Tasty Morsels of Critical Care 068 | Ventilator Triggering

Welcome back to the tasty morsels of critical care podcast. Today we are going to talk about triggering on the ventilator. Now given the ubiquity of the word “triggering” in contemporary discourse I must confess that i do find it quite “triggering” to walk up to a vent and see the pressure support set at 11 or some other horror show like a PEEP of 7… I mean, who would do such a thing. But let me clear we are talking about a very different type of triggering. If i was on a ventilator and somewhat engaged in the process of respiration at least at a brainstem level, I would feel a much more content if the ventilator cycled to inspiration whenever I requested it to. Indeed I would also find myself greatly contented if said ventilator did not randomly produce new inspirations any time it detected the slightest change in airway pressure. All of this is dependant on ventilator triggering. Let’s start with the basics, the ventilator can be triggered to cycle to inspiration in a number of ways: * time (in the case of mandatory ventilation, in fairness this is not really a trigger as the patient has no input) * pressure trigger. The patient must produce enough negative pressure in the circuit to trigger the vent * flow. The patient must produce a certain amount of inspiratory flow in the circuit to trigger the vent My experience has been overwhelmingly with the ubiquitous servo ventilators found in many ICUs in Ireland. On the servo-i when you scroll through the menus you’ll see a dial for trigger. This dial is defaulted to flow trigger with a dimensionless number from 1-10 based on a proprietary software from Maquet. The more clockwise you turn the knob the lower the flow in the circuit the patient has to generate and therefore the easier it is to trigger inspiration. Swing it all the way right for the poor GBS patient who struggles to trigger. As the dial is turned left (or anticlockwise) then the trigger will magically switch to a pressure trigger with actual numbers in cm H20. These define the negative pressure in the circuit that has to be generated before the vent will trigger a breath. Thus flow triggers are easier for the patient and pressure triggers harder. But when would you ever want to make the trigger harder for the patient? Typically it’s not actually that you want to make it harder for the patient, it’s more that you want to avoid autotriggering. A good example of auto triggering is commonly seen in the patient who has become dead by neurological criteria. The story at handover will typically be a  devastating brain injury with some haemodynamic instability and loss of pupilary and cough reflexes but the trainee notes that brain death cannot have occurred because they are still triggering the vent. In this scenario it is quite common for the ventilatory to be auto triggering due to the minor fluctuations of flow within the circuit caused by the substantial cardiac oscillations of the hyperdynamic circulation of the person undergoing  brain death. Simply switching from a flow trigger to a pressure trigger typically eliminates these auto triggers. Alternate sources of auto triggering can be the big air leaks of a bronchopleural fistula or a water logged circuit with a meniscus of rained out water oscillating back and forth in the tubing. Failure of triggering is very common. In this scenario there has been a neurological trigger that may have even initiated some diaphragmatic contraction but it was missed by the ventilator. An oesophageal balloon is probably the gold standard here and you can use it to see if a negative deflection on the balloon is matched by a breath. In the absence of a balloon (and aren’t we all?) we have to use some surrogates. It’s hard to detect but in some patients you can see a -ve deflecti...