Patient–Ventilator Asynchrony

Overview & definition

Patient-ventilator asynchrony is a mismatch between the patient's neural respiratory drive and the breath the ventilator delivers. It is common — over a quarter of ventilated patients have significant asynchrony — and it matters: it lengthens ventilation, worsens comfort and delirium, and is associated with higher mortality. It is diagnosed at the bedside by reading the waveforms (pressure, flow, volume) and quantified by the asynchrony index.[1][1]

The asynchrony index is the number of asynchrony events divided by the total number of patient trigger attempts, expressed as a percentage; over 10 per cent is significant.[1]

The types — by phase of the breath

Trigger asynchrony

• Ineffective triggering (a missed effort) — the commonest. The patient makes an inspiratory effort that does not trigger a breath. The usual cause is auto-PEEP (the patient must overcome the auto-PEEP threshold load before triggering, especially in COPD); other causes are a weak effort, an insensitive trigger setting, and over-sedation. On the waveform, a deflection appears in the pressure/flow trace (the effort) with no ventilator breath following.
• Auto-triggering — the ventilator triggers a breath without patient effort, from a too-sensitive trigger, a circuit leak, water in the circuit, or cardiac oscillations. The waveform shows regular "breaths" with no patient effort.
• Double triggering (breath stacking) — two breaths triggered close together, the second stacking on the first (a large effective tidal volume), when the ventilator's inspiratory time is shorter than the patient's neural inspiratory time (the patient is still inhaling when the ventilator cycles off, triggering a second breath).[1][1]

Flow asynchrony

• Flow starvation (insufficient flow) — in volume control with a fixed flow less than the patient's demand, the patient "sucks" against the ventilator. On the waveform the pressure trace dips or becomes concave during inspiration (the patient effort pulls the pressure down). The fix is to raise the flow, switch to a decelerating flow or pressure control, or use a demand-flow mode.
• Excess flow — too much flow over-distends or terminates the breath prematurely.[1]

Cycle asynchrony

• Premature cycling — the ventilator ends inspiration before the patient's neural inspiration ends, which can cause double triggering.
• Delayed (prolonged) cycling — the ventilator continues into the patient's neural expiration, so the patient actively exhales against the inspiratory flow ("fighting the ventilator"). On the waveform, the pressure rises at end-inspiration (the patient exhaling against flow).[1]

Mode asynchrony

• Reverse triggering — the ventilator's mechanical breath itself triggers diaphragmatic contraction (the reverse of normal triggering), seen in deeply sedated or paralysed patients; it can cause breath-stacking and patient-ventilator injury.[1]

Management — read the waveform, fix the cause

Each type has a specific fix:[1][1]

• Ineffective triggering — reduce the auto-PEEP (bronchodilation, a lower respiratory rate, external PEEP at about 75-80 per cent of the auto-PEEP), make the trigger more sensitive, reduce sedation, and increase the pressure support.
• Auto-triggering — make the trigger less sensitive and fix the leak or drain the water from the circuit.
• Double triggering — lengthen the inspiratory time to match the patient's neural inspiratory time, increase the flow, or switch to a mode that lets the patient set the inspiratory time (pressure support).
• Flow starvation — increase the inspiratory flow, switch to a decelerating flow or a pressure-control mode, or use a demand-flow mode.
• Cycle mismatch — adjust the cycling-off criterion (in pressure support, the percentage of the peak flow at which inspiration cycles off).
• Optimise sedation — both over-sedation (no drive) and under-sedation (excess drive) worsen asynchrony.
• Change the mode — neurally adjusted ventilatory assist (NAVA) (which triggers and cycles off the diaphragm's electrical activity) and proportional assist ventilation (PAV) (which amplifies the patient's own effort) reduce asynchrony by matching the ventilator to the patient's demand.[1]

[1]

