Patient-Ventilator Interaction and Dyssynchrony
Definition and Core Concept
Patient-ventilator interaction refers to the coordination between a patient's spontaneous respiratory effort and the mechanical breath delivered by the ventilator, with dyssynchrony occurring when there is a mismatch between the patient's neural respiratory timing and the ventilator's mechanical timing throughout the respiratory cycle. 1, 2
Patient-ventilator dyssynchrony (PVD) is prevalent in critically ill patients and causes increased work of breathing, which can lead to ventilatory muscle overload, fatigue, impaired sleep efficiency, prolonged mechanical ventilation duration, and increased morbidity and mortality. 3, 4, 1
Types of Dyssynchrony
Trigger Asynchrony (Breath Initiation Problems)
Ineffective triggering occurs when the patient makes an inspiratory effort but fails to trigger the ventilator, most commonly due to intrinsic PEEP creating an inspiratory threshold load that the patient cannot overcome. 5, 6, 2
Delayed triggering manifests as a lag between patient effort and ventilator response, visible on waveform analysis as a delay between the start of patient effort and the beginning of ventilator flow delivery. 5, 2
Auto-triggering happens when the ventilator initiates a breath without patient effort, often due to excessive trigger sensitivity, cardiac oscillations, or circuit leaks. 2
Double-triggering occurs when the patient's neural inspiratory time exceeds the ventilator's mechanical inspiratory time, causing the patient to trigger a second breath before completing exhalation from the first breath. 2
Flow Asynchrony (Breath Delivery Mismatch)
Flow starvation results when the ventilator's inspiratory flow rate does not match the patient's flow demand—typically requiring 60-100 L/min in distressed patients—causing the patient to perform additional work to "pull" more flow from the ventilator. 3, 2
Excessive flow delivery can cause patient discomfort and trigger premature termination of inspiration by the patient. 2
Cycling Asynchrony (Breath Termination Problems)
Delayed cycling occurs when the ventilator continues to deliver inspiratory pressure after the patient has begun exhalation, forcing the patient to actively exhale against ongoing inspiratory support. 5, 2
Premature cycling happens when the ventilator terminates inspiration before the patient's neural inspiratory time ends, leaving the patient with unmet inspiratory demand. 2
Mechanisms and Causes
Intrinsic PEEP (Auto-PEEP) as a Primary Driver
Intrinsic PEEP occurs when expiratory time is insufficient for complete lung emptying, leaving alveolar pressure elevated above atmospheric pressure at end-expiration despite the airway pressure returning to baseline. 7
In severe COPD, intrinsic PEEP may reach 10-15 cm H₂O, creating a substantial inspiratory threshold load that must be overcome before the patient can generate sufficient negative pressure to trigger the ventilator. 8, 5
The measured airway pressure may touch baseline at the airway opening, but alveolar pressure remains elevated because the pressure is "trapped" behind collapsed or narrowed airways—this discrepancy requires end-expiratory airway occlusion or esophageal pressure monitoring for detection. 7
Ventilator Settings That Promote Dyssynchrony
Inadequate pressure support causes increased respiratory rate, patient distress, and ineffective triggering as the patient struggles to meet ventilatory demands. 5
Excessive pressure support leads to hyperventilation during sleep, central apneas, and worsened asynchrony. 5
Excessively long expiratory times in assist modes create prolonged "lock-out" periods that prevent patient-triggered breaths, particularly problematic in SIMV mode. 8, 3
Inadequate expiratory time causes breath stacking, dynamic hyperinflation, barotrauma, and hemodynamic compromise. 3
Excessive backup rates in assist modes may override patient efforts and worsen synchrony. 3
Patient Factors
Respiratory muscle weakness or conditions that reduce respiratory drive (neuromuscular disease, sedation, metabolic alkalosis) increase the likelihood of ineffective triggering. 6
Obstructive lung disease (COPD, asthma) predisposes to intrinsic PEEP and dynamic hyperinflation. 8, 5
Advanced acute respiratory failure may cause patients to cease spontaneous effort when "captured" by the ventilator, or patients dependent on hypoxic respiratory drive may have unreliable triggering. 8
Assessment and Monitoring
Waveform Analysis (Primary Bedside Tool)
Waveform analysis is essential and the most practical method for detecting patient-ventilator asynchrony, requiring careful examination of pressure, flow, and volume waveforms displayed on modern ventilators. 5, 1, 9
Ineffective triggering appears as negative deflections in the pressure waveform or dips in the flow waveform that do not result in a triggered breath. 5
Delayed triggering shows a visible lag between the start of negative pressure deflection and the onset of positive pressure delivery. 5
Double-triggering manifests as two consecutive ventilator breaths without an intervening expiratory pause. 2
Flow starvation appears as a scooped-out or concave pressure waveform during inspiration, indicating the patient is actively pulling against the ventilator. 2
Delayed cycling shows continued positive pressure delivery while the expiratory flow waveform begins, with the patient actively exhaling against the ventilator. 2
Failure of the pressure curve to return to baseline before the next breath suggests dynamic hyperinflation and intrinsic PEEP. 7
Advanced Monitoring (When Available)
The most sensitive detection method involves simultaneous recordings of diaphragm electrical activity and esophageal pressure changes, though this is not routinely available. 5
