Patient ventilator asynchrony

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Patient-Ventilator Asynchrony

Patient-ventilator asynchrony (also called dyssynchrony) is a mismatch between the patient's neural respiratory drive and the mechanical breath delivered by the ventilator — specifically, disagreement in timing (trigger, flow delivery, or cycling) or magnitude of support. It is common, under-recognized, and clinically significant.

Definition

"Patient-ventilator dyssynchrony occurs when the trigger, flow, and/or cycling of a mechanical breath is not in time agreement with the initiation and termination of a patient's neural inspiration, respectively, or if the magnitude of mechanical assist does not respond to the patient's respiratory demand." — Fishman's Pulmonary Diseases and Disorders

In the context of NIV specifically, asynchrony is defined as a mismatch between the patient's neural inspiratory time and the ventilator insufflation time.

Prevalence and Clinical Impact

Frequent asynchronies (>10% of respiratory efforts) are present in 43% of patients undergoing NIV in observational studies.
Consequences include:
- Respiratory muscle overload — increased work of breathing
- Compromised ventilation — inadequate CO₂ clearance or oxygenation
- Sleep disruption — fragmented sleep architecture
- Patient discomfort — leading to poor tolerance and NIV failure
- In invasive MV: risk of ventilator-induced lung injury (VILI) from breath stacking and large tidal volumes

Types of Asynchrony

1. Trigger Asynchrony

Mismatch between the patient's inspiratory effort and when the ventilator triggers a breath.

Subtype	Description	Cause
Ineffective effort	Patient makes an effort but no breath is delivered	High auto-PEEP (e.g., COPD), weak muscles (NMD), insensitive trigger
Double-triggering	Ventilator delivers two breaths for one patient effort	Short inspiratory time relative to neural Ti
Auto-triggering	Ventilator delivers a breath without patient effort	Expiratory leaks causing false flow signals, circuit water, cardiac oscillations

Management:

Minimize mask leaks (most common cause in NIV)
Minimize upper airway resistance (e.g., treat OSA, glottic closure)
Adjust trigger sensitivity: increase to High for weak efforts; decrease to Low for auto-triggering
For double-triggering: increase pressure support and/or decrease cycle sensitivity; extend Ti in PC mode

2. Cycling Asynchrony

Ventilator cycling (end-inspiration) does not coincide with end of patient's neural inspiration.

Subtype	Description	Cause
Premature cycling (early)	Ventilator ends before patient finishes inspiring	Short Ti setting, mask leaks reducing flow quickly
Delayed/late cycling	Ventilator continues after patient wants to exhale → active exhalation against pressure	Long Ti setting, large leaks keeping flow above cycling threshold

Delayed cycling (prolonged inspiration) is the most common asynchrony during NIV, driven by inspiratory leaks — because the delivered flow remains above the cycling criterion, the ventilator continues to insufflate.

Management:

Minimize mask leaks first
Review Ti (or Ti max) settings
Adjust cycle sensitivity to Low (lower % of peak flow threshold) for premature cycling
Consider switching to pressure-control (PC) mode

3. Flow Asynchrony

The flow delivered does not match the patient's demand — either too slow (flow starvation, causing a scooped waveform on the pressure trace) or excessive (discomfort).

Leading Causes

Cause	Mechanism
Mask leaks	#1 cause in NIV — inspiratory leaks → prolonged inspiration; expiratory leaks → auto-triggering
Auto-PEEP / intrinsic PEEP	Raises triggering threshold → ineffective efforts (esp. COPD)
Inappropriate ventilator settings	Ti too short/long, trigger sensitivity off, PS level insufficient
Upper airway obstruction	OSA, glottic closure → resistance impairs triggering
Weak respiratory muscles	NMD patients — effort too small to trigger standard sensitivity

Monitoring and Detection

Flow and pressure waveform analysis at the bedside — key clinical tool
Esophageal electromyography (EAdi): diaphragm electrical activity monitoring allows detection of asynchrony, assessment of respiratory muscle unloading, and direct triggering/cycling (NAVA mode)
Polysomnography in home NIV patients — detects asynchrony during sleep

Management Summary (Troubleshooting Table)

