Entrainment Mechanism in Reverse Triggering
Entrainment in reverse triggering refers to a phenomenon where the ventilator's passive insufflation rhythmically triggers delayed diaphragmatic contractions in sedated patients, creating a synchronized pattern where the mechanical breath initiates the neural respiratory effort rather than the opposite. 1
Pathophysiological Mechanism
The entrainment mechanism represents a unique form of neuromechanical coupling that occurs predominantly in deeply sedated mechanically ventilated patients:
The ventilator's mechanical insufflation acts as the primary stimulus that triggers subsequent diaphragmatic muscle contractions, reversing the normal physiological sequence where neural effort precedes mechanical breath delivery 1
This creates rhythmic synchronization patterns between ventilator breaths and diaphragmatic contractions, with the most common pattern being a 1:1 ratio (one diaphragmatic contraction per mechanical breath), though 1:2 and 1:3 ratios have also been documented 1
The frequency of reverse-triggered breaths demonstrates remarkably low variability (mean coefficient of variability <5%), nearly identical to the variability of mechanical breaths themselves, indicating tight coupling between the ventilator and the respiratory control center 1
Clinical Context and Timing
The entrainment mechanism is particularly relevant during specific phases of mechanical ventilation:
Reverse triggering occurs most commonly during the transition phase from deep sedation to lighter sedation levels, when patients are moving toward spontaneous breathing but not yet fully triggering breaths independently 2
At 24 hours after intubation under assist-control ventilation, the median reverse triggering rate is 8% of all breaths, with 44% of patients experiencing ≥10% of breaths with reverse triggering 2
The phenomenon has been reported in 30-90% of all ventilated patients, making it a highly prevalent but frequently unrecognized form of patient-ventilator interaction 3
Distinguishing Features of Entrainment
The entrainment mechanism has specific characteristics that differentiate it from other forms of patient-ventilator interaction:
Phase delay exists between ventilator insufflation onset and neural breath commencement, with the mechanical breath consistently preceding the diaphragmatic contraction 1
Entrainment can persist over variable durations, ranging from 12% to 100% of total recording periods in individual patients 1
Multiple phenotypes exist, including reverse triggering with breath stacking (24.4% of cases) and mid-cycle reverse triggering (37.8% of cases), each with potentially different clinical implications 4
Mode-Specific Considerations
The entrainment mechanism manifests differently depending on ventilator mode:
During assist-control ventilation, entrainment is most commonly observed and represents the classic presentation of reverse triggering 1, 2
During pressure support ventilation (PSV), reverse triggered breaths occur with higher incidence compared to neurally adjusted ventilatory assist (NAVA), particularly at deeper sedation levels (RTB index 5.9% vs 1.7% during deep sedation) 5
The level of propofol sedation directly influences the incidence of reverse triggered breaths, with deeper sedation associated with higher rates of entrainment 5
Clinical Implications of the Entrainment Mechanism
Understanding entrainment is critical because of its potential effects:
Entrainment may affect lung function through mechanisms related to the level of breathing effort and eccentric activation of the diaphragm, potentially contributing to ventilator-induced lung injury 3
Patients with higher rates of reverse triggering (>10% of breaths) are more likely to progress to assisted ventilation modes or extubation within 24 hours (68% vs 35%), suggesting entrainment may indicate readiness for ventilator liberation 2
Detection requires specialized monitoring including esophageal pressure measurement, electrical activity of the diaphragm (EAdi), diaphragmatic ultrasound, or automated detection algorithms, as visual inspection of standard ventilator waveforms alone is insufficient 3, 4
Important Caveats
The exact pathophysiological mechanisms underlying entrainment remain incompletely understood, though it appears to involve complex interactions between the respiratory control center and mechanical ventilator rhythm 3
Optimal management strategies have not been established, but may include adjusting ventilatory parameters, modifying sedation depth, or in extreme cases, neuromuscular blockade 3
Automated detection software shows promise with specificity >95% and F1 scores >75%, but positive predictive values remain moderate (74%), indicating some false-positive detections still occur 4