Red flags

Epidemiology — how common, and why it matters

Asynchrony is the rule rather than the exception during assisted ventilation. In Thille's landmark study, around one quarter of assisted breaths showed some form of asynchrony, and 10–15 per cent of all trigger attempts were ineffective.[2] Blanch's multicentre cohort of fifty intensive care units showed that a high asynchrony index (over 10 per cent) is independently associated with increased ICU mortality, longer ventilation, and more tracheostomy.[3] de Wit showed that ineffective triggering alone predicts a longer duration of mechanical ventilation, independent of disease severity.[4]

It is therefore a quality marker: an ICU with persistent asynchrony is an ICU ventilating badly, and the asynchrony index is a tractable, modifiable target. The catch is that it is invisible without the waveforms — a single set of numbers on the screen (rate, tidal volume, oxygenation) cannot detect it, and bedside staff systematically under-recognise it. [1]

The phases of a breath — where asynchrony arises

Every mechanical breath has four phases, and asynchrony is classified by the phase that fails:[1][1]

Detection — waveform analysis (the bedside skill)

Asynchrony is diagnosed by reading the pressure, flow, and volume scalars displayed as time curves. Use curve loops, not numbers; review at least one minute of breathing and look at both flow–time and pressure–time traces simultaneously. Esophageal pressure (or diaphragm electrical activity, EAdi) adds the patient-effort signal that the ventilator traces lack, and is the reference standard for subtle asynchrony.[1]

Trigger asynchrony on the waveform

• Ineffective triggering — a downward deflection appears in the pressure trace and a simultaneous deflection in the flow trace during expiration, but no ventilator breath follows. The deflection is the patient's inspiratory effort; it fails to trigger because it cannot overcome the trigger threshold (most often auto-PEEP in COPD, or an insensitive flow/pressure trigger). Look at the expiratory portion of the flow curve — a notch of reverse (inspiratory) flow in expiration is the patient effort.
• Auto-triggering — the ventilator delivers a regular breath pattern with no preceding deflection (no patient effort). Suspect it when the displayed respiratory rate is higher than the patient's actual effort, or when the rate is suspiciously regular in a patient who should be variable. Causes: trigger set too sensitive, a circuit leak (especially around the tube or humidifier), water condensation sloshing in the circuit, or cardiac oscillations transmitted to the flow sensor (the "cardiac trigger").
• Double triggering (breath stacking) — two breaths within one patient effort: the first breath cycles off while the patient is still inhaling, the continuing demand immediately re-triggers a second breath, and the two tidal volumes sum. On the waveform there is no expiratory flow between the two breaths — the defining feature — and the combined delivered volume is large.[1][2]

Flow asynchrony on the waveform

• Flow starvation — the hallmark is a concave, dipping pressure trace during inspiration in volume control. In volume control the flow is fixed, so when the patient's demand exceeds it the patient "pulls" against the set flow and the airway pressure drops below the set value. In pressure control, flow starvation shows as a persistently high, flat inspiratory flow that does not decelerate (the patient is still demanding), or as a rising EAdi throughout inspiration.
• Excess flow — the pressure trace overshoots or the breath terminates early; the patient may actively exhale against the inspiratory flow early in the breath (a small pressure spike at the start). [1]

Cycle asynchrony on the waveform

• Premature cycling — the ventilator ends inspiration while the patient's neural inspiration continues; the demand then re-triggers a second breath, so premature cycling presents as double triggering. The clue is that the two breaths share a single patient effort (one EAdi deflection).
• Delayed (prolonged) cycling — the ventilator continues delivering inspiratory flow into the patient's neural expiration, so the patient actively exhales against the inspiratory flow. The waveform shows a pressure rise at end-inspiration (the patient's expiratory effort pressurises the circuit against the closing valve). In pressure support this is fixed by raising the cycling-off flow threshold; in volume control by shortening the inspiratory time. [1]

