Deranged Physiology » Required Reading » Respiratory Medicine and Ventilation » Chapter 2.5.4

Mechanisms of Ventilator Breath Triggering

This has come up in Question 11.1from the second paper of 2017 and in Question 18 from the second paper of 2015. The college wanted a thorough discussion of the mechanisms of triggering and cycling. Overall, the question seemed like a refugee from the primary exam. The trainees answered it well (maximum mark was 8.5 and 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
  • 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.

 

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.