Esophageal pressure monitoring allows quantification of the incremental work of breathing imposed by dyssynchrony and permits addressing trigger, flow, and cycle dyssynchrony. 4
Monitoring diaphragmatic electrical activity permits accurate assessment of relationships between neural drive and ventilator flow delivery and can assess the workload of the diaphragm, representing the closest approach to ideal ventilator monitoring. 4
Computerized algorithms that calculate a dyssynchrony index may assist clinicians with recognition of PVD. 4
Management Strategies
Systematic Approach to Optimize Synchrony
Step 1: Address Intrinsic PEEP in Obstructive Disease
Set EPAP/PEEP to offset intrinsic PEEP, typically 3-5 cm H₂O, which reduces the effort required to trigger a breath and improves patient comfort. 3, 5
Never set PEEP greater than intrinsic PEEP, as this worsens hyperinflation and can be harmful. 5
Although intrinsic PEEP in severe COPD may reach 10-15 cm H₂O, EPAP levels >5 cm H₂O are rarely tolerated. 8
Prolong expiratory time by reducing respiratory rate to allow complete lung emptying and reduce dynamic hyperinflation. 3, 5
Step 2: Optimize Trigger Settings
Use flow triggers instead of pressure triggers, as they reduce the incidence of asynchrony and generally provide better patient comfort. 8, 5
Flow sensors detect changes in machine-produced bias flow and have better trigger sensitivity and ventilator response times. 8
Avoid trigger thresholds that are set too high, which induce wasted efforts and ineffective triggering. 6
Step 3: Match Flow Delivery to Patient Demand
Match inspiratory flow rate to patient demand, typically 60-100 L/min in distressed patients, to prevent flow starvation. 3
Adjust flow-cycling thresholds in pressure support to align with the patient's neural timing. 3
Step 4: Titrate Pressure Support Appropriately
Titrate pressure support upward while monitoring patient comfort and respiratory rate—if the breathing rate falls after adjustment, the support was previously inadequate. 5
Balance reducing work of breathing while avoiding hyperventilation; excessive pressure support causes hyperventilation during sleep, central apneas, and worsened asynchrony. 3, 5
Step 5: Consider Mode Changes When Optimization Fails
Switch to assist-control (A/C) ventilation rather than SIMV for patients with persistent dyssynchrony, as SIMV introduces a mandatory-breath "lock-out" period that impedes patient-triggered breaths and worsens synchrony. 3
Switch to timed/assist-control mode for patients with advanced respiratory failure, neuromuscular disease, or dependence on hypoxic respiratory drive who may cease spontaneous effort or have insufficient respiratory effort to trigger breaths. 8, 5
Consider proportional assist ventilation (PAV) or neurally adjusted ventilatory assist (NAVA) for patients with persistent asynchrony despite optimization, as these adaptive modes improve synchrony and potentially reduce sleep fragmentation, though they have not yet demonstrated improved clinical outcomes regarding duration of mechanical ventilation or mortality. 3, 5
Step 6: Optimize Sedation Strategy
Use light sedation with dexmedetomidine, which preserves circadian rhythm and improves sleep efficiency better than midazolam or propofol, which can worsen synchrony. 3
Rule out asynchrony as the cause of agitation before treating with sedation, as sedation without addressing the underlying dyssynchrony can worsen patient outcomes. 5
Step 7: Continuous Monitoring and Reassessment
Check patient comfort and respiratory rate immediately after any ventilator adjustment. 5
Reassess waveforms continuously to ensure the intervention resolved the specific asynchrony type. 5
Disease-Specific Considerations
Obstructive Lung Disease (COPD, Asthma)
Prioritize prolonged expiratory time to prevent dynamic hyperinflation. 5
Never exceed intrinsic PEEP with applied PEEP, as this worsens air trapping. 5
Neuromuscular Disease and Chest Wall Disorders
Patients benefit from initially controlled ventilation (A/C) because their ability to trigger breaths is often limited. 3
Achieve adequate tidal volume with relatively low pressures (10-15 cm H₂O) in neuromuscular disease. 5
Timed mode is particularly important as respiratory effort may be insufficient to trigger a breath, particularly during sleep. 8
ARDS
Use low tidal volume strategy (4-8 mL/kg ideal body weight) with plateau pressure <30 cm H₂O. 3
Consider lung recruitment with PEEP for persistent hypoxia. 5
Critical Pitfalls to Avoid
Ignoring waveforms leads to subtle asynchronies being undetectable—waveform analysis must be routine practice. 5
Treating agitation with sedation first without ruling out asynchrony as the cause can worsen patient outcomes and prolong mechanical ventilation. 5
Setting PEEP too high in obstructive disease worsens air trapping rather than helping—never exceed intrinsic PEEP levels. 5
Excessive oxygen flow rates (>4 L/min) can cause mask leak and delayed triggering, promoting asynchrony in non-invasive ventilation. 5
Assuming baseline pressure equals alveolar pressure in all cases—this is only true when expiratory flow has ceased and airways remain patent; intrinsic PEEP creates a discrepancy. 7
Using SIMV with the expectation that it will facilitate weaning—direct transition from A/C to pressure support ventilation is equally effective and avoids the lock-out mechanism that worsens synchrony. 3
Misinterpreting expiratory muscle recruitment as intrinsic PEEP—active abdominal muscle contraction during expiration can elevate gastric pressure and mimic auto-PEEP patterns, requiring careful waveform analysis to differentiate. 7