Reverse triggering is a specific type of patient-ventilator interaction, stemming from the broader physiological concept of respiratory entrainment. This phenomenon, known since the 19th century, involves the synchronization or 'entrainment' of a biological rhythm—in this case, the patient's respiratory rhythm—with an external stimulus, such as the mechanical ventilation.
"Reverse triggering" describes an atypical interaction between a ventilator and a patient, where the ventilator's mandatory breath acts as an external stimulus that provokes a reflexive neural response in the patient. In this scenario, the ventilator effectively "triggers" the patient, leading to what is termed as "reverse-triggered" effort. In this phenomenon, the pressure and flow of the mandatory breath stimulate the respiratory control center, leading to an involuntary patient effort. This phenomenon commonly exhibits a 1:1 ratio of mechanical breaths to reverse-triggered efforts, although variations like 2:1, 3:1, and 1:2 have also been observed.
Mechanism of Reverse Triggering
The mechanism behind respiratory entrainment, and consequently reverse triggering, is thought to be the activation of stretch receptors in the upper airways, lungs, and chest wall by the flow and pressure delivered by the ventilator. The stimulation of these receptors sends feedback to the respiratory control center, leading it to adjust and match the phase and frequency of the ventilator's rhythm. This results in a repetitive respiratory pattern that aligns with the ventilator's cycle, manifesting as reverse triggering.
How to Recognize Reverse Triggering?
When observing inspiratory efforts following mechanical breaths, it's crucial to discern if these are spontaneous efforts or reverse triggering as a reaction to the mechanical breaths. Two key interventions can assist in this differentiation. Firstly, implementing an expiratory hold can be informative: if the efforts cease, it suggests reverse triggering; if they continue, it points to spontaneous efforts. Secondly, increasing sedation typically suppresses spontaneous efforts but does not affect reverse triggering, which tends to persist despite increased sedation.
In the following example, inspiratory efforts are observed late in the mechanical inspiration and into early expiration. The persistence of these efforts despite applying an expiratory hold, coupled with their elimination following increased sedation, indicates that these are spontaneous efforts, not reverse triggering.
Consequences of Reverse Triggering
Reverse triggering in mechanically ventilated patients can cause significant lung stress by increasing transpulmonary pressure, which is the difference between airway opening pressure and pleural pressure. Additionally, this patient effort may lower the airway pressure below the ventilator's trigger level, resulting in a subsequent mandatory insufflation before the expiration of the previous breath. This process can cause "breath stacking," which is the accumulation of two breaths without adequate expiration in between, This will cause increasing transpulmonary pressures and tidal volumes, potentially harming the lung and increasing the risk of ventilator-induced lung injury.
On the other hand, reverse triggering can lead to muscle fiber damage due to excessive activity and eccentric diaphragmatic contractions during expiration. While it might prevent muscle atrophy by maintaining some level of activity, the potential damage often outweighs this benefit.
Management of Reverse Triggering
The detrimental effects on both lungs and respiratory muscles highlight the need for careful management of reverse triggering in ventilated patients. Recognizing respiratory entrainment as a distinct form of patient-ventilator dyssynchrony is crucial because its management differs significantly from other types of dyssynchronies. To address respiratory entrainment effectively, consider the following strategies:
Avoid Increasing Sedatives: Since entrainment is more likely during sedation, increasing sedatives is not advised as a treatment method. While it might seem intuitive to sedate a patient more deeply to reduce their respiratory efforts, this can actually exacerbate the issue.
Adjust Ventilator Settings:
Tidal Volume and Pressure: Changing the tidal volume or pressure can disrupt the entrainment. Entrainment is more likely with longer mechanical insufflations, lower flow rates, and higher volumes. Thus, reducing the tidal volume can help break this pattern.
Mandatory Respiratory Rate: Modifying the mandatory respiratory rate is another effective method. Increasing this rate can abolish the patient's neural effort and disrupt entrainment, while decreasing it may allow the patient's effort to begin prior to mechanical inflation, effectively breaking the entrainment cycle.
Alter Trigger Sensitivity: While reducing the ventilator's trigger sensitivity can prevent breath stacking, it does not eliminate the muscular effort or the negative effects of intrathoracic pressure. Therefore, this alone might not be sufficient.
Use of Neuromuscular Blockers: As a last resort, administering a neuromuscular blocker can be considered. This approach stops the mechanical consequences of reverse triggering by eliminating the patient’s respiratory muscle activity.
Reverse triggering in mechanically ventilated patients is a complex phenomenon requiring careful identification and management. Understanding its mechanism and consequences is crucial for effective treatment. The management strategies focus on disrupting the entrainment pattern through ventilator setting adjustments and, in severe cases, the use of neuromuscular blockers. Proper management is vital to mitigate the risks of lung injury and muscle damage associated with this condition.