Ventilator Asynchronies 

1️⃣ Introduction

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.

2️⃣ Classification of Ventilator Asynchronies

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

3️⃣ Trigger Asynchrony

Occurs when ventilator fails to properly sense patient effort.

A. Ineffective Triggering (Missed Effort)

Definition:

Patient makes inspiratory effort but ventilator does NOT deliver a breath.

Causes:

• Auto-PEEP (most common)
• Weak respiratory muscles
• Over-sedation
• Low trigger sensitivity
• Neuromuscular disease

Waveform:

• Small negative deflection in pressure waveform
• No corresponding ventilator breath

Management:

• Reduce auto-PEEP
• Increase trigger sensitivity
• Reduce sedation
• Adjust inspiratory time
• Switch to pressure support mode

Most common cause = Auto-PEEP in COPD

B. Auto-Triggering

Definition:

Ventilator delivers breath WITHOUT patient effort.

Causes:

• Circuit leak
• Water in tubing
• Too sensitive trigger
• Cardiac oscillations

Waveform:

• Breath delivered without pressure deflection

Management:

• Reduce sensitivity
• Drain water
• Fix leaks

4️⃣ Flow Asynchrony (Flow Starvation)

Definition:

Delivered inspiratory flow does not match patient demand.

Seen in:

• Volume-controlled ventilation
• High respiratory drive
• ARDS

Waveform:

• Scooped pressure-time waveform
• Patient “pulling” against ventilator

Management:

• Increase inspiratory flow
• Switch to pressure control
• Increase pressure support
• Reduce sedation mismatch

5️⃣ Cycling Asynchrony

Occurs when ventilator ends inspiration too early or too late.

A. Premature Cycling (Short Inspiration)

Definition:

Ventilator ends inspiration before patient neural inspiration ends.

Causes:

• Low inspiratory time
• Low cycling threshold (PSV)

Waveform:

• Double triggering may follow

Management:

• Increase inspiratory time
• Decrease cycling percentage in PSV

B. Delayed Cycling (Prolonged Inspiration)

Definition:

Ventilator continues inspiration after patient wants to exhale.

Seen in:

• COPD
• High inspiratory time
• Low cycling threshold

Waveform:

• Pressure spike at end inspiration

Management:

• Shorten inspiratory time
• Increase cycling threshold

Classic in COPD patients.

6️⃣ Double Triggering & Breath Stacking

Definition:

Two consecutive breaths with minimal expiration between them.

Seen in:

• ARDS
• Low tidal volume ventilation
• High respiratory drive

Consequence:

• Large tidal volume
• Volutrauma risk

Management:

• Increase tidal volume slightly
• Increase sedation
• Switch to pressure control
• Neuromuscular blockade (if severe ARDS)

7️⃣ Reverse Triggering 

Definition:

Ventilator-initiated breath triggers diaphragm contraction.

Seen in:

• Deeply sedated patients
• ARDS

Mechanism:

Entrainment phenomenon

Complication:

• Breath stacking
• Self-inflicted lung injury (P-SILI)

Management:

• Reduce sedation
• Adjust ventilator mode
• Neuromuscular blockade (short term)

📌 Increasingly recognized in modern ICU.

8️⃣ Asynchrony Index (AI)

AI=Total breathsAsynchronous breaths ×100

• AI > 10% → clinically significant
• Associated with prolonged ventilation

9️⃣ Mode-wise Asynchrony

🔟 Practical Bedside Approach (ICU Algorithm)

Step 1: Look at patient

• Accessory muscle use?
• Tachypnea?
• Agitation?

Step 2: Check waveform

• Pressure
• Flow
• Volume

Step 3: Identify phase problem

Trigger? Flow? Cycling?

Step 4: Fix ventilator before increasing sedation

Important: Do NOT blindly increase sedation before correcting ventilator settings.