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.