VTs, so the vent lowers pressure exactly when the patient is working hardest | De-recruitment if T-low is too long; hypercapnia; unfamiliar settings invite misuse |
| Typical use | Default initial full support; deep sedation or paralysis; most respiratory failure | Largely historical; occasional post-op or transition use | Lung protection when compliance is changing — pressure cap plus volume target | Rescue oxygenation in severe ARDS; open-lung strategy with spontaneous breathing |
| Key settings | VT, rate, flow (and waveform), PEEP, FiO₂ | VT, mandatory rate, PS level, PEEP, FiO₂ | Target VT, rate, Ti, Pmax alarm, PEEP, FiO₂ | P-high, T-high, P-low, T-low, FiO₂ |
| Weaning approach | Transition to PS or daily SBTs | Historically: reduce mandatory rate — slower than PS/SBT | Same as AC: transition to PS / SBT once improving | 'Drop and stretch' — lower P-high while lengthening T-high toward CPAP |
Educational reference for students — mode names and behavior vary by manufacturer (e.g. PRVC ≈ VC+ ≈ AutoFlow ≈ APV). Always confirm on the ventilator in front of you.
Volume-Control Assist-Control is the default mode in most ICUs. The clinician sets a tidal volume and a backup rate; the ventilator delivers exactly that volume every breath — whether the breath was triggered by the patient or timed by the machine. Because every patient effort is rewarded with a full machine breath, AC provides the most predictable minute ventilation and the lowest work of breathing of any full-support mode.
The main hazard is ventilator-induced lung injury from stacked breaths if the patient's respiratory drive is high. Watch for double triggering: the patient triggers, the machine delivers the full VT, and then the patient's neural inspiratory effort continues into the exhalation phase and triggers a second breath — the stacked volume can approach 2× the set VT. Managing drive with sedation analgesia and optimizing Ti are the bedside fixes. Flow starvation — a concave pressure scoop — occurs when set flow is insufficient for the patient's demand; switch to a pressure-targeted mode or increase flow.
Synchronized Intermittent Mandatory Ventilation delivers a set number of full machine breaths per minute, synchronized to coincide with the patient's own inspiratory efforts when possible. Any extra breaths the patient initiates between mandatory cycles receive only pressure support — a sustained pressure boost that assists the breath without guaranteeing a volume.
SIMV was developed as a weaning tool (reduce the mandatory rate gradually) but randomized trials showed it weans patients more slowly than pressure-support weaning or daily spontaneous breathing trials. It remains in use for post-operative patients being rapidly transitioned and in units with well-established SIMV protocols, but AC or PSV is preferred in most modern weaning guidelines.
PRVC (also called VC+, AutoFlow, or APV depending on the manufacturer) combines the pressure-controlled flow waveform — decelerating, demand-following — with a tidal volume target. The ventilator delivers a test breath, measures the VT actually achieved, and adjusts the inspiratory pressure up or down on the next breath to converge on the target. The pressure is capped at a user-set Pmax.
The key pitfall: if the patient's own effort increases, the ventilator reads a larger VT and reduces its pressure support for the next breath — exactly when the patient is working harder. This “support runaway” behavior can increase work of breathing in patients with highly variable drive. Monitor the delivered ΔP and the patient's WOB; if PRVC is chasing a moving target, the mode may not be the right fit.
APRV applies a sustained high pressure (P-high) for most of the respiratory cycle (T-high), maintaining alveolar recruitment, then releases briefly to a low pressure (P-low) for a short window (T-low) to allow exhalation and CO₂ clearance. The release is intentionally brief — T-low is set to terminate at approximately 50–75% of the peak expiratory flow rate so that the airways do not have time to de-recruit before the next pressure cycle.
The defining advantage of APRV over conventional inverse-ratio ventilation is that spontaneous breathing is not just permitted but encouraged throughout the cycle. Patient effort recruits dependent lung regions and reduces the need for heavy sedation. APRV is used primarily as a rescue strategy in severe ARDS when plateau pressures on conventional modes are unacceptably high or oxygenation goals cannot be met.
Ventilator mode names are not standardized across manufacturers. What one calls PRVC, another calls VC+, AutoFlow, or APV (Adaptive Pressure Ventilation). SIMV is nearly universally named, but the behavior of spontaneous breaths between mandatory cycles varies by platform. AC volume-control may appear as CMV, VC-CMV, or simply A/C on different machines. Always consult the ventilator in front of you and its manufacturer manual — the control variable, the adaptation algorithm, and the alarm logic all differ.
Educational use only. This comparison is designed for RT, nursing, and medical students learning ventilator principles. Clinical decisions — mode selection, parameter titration, weaning — should be made by qualified clinicians using institutional protocols and real-time patient assessment.
In Assist-Control (AC), every patient effort — even one above the set rate — triggers a full machine breath at the set tidal volume. In SIMV, only a fixed number of mandatory breaths are delivered per minute; additional patient efforts receive only pressure support, not a full machine breath. AC provides more consistent support and better patient synchrony in most patients; SIMV is largely historical.
PRVC is a pressure-controlled mode that targets volume. The ventilator delivers a pressure breath, measures the resulting tidal volume, and adjusts the pressure up or down on the next breath to hit the set VT target. Because it's pressure-controlled, flow decelerates like PC-AC — but because it's volume-targeted, you get the lung-protection guarantee of volume control. Different manufacturers call it VC+, AutoFlow, or APV.
APRV is used as a rescue oxygenation strategy in severe ARDS when conventional modes are failing. It works by applying a sustained high airway pressure (P-high) for most of the cycle to recruit alveoli, then releasing briefly (T-low) to allow CO₂ clearance. Its defining advantage is that patients can breathe spontaneously throughout the cycle without heavy sedation — though this requires careful T-low titration to avoid de-recruitment.
The standard approach is 'drop and stretch': gradually lower P-high (improving comfort and reducing barotrauma risk) while lengthening T-high toward the full respiratory cycle. As T-high approaches the entire cycle, APRV transitions toward high-CPAP, then continuous CPAP, and finally extubation or a conventional low-support mode.
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