Mode asynchrony — reverse triggering

Reverse triggering is a physiologically distinct entity: the ventilator's mechanical breath itself entrains rhythmic diaphragmatic contractions at a 1:1, 2:1, or 3:1 ratio with the ventilator rate.[6] It is invisible on standard scalars (the effort is entrained, not spontaneous) and was only recognised once EAdi and oesophageal pressure monitoring became routine in research. It occurs in deeply sedated or paralysed patients on controlled ventilation, and matters because it can generate breath-stacking (a second, entrained effort stacks a second tidal volume) and is associated with diaphragm injury and patient–ventilator dyssynchrony that conventional monitoring misses.[6][10]

The asynchrony index

The asynchrony index (AI) quantifies the burden: the number of asynchrony events divided by the total number of trigger attempts (patient efforts plus ventilator triggers), expressed as a percentage. An AI over 10 per cent is the conventional threshold for significant, harmful asynchrony and the threshold at which mortality rises in observational data.[3] It should be assessed over a representative window (at least 15–30 minutes of assisted ventilation, excluding suctioning and procedures), and re-measured after any ventilator change. Automated waveform analysis (as used in research, e.g. for reverse triggering detection) improves reliability over visual estimation.[11]

Diaphragm electrical activity (EAdi) and NAVA monitoring

The diaphragm electrical activity (EAdi) signal, captured by a dedicated nasogastric tube with electrode array, gives the patient's neural respiratory drive directly — the signal the ventilator is trying to follow. It serves two purposes:[1]

1. Diagnostic — EAdi reveals asynchrony that scalars hide, especially reverse triggering (entrained EAdi bursts), ineffective efforts hidden within expiration, and the timing mismatch between neural and mechanical inspiration. It is the reference standard for asynchrony research.
2. Therapeutic (NAVA) — neurally adjusted ventilatory assist (NAVA) uses EAdi both to trigger the breath and to proportionally deliver pressure in proportion to the diaphragm's electrical output, so the patient's own neural drive sets the rate, the timing, and the size of every breath. Because trigger and cycling follow the neural signal rather than a flow/pressure threshold, NAVA markedly reduces trigger and cycle asynchrony in patients with a preserved drive.[1]

EAdi requires an intact diaphragm and neural output — it is unsuitable for high spinal cord injury, phrenic nerve disruption, or profound neuromuscular weakness, and it adds the cost and placement of a specialised tube. [1]

Consequences — why asynchrony must be treated

Persistent asynchrony is not merely uncomfortable; it is harmful through several mechanisms:[3][4]

• Increased work of breathing — in ineffective triggering the patient performs inspiratory work that produces no ventilator support (wasted effort); in flow starvation the patient pulls against a fixed flow, doubling the load; in delayed cycling the patient actively exhales against inspiratory flow. Each unwasted effort is muscle work the fatiguing respiratory pump must still perform.
• Patient discomfort and agitation — the mismatch between demand and delivery is sensed as air hunger, driving agitation and the "fighting the ventilator" appearance that is then (wrongly) treated with more sedation, deepening the cycle.
• Prolonged mechanical ventilation — ineffective triggering independently predicts longer ventilation, in part because the wasted effort delays weaning and reflects a patient whose drive, load, and ventilator settings are mismatched.[4]
• Sleep disruption — asynchrony is markedly worse at night, when the sleep state destabilises the respiratory pattern; auto-triggering and ineffective efforts fragment sleep, contributing to ICU delirium and fatigue.
• Ventilator-induced injury — double triggering and reverse triggering can deliver stacked, excessive tidal volumes (volutrauma) and high transpulmonary pressures, generating the very lung injury ventilation is meant to avoid; reverse triggering is also linked to diaphragm contractile injury (load-induced myotrauma).
• Increased mortality — after adjustment for severity, an asynchrony index over 10 per cent is independently associated with higher ICU mortality.[3]

Management — a structured, cause-specific approach

The principle is read the waveform, classify the type, and fix the cause — not to deepen sedation reflexively. The general steps, then the type-specific fixes.[1][1]

