Reverse Triggering

Reverse triggering is a type of dyssynchrony that occurs when a patient effort occurs after (‘is triggered by’) the initiation of a ventilator (non-patient triggered) breath.  Usually, it is a phenomenon occurring over many consecutive breaths and also referred to as ‘entrainment’.

Diagnosis

The visual detecting of reverse triggering is slightly different between modes of ventilation.  The recognition begins with the fact that the ventilator breath was not triggered by the patient.

Detecting Effort During Ventilator Insufflation

The ‘triggered’ effort frequently starts somewhere during the insufflation and continues during the expiration. The detection of patient effort that is ‘triggered’ by a ventilator breath is simpler to detect during pressure-controlled ventilation, because the flow will change when the effort occurs early enough in the inspiratory phase (FIGURE 1; red arrows).  With this easier recognition comes the potential for increased tidal volume above your target (if the goal is lung protective ventilation).  During volume-controlled ventilation the effort can be detected in the pressure-time tracing if flow is constant since flow will not usually change in response to patient effort (FIGURE 2).  If the breath is late enough during inspiration and there is a pause set, you may see a loss of plateau (FIGURE 2; red arrows).  It can be difficult to diagnose when flow is set high enough to match the patient effort because the pressure may not drop sufficiently enough to be obvious, but If effort continues into the expiratory phase, it is usually noticeable in the expiratory waveform (next section).^{1, 2}(FIGURE 2; yellow arrows)

Figure 1: Noticeable flow changes during the inspiratory phases commonly seen when reverse triggering occurs during pressure-controlled ventilation (red arrows).  The yellow arrows demonstrate the continued patient effort detected in the expiratory flow waveform.

Figure 2: Noticeable pressure (plateau) changes during the inspiratory phases commonly seen when reverse triggering occurs in volume-controlled ventilation (red arrows).  The yellow arrows demonstrate the continued patient effort detected in the expiratory flow waveform.

Detecting Effort During Ventilator Exhalation

A common finding with reverse triggering (when patient effort continues after insufflation ends) is alteration of the peak expiratory flow at the onset of exhalation (FIGURE 1&2; yellow arrows). The alteration in flow is like that of short cycling; in fact it technically is short cycling because the machine breath terminates while the patient is still making inspiratory effort. 

When does it happen?

The presence of reverse triggering is likely more common than previous thought.  It appears in (heavily) sedated patients, with or without lung injury.  It seems particularly common in those transitioning from sedated to awakened states.  However, reverse triggering has also been described in patients after brain-death.³  Therefore, the exact mechanism of reverse triggering is unclear at this time, and there may in fact be multiple mechanisms.

Correction

Reverse triggering poses a unique challenge when it comes to correction.  Because it normally occurs in patients heavily sedated, the decision must be made whether sedation is still necessary (plan to liberate the patient from the ventilation), or whether the patient is still in the acute phase combined with the breath profile (ex. presence of breath-stacking).  It should be noted that increasing sedation will often not resolve reverse triggering, and paralysis may be necessary if lung injury is of concern.

Reverse triggering and risk of injury

Reverse triggering is a cause of ‘double breaths’ and breath-stacking; it is likely to be injurious if the ventilator delivers a second breath (Figure 3).  The selection of mode does not seem to change the resulting effects (Figure 4).  One solution can be to turn down the set rate and decrease sedation. It has to be differentiated from short cycling and ’double triggering’ where the patient may need lengthening the inspiratory time and / or increased sedation. See (Figure 5) for differentiating the two.

Alternatively, large efforts resulting from reverse triggering could be injurious as it can be eccentric contraction when occurring during the expiratory phase, and may result in pendelluft (movement of air from one region of the lung to another) and excessive regional lung stress when it occurs during the inspiratory phase.^{4, 5}

Figure 3: In this example the patient effort starts long enough after the machine breath, that it results in an additional breath being delivered by the ventilator. Top waveform is pressure, middle is flow, bottom is esophageal pressure.

Figure 4: In this example, effort begins just prior to cycling to exhalation, and the effort is strong enough to result in an additional breath delivered by the ventilator.  Even by using time cycled instead of flow cycled mode, it would be difficult to prevent the breath-stacking unless the iTime was greater than 2 seconds. Top waveform is pressure, middle is flow, bottom is esophageal pressure.

Figure 5: Breath-stacking:  On the left is short-cycling, the patient initiates the first breath.  On the right is reverse triggering, the machine breath is delivered, and patient effort follows after. Top waveform is pressure, middle is flow, bottom is esophageal pressure.

Final Thoughts

Reverse triggering is a common but likely under-recognized or mis-interpreted form of patient-ventilator dyssynchrony.  Its mechanism is still unclear but could include more than one.  Whether the form of dyssynchrony is harmful or beneficial still needs further study, but likely depends on several factors such as the strength of contraction, the phase of breath in which it appears, and whether a second breath is delivered (breath stacking).  Currently, we are undertaking a large multicenter observational study to learn more about dyssynchrony, particularly reverse triggering.  However, much work still needs to be done to understand this complex phenomenon.

Suggested Reading

1.           Akoumianaki E, Lyazidi A, Rey N, Matamis D, Perez-Martinez N, Giraud R, et al. Mechanical ventilation-induced reverse-triggered breaths: a frequently unrecognized form of neuromechanical coupling. Chest 2013;143(4):927-938.

2.           Murias G, de Haro C, Blanch L. Does this ventilated patient have asynchronies? Recognizing reverse triggering and entrainment at the bedside. Intensive Care Med 2016;42(6):1058-1061.

3.           Delisle S, Charbonney E, Albert M, Ouellet P, Marsolais P, Rigollot M, et al. Patient-Ventilator Asynchrony due to Reverse Triggering Occurring in Brain-Dead Patients: Clinical Implications and Physiological Meaning. Am J Respir Crit Care Med 2016;194(9):1166-1168.

4.           Yoshida T, Nakamura MAM, Morais CCA, Amato MBP, Kavanagh BP. Reverse Triggering Causes an Injurious Inflation Pattern during Mechanical Ventilation. Am J Respir Crit Care Med 2018;198(8):1096-1099.

5.           Pham T, Telias I, Piraino T, Yoshida T, Brochard LJ. Asynchrony Consequences and Management. Crit Care Clin 2018;34(3):325-341.