Mechanisms of ventilator breath triggering

By Alex Yartsev - Dec 18, 2015

This has come up in multiple past paper questions:

Question 6 from the second paper of 2020
Question 11.1from the second paper of 2017
Question 18 from the second paper of 2015

The college wanted a thorough discussion of the mechanisms of triggering and cycling, or a table comparing the advantages and disadvantages. Overall, some of these questions seemed like refugees from the primary exam. The trainees answered them well (maximum mark for one of them was 8.5, with 77% passed), but the college still complained that "overall there was a lack of knowledge". Triggering of ventilation is subjected to a more thorough dissection in the Mechanical Ventilation section among the core topics. The best published resource on this topic is probably the 2011 article by Catherine Sassoon. Here, only a brief summary is offered. As many of these things, it works best in a tabulated format.

Triggering method	Mechanism	Advantages	Disadvantages
Pressure	triggered by a patient-generated drop in pressure, from PEEP.	Prevents cardiac auto-triggering By gradually increasing respiratory workload, one may theoretically "train" the respiratory muscles to perform more work May be useful as a part of extubation assessment (a high pressure trigger is lke a quasi-MIP measurement)	Requires the patient to inhale against a closed inspiratory valve. This increases the work of breathing. Tracheal triggering is apparently better than conventional, but The patient may not be able to generate such pressure, and may be unable to trigger Between the initiation of effort and the actual delivery opf gas, there is a delay (however long it takes for the patient to generate that sort of pressure)
Flow	Triggered by a patient-generated change in fresh gas flow though a circuit	Little effort is required: the patient's respiratory workload is decreased (compared to pressure triggering) Rapid triggering: little delay between initiated effort and the triggered breath	May have auto-triggering by cardiac oscillations Still no rapid enough (some delay exists between initiated effort and the delivered breath)
NAVA	Triggered by a change in diaphragmatic EMG, detected by a properly positioned electrode array on a special NGT	Least amount of patient effort is wasted Patient-ventilator synchrony is improved compared to PSV Dynamic hyperinflation is prevented	A special NG tube is required It must be positioned correctly; if it is slightly dislodged the system does not work.

NAVA is probably the only element of this table which needs to be explored in slightly more detail. Most trainees this side of 2010 will not have seen one of these, nor even ever heard of them. NAVA devices are, however, still available, and the elderly examiner will be usually be tickled pink by the mention of them. It is unfortunate that NAVA, which should theoretically make weaning faster and easier, does not seem to do either of those things. Demoule et al (2016) implemented it in an RCT and determined that it is "safe and feasible" but could not find any statistically significant outcome differences, other than a predictable decrease in the incidence of patient-ventilator asynchrony

References

BANNER, MICHAEL J., PAUL B. BLANCH, and ROBERT R. KIRBY. "Imposed work of breathing and methods of triggering a demand-flow, continuous positive airway pressure system." Critical care medicine 21.2 (1993): 183-190.

Sassoon, Catherine SH. "Triggering of the ventilator in patient-ventilator interactions." Respiratory Care 56.1 (2011): 39-51.

Demoule, A., et al. "Neurally adjusted ventilatory assist as an alternative to pressure support ventilation in adults: a French multicentre randomized trial." Intensive care medicine 42.11 (2016): 1723-1732.

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