Type-specific fixes

• Ineffective triggering — the cause is almost always auto-PEEP. Reduce the auto-PEEP with bronchodilation, a lower respiratory rate (longer expiratory time), and external PEEP set at about 75–80 per cent of the measured auto-PEEP (this counteracts the inspiratory threshold load without adding to hyperinflation). Make the trigger more sensitive, reduce excessive sedation, and increase pressure support if the effort is weak.[1][1]
• Auto-triggering — make the trigger less sensitive (raise the flow trigger threshold or lower the pressure trigger sensitivity), inspect and fix the circuit leak, drain condensate from the circuit, and reposition the patient if cardiac oscillations are the source.
• Double triggering — the ventilator's inspiratory time is shorter than the patient's neural inspiratory time. Lengthen the inspiratory time (or increase the flow in volume control, which shortens the time to deliver the volume and paradoxically can worsen — so prefer lengthening Ti), or switch to a patient-cycled mode (pressure support) where the patient sets the inspiratory time.[1]
• Flow starvation — increase the set inspiratory flow, change to a decelerating flow pattern, or switch from volume to pressure control/pressure support (which delivers the flow the patient demands). The most effective single move is usually to abandon fixed-flow volume control.
• Delayed cycling — in pressure support, raise the cycling-off flow threshold (e.g. from 25 per cent to 40–50 per cent of peak inspiratory flow) so the ventilator cycles earlier; in volume control, shorten the set inspiratory time. The goal is for the ventilator to end inspiration as the patient's neural inspiration ends.
• Premature cycling — it presents as double triggering; lower the cycling-off threshold (pressure support) or lengthen the inspiratory time so the breath does not end before neural inspiration.
• Reverse triggering — reduce the depth of sedation (allow the patient's own drive to resume and entrain normally), reduce the controlled respiratory rate if the patient has a drive, and consider a mode that allows patient triggering (pressure support, PAV+, NAVA). Do not ignore it as a benign deep-sedation phenomenon.[6][10]

Adaptive modes — when standard adjustments fail

When trigger, flow, and cycle adjustments do not resolve asynchrony in a patient with preserved respiratory drive, an adaptive mode that lets the ventilator follow the patient is the next step.[1][7][8]

PAV+ has been shown to reduce asynchrony and improve synchrony compared with pressure support in critically ill adults.[7] The Bosma pilot randomised trial compared weaning on PAV versus pressure support and found PAV feasible, with favourable synchrony, though definitive outcome trials are still awaited.[8] NAVA is particularly effective for double triggering and reverse triggering, because the breath is timed to the neural signal rather than to a flow threshold. ASV provides a closed-loop safety net that adapts the pattern to changing mechanics and drive.

Sedation and the patient–ventilator interface

Sedation is both a cause and a treatment of asynchrony, and the relationship is U-shaped: both extremes worsen synchrony.[5]

• Over-sedation abolishes or attenuates the respiratory drive, causing ineffective triggering (the effort is too weak to trigger) and, in deeply sedated or paralysed patients on controlled ventilation, reverse triggering (the ventilator breath entrains the suppressed diaphragm). It also prolongs ventilation and delirium.
• Under-sedation (or unrelieved pain/agitation) generates an excessive, disordered drive that the set pattern cannot follow — double triggering, flow starvation, and auto-triggering from movement and leaks.
• The right depth is the lightest sedation compatible with comfort and synchrony, with analgesia prioritised over hypnotics (analgesia-first, daily sedation interruption, avoidance of benzodiazepines where possible), and regular reassessment against a target (e.g. RASS 0 to −1 once the patient is triggering).
• Treat pain before adjusting the ventilator or the sedation — an intubated patient in pain breathes badly, and the asynchrony resolves only when the pain does. [1]

The interface matters too: an obstructed or kinked endotracheal tube, a cuff leak, or excessive circuit dead-space all generate asynchrony that no ventilator setting will fix. Check the tube and circuit before chasing the waveform. [1]