Asynchrony	First Action	Second Action	Third Action
Ineffective triggering	Minimize mask leak	Minimize UA resistance	Increase trigger sensitivity / switch to Auto-Trak
Double-triggering	Increase PS, decrease cycle sensitivity	Extend Ti in PC mode	—
Auto-triggering	Set trigger sensitivity to Low	Check for expiratory leaks	NIV-dedicated ventilator algorithms
Premature cycling	Minimize mask leak	Review Ti/Ti max	Set cycle sensitivity to Low
Late cycling	Minimize mask leak	Shorten Ti max	Set cycle sensitivity to High

Special Considerations

NAVA (Neurally Adjusted Ventilatory Assist): Triggers, limits, and cycles the ventilator directly from diaphragm electrical activity — theoretically eliminates most asynchrony and is not affected by leaks.

NIV-specific modes: Modern ICU ventilators and dedicated NIV devices include leak-compensation algorithms that markedly reduce both prolonged inspiration (from inspiratory leaks) and auto-triggering (from expiratory leaks) compared to older standard ICU ventilators.

ARDS patients: Neuromuscular blocking agents may be beneficial when patient-ventilator asynchrony leads to breath stacking or very large inspiratory efforts that risk VILI.

Sources: Fishman's Pulmonary Diseases and Disorders (Patient-Ventilator Dyssynchrony, Table 148-4); Murray & Nadel's Textbook of Respiratory Medicine (Asynchrony under BPAP, NIV chapter)

Weaning from mechanical ventilation

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Weaning from Mechanical Ventilation

"Weaning" refers to all methods of discontinuing mechanical ventilation (MV) — from gradual reduction in support to abrupt removal. A more modern framing calls it liberation: an active, daily process of assessing readiness rather than a slow, passive taper. Approaching it as a prolonged wean unnecessarily extends MV time by up to 40%.

1. When to Consider Weaning: Readiness Criteria

Before initiating a spontaneous breathing trial (SBT), assess all of the following daily:

Domain	Criterion
Underlying disease	Primary cause of respiratory failure has improved
Neurological	Patient is awake, alert, minimal or no sedation
Oxygenation	FiO₂ ≤ 0.5; PEEP < 8 cmH₂O; SaO₂ > 88%
Hemodynamics	Stable — no vasopressors or low-dose only
Airway	Manageable secretions; adequate cough

2. Weaning Predictor Tests

These are screening tools — not confirmatory. Their role is to flag patients who are ready before the clinician would otherwise think to trial them, offsetting cognitive errors in bedside decision-making.

Rapid Shallow Breathing Index (RSBI / f/Vt ratio — the Tobin Index)

Formula: Respiratory frequency (f) ÷ tidal volume in litres (Vt)
Interpretation: f/Vt < 105 breaths/min/L predicts weaning success
Sensitivity ≥ 0.90 in multiple studies — excellent as a screening test
Must be measured during unassisted spontaneous breathing (not on PS or CPAP, which falsely lowers the ratio)
The higher the f/Vt, the worse the prognosis: patients who fail weaning immediately develop rapid, shallow breathing (↑ RR + ↓ Vt) upon ventilator disconnection

Other indices (less relied upon than SBT outcome)

Minute ventilation (Ve)
Negative inspiratory force (NIF / MIP) — reflects respiratory muscle strength
Vital capacity
P0.1 (airway occlusion pressure) — reflects neural drive

Relying on these physiologic variables rather than the outcome of an SBT leads to unnecessary delays in extubation. — Harrison's Principles of Internal Medicine

3. Spontaneous Breathing Trial (SBT)

The SBT is the cornerstone of the weaning process. Positive-pressure support is reduced to a minimum and the patient breathes near-spontaneously for 30–120 minutes.

SBT Methods

Method	Description
T-piece trial	Patient disconnected from ventilator; breathes through humidified T-circuit with supplemental O₂. No ventilator support.
Low-level CPAP	CPAP 5 cmH₂O — maintains airway patency, compensates for ET tube resistance
Low-level PSV	PSV 5–7 cmH₂O — most common; compensates for ET tube and circuit resistance
SIMV reduction	Gradual stepwise reduction of mandatory breath rate (least effective method)

In patients not anticipated to have weaning difficulty, a 30-minute SBT is as effective as a 2-hour trial.

