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With advanced monitoring, including esophageal pressure, we can confirm the upward deflection on the flow waveform at the onset of exhalation, and a bit of breath-stacking at the end (arrow) is due to a patient effort (downward deflection in esophageal pressure that corresponds with the abnormal slope in expiratory flow) that occurs after a machine delivered breath .
Based on these findings, the pattern is referred to as respiratory entrainment, or reverse triggering. A machine delivered breath (not triggered by the patient) elicits a response (effort) by the patient, and this occurs during, or very shortly after, insufflation by the ventilator, similar to a reflex. This typically happens when patients are heavily sedated or transitioning out of sedation. The ventilator will indicate that this is fully controlled ventilation.
Although the final breath (#5) is patient triggered (resulting in some breath-stacking), the classification of reverse triggering does not require a breath to be delivered, it simply is referring to the patient effort as a response to a machine breath.
Possible corrective measures
One approach (usually my first attempt) is to lower the respiratory rate (I choose 10 breaths per-minute). If a patient effort is truly being ‘triggered’ by the ventilator, then lowering the respiratory rate will result in the patient following the machine rate (the stimulus of the machine delivered breath changes and so does the patient’s response). However, sometimes the patient has a respiratory drive that is sufficient enough to now trigger every breath. The clinical question then becomes, is their underlying drive sufficient enough to maintain an acceptable minute ventilation and can they now transition to a spontaneous mode?
If the patient does not have sufficient drive and a higher respiratory rate is required (one in which reverse triggering is present), sometimes a slightly higher tidal volume (if safe to do so) or an increase in iTime, and using pressure assist-control can help limit full breath-stacking (the same volume twice) when it is occurring.
Because this often happens at the transition between deep sedation and awakening another simple solution is to reduce or stop sedation so that the patient can trigger all breaths (and you can stay on pressure controlled with a low back-up rate or switch to pressure support).
Should we intervene at all?
At this time we don’t truly know the potential harms (or benefits) of reverse triggering. Continued inspiratory effort at the onset of exhalation can result in eccentric contraction of the diaphragm, and can also lead to potentially injurious distribution of ventilation. Since reverse triggering is more common in patients receiving sedation, the response of increasing sedation is less likely to resolve the issue, and the harms of heavy sedation and prolonged mechanical ventilation are more established than those of reverse triggering. Also, studies have shown that patients with a higher prevalence of reverse triggering were more likely to be switch to a spontaneous mode or be extubate within 24 hours (transitioning out of sedation).
One potential reason for intervening with neuromuscular blockade would be in the case of severe ARDS and persistent breath-stacking caused by reverse triggering as this can make control of tidal volume and plateau/driving pressure more difficult.
Asynchrony often results in waveforms we don’t understand, and would like to correct. Oftentimes we can make slight corrections to make the waveforms look nicer, or prevent undesirable things like breath-stacking. At the end of every strange waveform is a patient and we have to always ask ourselves, if we can’t correct it, should we increase sedation or paralyze to prevent it? The answer to this question is almost always no! Allowing the patient to regain respiratory control by being more awake is the more responsible approach, even if we acknowledge that sometimes it is not (i.e., severe ARDS).
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