Asynchrony during non-invasive ventilation

Asynchrony is even more common during non-invasive ventilation (NIV) than during invasive ventilation, because the interface (mask, helmet) introduces leaks and a large dead-space, and the patient is often awake with a high, variable drive. Carteaux's bench-and-clinical study showed that leaks drive auto-triggering and delayed cycling, and that the choice of interface and the cycling-off setting strongly influence synchrony.[9] Practical fixes for NIV asynchrony: use the ventilator's leak-compensation mode, set a higher cycling-off threshold to overcome leak-induced delayed cycling, choose the least-leaky interface that the patient tolerates (oronasal mask generally better than total-face mask for synchrony), and consider NAVA-NIV (which triggers off EAdi and is largely leak-proof) for refractory cases.

Landmark trials and evidence

[1] [1]

Clinical pearls — high-yield for the exam

Additional red flags

One-line summary

Patient-ventilator asynchrony is a mismatch between neural drive and the delivered breath, common (a quarter of ventilated patients), harmful (longer ventilation, higher mortality at an asynchrony index over 10 per cent), and diagnosed only on the waveforms. Classify by phase — trigger (ineffective triggering from auto-PEEP is the commonest; auto-triggering; double triggering), flow (starvation — a concave pressure trace), cycle (delayed cycling — pressure rises at end-inspiration), and mode (reverse triggering in the deeply sedated) — and fix the cause: reduce auto-PEEP, adjust the trigger/flow/inspiratory time/cycling-off criterion, optimise sedation (U-shaped; treat pain first), and switch to NAVA, PAV+, or ASV for refractory cases with preserved drive. [1]

References

1. [1]Hodzic-Santor B, Telias I, et al. Asynchrony: Clinical Relevance, Detection, and Resolution Respir Care, 2025.PMID 40971310
2. [2]Thille AW, Rodriguez P, Cabello B, et al. Patient-ventilator asynchrony during assisted mechanical ventilation Intensive Care Med, 2006.PMID 16896854
3. [3]Blanch L, Villagra A, Sales B, et al. Asynchronies during mechanical ventilation are associated with mortality Intensive Care Med, 2015.PMID 25693449
4. [4]de Wit M, Miller KB, Green DA, et al. Ineffective triggering predicts increased duration of mechanical ventilation Crit Care Med, 2009.PMID 19886000
5. [5]de Wit M, Pedram S, Best AM, et al. Observational study of patient-ventilator asynchrony and relationship to sedation level J Crit Care, 2009.PMID 19272542
6. [6]Akoumianaki E, Lyazidi A, Rey N, et al. Mechanical ventilation-induced reverse-triggered breaths: a frequently unrecognized form of neuromechanical coupling Chest, 2013.PMID 23187649
7. [7]Xirouchaki N, Kondili E, Vaporidi K, et al. Proportional assist ventilation with load-adjustable gain factors in critically ill patients: comparison with pressure support Intensive Care Med, 2008.PMID 18607562
8. [8]Bosma KJ, Read BA, Bahrgard Nikoo MJ, et al. A Pilot Randomized Trial Comparing Weaning From Mechanical Ventilation on Pressure Support Versus Proportional Assist Ventilation Crit Care Med, 2016.PMID 26807682
9. [9]Carteaux G, Lyazidi A, Cordoba-Izquierdo A, et al. Patient-ventilator asynchrony during noninvasive ventilation: a bench and clinical study Chest, 2012.PMID 22406958
10. [10]Mellado Artigas R, Damiani LF, Piraino T, et al. Reverse Triggering Dyssynchrony 24 h after Initiation of Mechanical Ventilation Anesthesiology, 2021.PMID 33662121
11. [11]Pham T, Montanya J, Telias I, et al. Automated detection and quantification of reverse triggering effort under mechanical ventilation Crit Care, 2021.PMID 33588912