Passing the SBT — All of the following:

Parameter	Pass Criterion
Respiratory rate	< 35 breaths/min
SpO₂	> 90%
Systolic BP	90–180 mmHg
Heart rate	Stable, change < 20%
Clinical appearance	No marked anxiety, dyspnoea, diaphoresis, or use of accessory muscles

Patients passing an SBT have a >70% chance of successful extubation.

Incorporating a daily readiness screen + SBT protocol leads to:

25% fewer ventilator days
10% decrease in ICU length of stay

4. Algorithm for Discontinuing Mechanical Ventilation

Algorithm for discontinuing mechanical ventilation. — Harrison's Principles of Internal Medicine, 22e

5. Causes of Weaning Failure

Up to 25% of patients experience respiratory distress requiring ventilator reinstitution after disconnection. The hallmarks of failed weaning are:

Rapid, shallow breathing — immediately upon disconnection (↑ RR, ↓ Vt)
Progressive respiratory effort over 30–60 min, reaching >4× normal by the end of a failed trial

Why mechanics worsen during a failed trial:

Parameter	Change During Failed Trial
Respiratory resistance	Increases ~7× normal
Lung stiffness	Increases ~5× normal
Auto-PEEP (PEEPi)	More than doubles

Importantly, respiratory mechanics before the trial are similar in patients who succeed and those who fail — the worsening is triggered by the act of spontaneous breathing itself (via unknown mechanisms), not pre-existing abnormality.

Cardiovascular stress of weaning failure:

Extreme negative intrathoracic pressure swings → ↑ RV and LV afterload
Pulmonary artery pressure ↑ 39%; systemic artery pressure ↑ 27%
Patients who fail cannot increase O₂ delivery to meet demand → mixed venous O₂ saturation falls (occult cardiac dysfunction is unmasked)

Common causes of weaning failure:

Unresolved underlying disease
Respiratory muscle weakness / fatigue
Excessive respiratory load (↑ resistance, ↓ compliance, auto-PEEP)
Cardiovascular dysfunction / occult heart failure
Neurological impairment / impaired drive
Metabolic derangements (hypophosphataemia, hypomagnesaemia → muscle weakness)
Psychological dependence (in prolonged ventilation)

6. Risk Factors for Extubation Failure Despite Passing SBT

Even after a successful SBT, extubation may fail. High-risk features:

Age > 65
Congestive heart failure
COPD
APACHE-II score > 12
BMI > 30
Significant secretions
2 medical comorbidities
7 days on mechanical ventilation

In these patients, transition immediately post-extubation to high-flow nasal oxygen (HFNO) or non-invasive ventilation (NIV) significantly reduces reintubation rates.

7. Prolonged Weaning

Patients who fail ≥ 3 SBTs or require > 7 days of weaning after the first SBT are classified as difficult or prolonged weaning. Strategies include:

Tracheostomy — reduces dead space, improves comfort, facilitates secretion clearance
Progressive SBT duration increases — gradually extend unassisted breathing time each day
Inspiratory muscle training
Optimizing nutrition — avoid overfeeding (↑ CO₂ production worsens ventilatory demand)
Management of comorbidities (heart failure, infection, thyroid, electrolytes)

Sources: Fishman's Pulmonary Diseases and Disorders (Weaning, Causes of Weaning Failure, Timing, Weaning Trials — Chapter 147); Harrison's Principles of Internal Medicine 22e (Liberation from Mechanical Ventilation, Fig. 313-4)

Ventilation in ARDS

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Ventilation in ARDS

Overview

Mechanical ventilation (MV) is the cornerstone of ARDS management — but MV itself can aggravate or even induce lung injury (ventilator-induced lung injury, VILI). The central goal of ventilatory management is therefore not to normalise blood gases, but to support gas exchange while minimising further lung damage — "lung-protective ventilation."

