Ventilator Asynchronies

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Ventilator Asynchronies 

Ventilator asynchrony refers to a mismatch between the patient’s neural respiratory drive and the ventilator’s delivered breath (trigger, flow, cycling, or mode).

It leads to:

  • ↑ Work of breathing (WOB)
  • Patient discomfort
  • Sedation escalation
  • Delirium
  • Prolonged mechanical ventilation
  • Increased ICU stay
  • Possibly increased mortality
    Asynchrony is present in up to 20–40% of ventilated ICU patients, and severe asynchrony index (>10%) is associated with worse outcomes.

Classification of Ventilator Asynchronies

Ventilator asynchronies are classified according to the phase of the breath cycle:

Phase

Type of Asynchrony

Trigger phase

Trigger delay, ineffective trigger, auto-triggering

Inspiratory phase

Flow starvation (flow mismatch),Excessive flow

Cycling phase

Premature cycling, delayed cycling

Breath delivery

Double triggering, breath stacking

Mode-related

Reverse triggering

Asynchrony Index (AI)

AI=Number of asynchronous events/Total respiratory rate ×100

Where:Total respiratory rate = ventilator breaths + ineffective efforts

AI

Interpretation

<10%

Acceptable

>10%

Significant asynchrony

>20%

Severe asynchrony

AI >10% has been associated with prolonged ventilation and worse outcomes.

Ineffective Triggering (Missed Trigger)

Definition: Patient effort fails to trigger ventilator breath.

Causes: 

  1. high tidal volume
  2. long inspiratory time,
  3. high pressure support,
  4. auto-PEEP (COPD/asthma)
  5. respiratory muscle weakness
  6. heavy sedation
  7. insensitive trigger settings.

Pressure waveform:Small negative deflection not followed by a breath.

Flow waveform:Small inspiratory dip without breath delivery.

Most common ventilator asynchrony.

  • Reduce pressure support and tidal volume
  • shorten inspiratory time if excessive
  • treat auto-PEEP
  • increase trigger sensitivity
  • reduce sedation
  • apply external PEEP in COPD/asthma
  • optimize respiratory muscle function.

Auto-Triggering

Definition: Ventilator delivers breaths without patient effort.

Causes: 

  1. Circuit leak
  2. ETT cuff leak
  3. NIV mask leak
  4. water/condensate in tubing
  5. cardiac oscillations (cardiomegaly/high cardiac output)
  6. overly sensitive trigger setting.

Breaths occur without pressure deflection, inspiratory effort, or diaphragmatic activity. Respiratory rate appears higher than actual patient effort.

  • Remove condensate
  • correct circuit/cuff/mask leaks
  • reduce trigger sensitivity
  • inspect ventilator circuit.

Double Triggering

Definition: One inspiratory effort triggers two consecutive breaths separated by minimal expiratory time.

Causes: 

  1. Neural inspiratory time exceeds ventilator inspiratory time;
  2. ARDS
  3. high respiratory drive
  4. low tidal volume ventilation
  5. pain or anxiety
  6. metabolic acidosis.

Two consecutive breaths with little or no exhalation between them (“breath stacking”).

May result in excessive tidal volume.

  • Increase inspiratory time
  • increase inspiratory flow delivery
  • treat pain/anxiety; correct metabolic acidosis
  • optimize sedation
  • consider neuromuscular blockade in severe ARDS.

Reverse Triggering

Definition: Ventilator breath induces diaphragmatic contraction (patient follows ventilator).

Causes: 

  1. Deep sedation
  2. ARDS
  3. controlled ventilation
  4. ventilator insufflation triggers respiratory center reflex.

Patterns: 1:1 or 1:2 entrainment.

Patient effort begins aftermachine breath starts.

Regular entrainment pattern (1:1 or 1:2).

May cause breath stacking and high transpulmonary pressures.
  • Reduce deep sedation
  • change ventilator mode
  • adjust mandatory respiratory rate
  • allow spontaneous breathing when appropriate
  • optimize ventilator-patient synchrony.

Flow Starvation 

Definition: Ventilator delivers less inspiratory flow than the patient demands.

Most common in:Volume-Control Ventilation (VCV).

Causes:

  1. Low set inspiratory flow
  2. high respiratory drive
  3. pain, fever, anxiety
  4. metabolic acidosis
  5. hypoxemia.

Pressure-time waveform:Characteristic “scooped” or concave downward appearance during inspiration due to patient pulling additional flow.

inspiratory pressure curve dips inward (“fish-hook” appearance).

 

  • Increase inspiratory flow rate
  • increase rise time (if applicable)
  • switch to pressure-targeted mode (PSV/PCV)
  • shorten inspiratory time if appropriate
  • treat pain, fever, anxiety, acidosis, and hypoxemia
  • optimize sedation if required.

Excessive Flow (Flow Overshoot)

Definition: Ventilator delivers inspiratory flow faster than the patient desires.

Causes:

  1. Excessively high flow setting
  2. excessive pressure support
  3. rapid rise time in pressure modes.

Pressure-time waveform: Early inspiratory pressure spike (“overshoot”) followed by stabilization.

Patient may actively exhale during inspiration.

 

  • Reduce inspiratory flow rate
  • reduce pressure support level
  • slow rise time
  • adjust inspiratory flow pattern
  • improve patient-ventilator matching.

 

Premature Cycling (Early Cycling-Off)

Definition

Ventilator terminates inspiration before the patient has completed inspiratory effort.

Mechanism

Neural inspiratory time exceeds ventilator inspiratory time.

Causes

• High cycling threshold in PSV 

• Short inspiratory time 

• High respiratory drive 

• ARDS 

• Restrictive lung disease

Pressure Waveform

Negative pressure deflection immediately after cycling to expiration as the patient continues to inhale.

Flow Waveform

Persistent inspiratory effort after ventilator cycles off.

Associated Findings

May trigger a second breath, resulting in double triggering and breath stacking.

Management

• Lengthen inspiratory time 

• Lower cycling threshold (% peak inspiratory flow) 

• Increase pressure support if appropriate 

• Treat causes of high respiratory drive (pain, anxiety, acidosis, hypoxemia)

 

Delayed Cycling 

Definition

Ventilator continues inspiration after the patient wishes to exhale.

Mechanism

Ventilator inspiratory time exceeds neural inspiratory time.

Common In

• COPD 

• Pressure-support ventilation (PSV) 

• Excessive pressure support

Pressure Waveform

Terminal pressure spike near end inspiration due to active expiratory effort.

Flow Waveform

Abrupt drop in inspiratory flow as the patient attempts exhalation.

Associated Findings

Patient may actively exhale against the ventilator.

Management

• Shorten inspiratory time 

• Increase cycling threshold (earlier cycling) 

• Reduce pressure support 

• Optimize settings in obstructive lung disease 

• Improve patient–ventilator synchrony

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