Our hospital recently aquired the latest SERVOI. During company inservice, the rep. explained that flow and pressure trigger is adjusted in sensitivity, Putting the sens to (-2 or somewhere near) is the pressure trigger,(this is our old way of setting the sensitivity isnt?) and putting the sens to more than (0)but still on the green side of the sens is the flow trigger, Recently we have an anesthetist who sets the sens to the RED mark like oversensitive on the old vent. ---Question is this right? please help me on learning more about flow trigger vs. pressure trigger.
Setting pressure trigger at -2 cmH2O is an old habit that needs breaking! Every trigger setting, flow or pressure, should be adjusted to the point of auto-triggering and then backed off just enough to stop auto-triggering.
There are peer reviewed journal articles out there that document that when set correctly, pressure trigger can provide a faster response to patient effort than flow trigger, even when the flow trigger is also set correctly.
Regarding the peer reviewed articles on flow vs. pressure triggering, you stated that in some cases, correctly setting Pressure triggering was more responsive than flow triggering. The only scenario, I can think of where pressure triggering might be advantageous is if there is an excessive leak which would cause the ventilator to auto-cycle. I can't see how pressure triggering could possible be faster, since in most vents, the patient has to deflect the flow prior to than generating an additional negative pressure of -1 or -2.
Most of the published studies that compare flow and pressure triggering use a pressure trigger setting of -2 cmH2O, but set flow trigger at 1 or 2 L/min. And so therefore the pressure trigger response time stinks in these studies since the pressure trigger setting is horrible.
Brochard published an article: Am J Respir Crit Care Med Vol 157. pp 135–143, 1998 in which he compared pressure and flow triggering with each set at different thresholds. If you look at the comparison tables for response time, you will find that when set correctly (-0.5 cmH2O) the Newpoert Wave with pressure triggering (I work for Newport) triggered the fastest compared to all other flow and pressure triggers tested. Holbrook, in: RESPIRATORY CARE .OCTOBER '97 VOL 42 No 10, found the same results.
A ventilator's response to trigger is affected by many variables, including leak compensation, the ventilator's ability to control baseline pressure and flow for stability, etc. Using a flow instead of a pressure signal as a trigger input can not, in and of itself, make the response to patient effort faster. A ventilator's management of the trigger signal, how it handles baseline pressure/flow and how it responds to the trigger can.
When set appropriately, SOME pressure triggers are as fast or faster than flow triggers.
All my best,
I have not tested the Servo I but I know that the Servo 300 used to show an interesting characteristic. If you pinched off the pressure line to the pressure tranducer, the flow trigger would not sense. This may play into the fact that it is probably looking at pressure always. You do not want a ventilator autocycling because it creates dys-syncrony and work. I would ask the MD to breathe on the ventilator with a mouthpiece and set it up to autocycle. The expiratory work would be quite obvious. If the MD won't do this explain that an autocyling ventilator applied expiratory work to the patient and will actually increase the work of breathing. Good luck.
|<Gerry Smetana, MD>|
A presure triger set at .5 cmH2O is not clinically feasible. The autocycling that would occur presents a patient risk. Pressure triggering vs. incorrectly set flow triggering? I would hope that it responds faster.
It is my understanding that the original reason that PB put flow triggering on the 7200 was because the pressure triggering they had was deficient. And also then added a breath termination criteria of 2cwp PS. So it wasn't so much what happened at the begining of the breath, but what happened at the end.
I like to set a pressure sensitvity of negative 1, to start. Any more sensitive (be it pressure 0.5 of flow sensitivity) and you needlessly risk auto-cyling. You may cause auto-cycling and then back off, but what if the patient's cardiac osciallation increase between the check?
In the days of yore, when we "weaned" our patients on the constant borderline of frank fatigue, it might have made a difference in terms of them "treading air" how sensitive we set the trigger. But nowadays, if we're following the ventilator "weaning" and discontinuation guidelines, there is no reason not to keep the patient on robust full support (A/C in whatever PC/VC incantation floats your and their boat) and then just do a daily SBT. If the patient still has trouble triggering then maybe it's time to either unload them more, reduce auto-peep.
We may be chasing after technology (supere sensitivity) in service of a strategy (constant "weaning") we would be better off to abandon?
We had the same question come up at our hospital. Here is the whole story. The servo-i has a bias flow in the adult setting of 2 lpm. The trigger setting can be pressure or flow. In the pressure setting your patient has to pull a negitive pressure of what ever you have set to trigger a breath. If your patient is set up in the flow triggering side you patient pulls a percent of the bias flow to trigger a breath. If you have the flow trigger set at 5 (50%) your patient has to have a peak insp. flow of 50% of the bias flow ( 1 lpm)to trigger a breath. If the trigger is set in the red (70-100 %)your patient has to have a peak insp. flow of 600cc/min (70%) to 0cc/min (100%) to trigger a breath. As Dr. Smetana pointed out, allowing your patient to autocycle could pose problems for your patient.
Jeff made the point later in his comments that hits the nail on the head (auto-cycling). If you are in pressure triggering (-2) and you patient has auto-peep of say 5 cm/H2O.. then the patient would then have to pull back -7 to trigger the ventilator. When set to "flow" triggering, your patient now only has to pull back on the bias flow circulating in the circuit... (Much easier).
As far as setting the flow sensitivity in the red.... would be ok I guess as long as the vent wasn't obviously auto cycling, which if set in red is very possible. I am not sure why the Dr would want to set in the red... if the patient is unable to trigger the vent while it is in the green area, then something is wrong.
I know it's been a while since the last post on this topic, but to clarify old timer in the second to last post as to the flow triggering parameters I think the following should be noted. As he stated, bias flow in the adult mode is 2 lpm. Bias flow in infant mode for those of us who do peds is .5 lpm. He is correct in that a flow setting of 5 means 50% of the bias flow must be met in order to trigger a breath. However, the further right you turn the dial, the more sensitive the trigger is. At a flow setting of 1, you must generate 90% of bias flow, 2 = 80%, 3 = 70%,
4 = 60%, 5 = 50%, etc. Strange that the LEAST sensitive you can get in the infant mode is .4 lpm which leaves you wide open for autocycling.