Pathophysiologic Basis: Why the ARDS Lung is Vulnerable

CT imaging transformed understanding of ARDS. Rather than diffuse homogeneous injury (as plain radiographs suggest), ARDS lungs show heterogeneous patchy injury — consolidated dependent regions interspersed with more normal-appearing lung. Positive pressure therefore preferentially inflates the "baby lung" (the smaller volume of recruitable normal lung), placing it at risk of overdistension even with seemingly normal tidal volumes.

Mechanisms of VILI

Mechanism	Description
Volutrauma	Overdistension from large tidal volumes → increased alveolar-capillary permeability → pulmonary edema. The dominant determinant — high Vt causes injury regardless of whether airway pressure is high or low
Barotrauma	High airway pressures → pneumothorax, pneumomediastinum, subcutaneous emphysema
Atelectrauma	Repetitive cyclic opening and closing of collapsed terminal units → high shear stress at the interface of aerated and collapsed tissue
Biotrauma	Mechanical ventilation triggers local and systemic inflammation; high Vt + zero PEEP causes synergistic elevation of inflammatory cytokines in BAL and blood → multiorgan dysfunction

Lung-Protective Ventilation: The ARDSNet Protocol

The landmark ARDSNet ARMA trial (861 patients) demonstrated that low tidal volume ventilation reduces mortality from 39.8% → 31.0% (P = 0.007) and increases ventilator-free days. This is now the standard of care.

Part I — Ventilator Setup

Step	Detail
Mode	Assist-Control (volume-cycled)
Initial Vt	8 mL/kg predicted body weight (PBW) — reduce by 1 mL/kg every ≤2 h
Target Vt	6 mL/kg PBW (reduce to 4 mL/kg if Pplat > 30 cmH₂O)
Respiratory rate	Set to approximate baseline minute ventilation (max 35 bpm)
Inspiratory flow	>80 L/min; I:E ratio 1:1.0–1.3

Use predicted (ideal) body weight, not actual body weight — actual BW can be ~20% higher due to fat and oedema, leading to dangerous over-ventilation.

Part II — Oxygenation Goal: PaO₂ 55–80 mmHg or SpO₂ 88–95%

Use these preset FiO₂/PEEP combinations (titrate upward as needed):

FiO₂	0.3	0.4	0.4	0.5	0.5	0.6	0.7	0.7	0.8	0.9	0.9	1.0	1.0
PEEP	5	5	8	8	10	10	10	12	14	14	16–18	20	22–24

Part III — Plateau Pressure Goal: ≤30 cmH₂O

Check Pplat with a 0.5-second inspiratory pause
If Pplat > 30 cmH₂O → reduce Vt by 1 mL/kg (minimum 4 mL/kg PBW)
If Pplat < 25 cmH₂O and Vt < 6 mL/kg → increase Vt by 1 mL/kg

No plateau pressure is known to be truly "safe" — the data show a continuous relationship between Pplat and mortality with no safe threshold.

Part IV — pH Goal: 7.30–7.45

pH	Action
< 7.30	Increase RR (max 35); consider sodium bicarbonate infusion
> 7.45	Decrease RR

Permissive hypercapnia — accept elevated PaCO₂ to maintain low Vt. This is the physiologic cost of lung protection.

PEEP in ARDS

PEEP serves two purposes:

Prevent atelectrauma — keeps terminal units open, avoiding cyclic collapse/reopening
Improve oxygenation — recruits collapsed alveoli, reduces shunt

Evidence on High vs. Low PEEP

ALVEOLI, LOVS, EXPRESS trials: Higher PEEP (combined with low Vt) did not reduce 60-day mortality vs. lower PEEP strategies
Meta-analysis of the three trials: Higher PEEP improved survival in moderate-to-severe ARDS (PaO₂/FiO₂ < 200) but not in mild ARDS
Higher PEEP improved secondary outcomes: reduced rescue therapy use (LOVS), shorter MV duration and less organ failure (EXPRESS)

The optimal PEEP level in ARDS remains unclear. Higher PEEP is safe and may benefit severe ARDS; PEEP increments should only be applied when they produce reliable alveolar recruitment. — Fishman's Pulmonary Diseases and Disorders

Recruitment Maneuvers

Transient increases in airway pressure (e.g., sustained inflation at 40 cmH₂O for 40 seconds, or stepwise PEEP increments) to open collapsed alveoli. Benefit is transient unless adequate PEEP is applied afterward to prevent derecruitment. Used as a rescue strategy — not routine.

