Vent trigger issues

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December 25, 2006, 05:22 PM
Vent trigger issues
I am currently using The Drager(evita XL). I have noted patients(COPD/CHF) attempting to trigger extra breaths without success. I make all attempts(<I Time, suction, Bronchodilate, Flow trigger to 0.3, vent patient to atmosphere,exp ATC off)to improve their trigger ability without success. These patients do have some auto-peep, but should that matter in flow trigger? Any suggestions that I can try that I'm not already doing without increasing set peep(our docs are resistant of using higher peep)?


This message has been edited. Last edited by: GJ,RRT,

Chris Hanson RN, RRT-NPS, CPFT, AE-C
ER Registered Nurse
Grand Junction, Colorado
December 25, 2006, 07:53 PM
The answer is PEEP. You need to match intrinsic peep to help splint AW's open to allow for gas flow. I always had success with showing the Docs the intrinsice PEEP levels then explaing that all i want to do is match this level so the patient can actually trigger easier. I then increase the PEEP and show them the trigger abilities of the patient now vs before. good luck because PEEP is the answer.

December 26, 2006, 09:00 AM

Somehow I new that was the only answer. It's amazing that with flow sensitivity at 0.3 that the patient still can't assist. I have an uphill battle with moving into 21st century vent management at my current hospital. But, like you I enjoy respiratory and am willing to take it on for the benefit of my patients.


Chris Hanson RN, RRT-NPS, CPFT, AE-C
ER Registered Nurse
Grand Junction, Colorado
December 26, 2006, 02:16 PM
Unless it just changed and I do not believe it has, the Drager XL does not offer the option of flow triggering. That is according to the manual.
December 26, 2006, 03:43 PM
The Drager Evita is flow triggered ventilator; pressure trigger is not an available option.
December 27, 2006, 02:16 PM
Your issue with vent triggering in the Evita is that, correct me if I'm wrong, the Evita doesn't have an adjustable expiratory sensitivity/trigger setting.

The default expiratory trigger setting on ventilators are not appropriate for most COPD patients. Usual default setting is 25% of peak expiratory flow. This causes aysynchrony beacuse the patient is attempting to exhale while the vent has not yet terminated the breath. This causes an increased autoPEEP, missed or untriggered breaths and an increased respiratory muscle workload.

Choosing a correct expiratory sensitivity will correct the asynchrony and eliminate any missed breaths. Some COPD patients may require an expiratory sensitivity setting as much as 70%.

A good article that goes over this in COPD patients specifically is:
Tassauk D, Gainnier M, Battisti, et al. Imapct of Expiratory Trigger setting on Delayed Cycling and Inspiratory Muscle Workload. Am J Respir Crit Care Med. 2005; 172: 1283-1289.

So choosing a ventilator that has an expiratory sensitivity/trigger setting can be crucial in COPD patients. There is a good chance that it may even shorten their ventilator time.

Ventilators that I know have this setting are the Maquet Servo-i, PB 840, Viaysis Avea and Galileo Gold. The Avea setting only goes to 40% and may not be a good choice because some patients require >40%.

On the 840, PAV can also be used and it will automatically adjust the expiratory termination for the patient on a breath by breath basis. This is the ideal choice.

Also, any adjustment made to rise time will affect your expiratory sensitivity, so after a rise time adjustment you may need to adjust the expiratory sensitivity.

Hope this helps you and your patients. Let me know if you have any other questions.


December 27, 2006, 06:03 PM
The way I recall the Drager trigger mechanism being explained to me...

The Evita does not send out any continuous flow. Therefore it's not true flow triggering. When you enter the trigger amount it's when the pressure drops sufficiently that, in the vent's efforts to maintain peep, that volume of air leaves the vent outlet into the patient circuit. (I believe this is why the Drager can work better with some of the metabolic monitors?). So if CMV is the mode being used and flow trigger is off, Autoflow is on, it's really IMV as the patient can still breath spontaneously around the mandatory breaths.

Please correct if this is wrong.
December 27, 2006, 06:08 PM
Forgot to add..

If auto-peep is causing the patient's vent signal not to be sensed by the vent switching from pressure sensitivity to flow triggering isn't any fix at all. The Hamilton folks send out an Intelligent Ventilation newsletter which described how flow triggering led to cardiac beats triggering the vent on a potential organ donor, delaying donation as people thought the patient was actually breathing and triggering the vent. One of the RT students (I emailed her a copy) described how EXACTLY the same thing happened just recently at her current clinical site.

I have never seen a patient whom couldn't pressure trigger the vent (negative 1.0) then be able to pressure trigger. I have seen MANY times where flow triggering was too sensitive and people were vainly trying to fix the resultant resp alkalosis.

On the new vents pressure triggering (at least for adults) is not second fiddle to flow triggering. The old 7200 had lousy pressure triggering and so flow triggering was "the fix". The also added PS of two over baseline whereas just CPAP without flow triggering ended the breath at the CPAP level.
December 27, 2006, 06:17 PM
Below is from the Hamilton Intelligent Ventilation newsletter.

"Several issues of our e-news have addressed imposed WOB during mechanical ventilation due to triggering issues. We'd like to thank Mr. France for submitting this case study regarding 'inadvertent' triggering. Introduction: An 80-year-old black male was admitted to our 32 bed ICU after MI post- massive GI bleed. The patient was resuscitated at the nursing home and brought to the ER where he was stabilized and transferred to ICU. He was diagnosed with a significant anoxic brain injury. The patient was ventilated for 4 days before being declared brain dead and removed from mechanical ventilation. This case study is intended to document an incidence of self-cycling while using flow triggering (FT) on a ventilator. Case report: 80-year-old black male admitted with anoxic brain injury s/p GI bleed and MI. The patient has a history of prostate cancer, hypertension and Alzheimer's Disease. On admission vital signs were HR of 130, BP 100/60. Vent settings were: AC/VC+ mode, Vt 500, FIO2 1.0, PEEP 5, Rate 12 and Flow Triggering of 3lpm. ABG results obtained at 2315 were pH 6.99, PCO2 56, PaO2 66, BE –18, HCO3 14 and O2 saturation 85%. Set RR was increased to 16. Blood gases obtained at 0310 were pH 7.31, PCO2 36, PaO2 505, BE –8 and HCO3 18 with Saturation of 100%. EEG was obtained on with a finding of "Severe hypoxic encephalopathy with severe grade 3 suppression". No response to photic stimulation. Electrocerebral silence could not definitively be established". EKG showed sinus tachycardia with a HR of 139. Because of the inconclusive EKG and the presence of spontaneous respirations it was thought that the patient had some brain stem function left. Over the next two days the patient spontaneous rate averaged 24, 8 breaths greater than the set rate. On day four it was noted that the patient heart pulsations were visible through his chest. This coupled with the fact that he exhibited all the signs of brain death except for the presence of spontaneous breathing, prompted a switch from a flow trigger system to a pressure trigger system. After this change, the patients RR dropped to the set vent rate, e.g. ˜spontaneous' breathing ceased. The patient's chest was palpated to see if it was moving. It was ascertained from the exam that the patient was indeed apneic. An apnea test was performed that was positive. Shortly thereafter he was declared brain dead.
Discussion: Flow triggering is an extremely sensitive trigger system that enables a patient to initiate a breath with less work than pressure triggering. Pressure trigger systems sense the negative pressure drop caused by the patient's inspiratory effort. Flow trigger provides continuous flow of gas running through the vent circuit during the expiratory phase, when the patient inhales the equivalent of the flow trigger setting a breath is delivered. This small flow change at the airway requires less effort than a pressure trigger system. For this reason FT is widely used as the default triggering system for mechanical ventilation. This case study was completed to bring attention to the possibility that a brain dead patient was ventilated for 4 days because the ventilator was being cycled by the patient's heart palpitations which can cause sufficient pressure changes in the thorax to cause small flow changes in the airway. These pressure changes may be sufficient to cause false triggering. (note, this is possible, but less likely with a pressure trigger system). Figure 1 above demonstrates heart rate oscillations within the ventilator pressure waveform while on flow trigger of 3lpm. Note spontaneous respirations. Figure 2 demonstrates heart rate oscillations, however because pressure trigger is being used auto- cycling does not occur. Because of this case we instituted a policy that all patients suspected of being brain dead must be assessed for spontaneous breathing on a pressure trigger setting to rule out autocycling."

I sent the above to one of our RT students (whom was then at another clinical site). She described exactly the same thing happening--an organ donation being delayed by flow triggering.

I personally have seen many cases where auto-cycling arises from flow triggering (or the quest to make the vent super sensitive, i.e. pressure sens. 0.5). The new vents have pressure triggering which is sufficient for adult patients. I have NEVER seen an adult patient whom couldn't trigger by pressure whom then could by flow.

The 7200's had screwy pressure triggering and so flow triggering was the fix. But they also added PS when you got flow triggering, so that was the real benefit.