Prone Positioning

One of the most evidence-supported rescue interventions in severe ARDS:

Redistributes perfusion toward non-dependent (now ventral) lung regions, better matching ventilation to perfusion
Reduces dorsal compression atelectasis
PROSEVA trial: Prone positioning ≥16 hours/day in severe ARDS (PaO₂/FiO₂ < 150) reduced 28-day mortality from 32.8% → 16.0%
Indicated in moderate-to-severe ARDS (PaO₂/FiO₂ < 150 on FiO₂ ≥ 0.6, PEEP ≥ 5, on lung-protective ventilation)

Rescue / Salvage Strategies

For patients with severe refractory hypoxaemia despite standard lung-protective ventilation and PEEP:

Intervention	Mechanism	Notes
Prone positioning	V/Q matching, recruits dorsal lung	Strongest mortality evidence (PROSEVA)
Neuromuscular blockade	Eliminates patient-ventilator asynchrony, reduces VILI from effort-driven injury	Beneficial in severe ARDS (PaO₂/FiO₂ < 150); reduces breath stacking
Inhaled NO / prostacyclin	Selective pulmonary vasodilation → V/Q improvement	Improves oxygenation transiently; no mortality benefit; used as bridge
High-frequency oscillatory ventilation (HFOV)	Ultra-small Vt at very high frequency	No mortality benefit in RCTs (OSCILLATE, OSCAR); not routinely recommended
ECMO (VV-ECMO)	Extracorporeal gas exchange — permits "ultra-lung-rest"	For refractory ARDS failing all conventional measures; EOLIA trial
ECCO₂R	Extracorporeal CO₂ removal	Permits further reduction of Vt below 6 mL/kg
Corticosteroids	Anti-inflammatory	Limited evidence; may be used in fibroproliferative phase
PC-Inverse Ratio Ventilation (PC-IRV)	Prolongs inspiration (I:E > 1) → ↑ mean airway pressure → improved oxygenation	Risk of auto-PEEP, dynamic hyperinflation, haemodynamic compromise; salvage only
APRV (Airway Pressure Release Ventilation)	High baseline PEEP with brief release phases for CO₂ clearance; allows spontaneous breathing	Limited controlled evidence; promotes recruitment

Modes Allowing Spontaneous Breathing

APRV and BIPAP allow spontaneous breathing at any phase of the ventilatory cycle:

APRV: sustained high pressure (P-high) to recruit + brief release to a low pressure (P-low) for CO₂ clearance, without allowing derecruitment
Theoretical benefits: reduced sedation, preserved diaphragm function, improved haemodynamics
Controlled data are limited; risk of patient self-inflicted lung injury (P-SILI) in vigorous spontaneous breathers

Fluid Management

The FACTT trial (ARDSNet) demonstrated that a conservative fluid strategy (targeting lower CVP/PAOP) vs. liberal strategy reduced ventilator days and ICU length of stay without increasing non-pulmonary organ failure. Conservative fluid management is therefore preferred once haemodynamic stability is achieved.

Summary: Priorities in ARDS Ventilation

Vt 6 mL/kg PBW — non-negotiable foundation
Pplat ≤ 30 cmH₂O — reduce Vt further if needed
Permissive hypercapnia — tolerate high PaCO₂ to protect the lung
Adequate PEEP — prevent atelectrauma; higher PEEP in moderate-severe disease
Prone positioning — for PaO₂/FiO₂ < 150 early and sustained
Conservative fluids after resuscitation
Rescue strategies — stepwise escalation for refractory hypoxaemia

Sources: Murray & Nadel's Textbook of Respiratory Medicine (VILI, Mechanical Ventilation in ARDS, Role of PEEP and Recruitment Maneuvers — Chapter 134); Fishman's Pulmonary Diseases and Disorders (ARDSNet ARMA Trial, NIH NHLBI Low Tidal Volume Strategy Table 141-9, Rescue Interventions Table 141-12 — Chapter 141)

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