The Case:
Neurologically devastating injury. Radiographically, brain death is beyond plausible. All sedatives have been off for 5 half-lives. There is no concern for prior drug use before admission.
Exam demonstrates:
👉No response to noxious stimulus
👉No pupillary response
👉Absent corneals.
👉Absent oculovestibular and oculocephalic reflexes.
👉No gag.
👉No cough.
👉There is no movement to deep stimulus in any extremity.
The vent rate is set to 14 and yet…….the vent is delivering 27 breaths/min:
It’s tempting to say at this point “well, the patient is still over-breathing the vent. We’ll assess candidacy for brain death testing later”
DO NOT DO THIS. The “over-breathing-the-vent-patient” may still be dead by brain criteria!
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Wait…. What?
Let’s be clear: To be dead by neurologic criteria a patient must have cessation of ‼️WHOLE‼️ brain function — including the absence of respiratory drive.
🚨No breathing allowed!🚨
So then — what’s going on here?
Potentially, auto-triggering.
There are, as you can see, lots of reasons for vent auto-trigger. The most important cause for the brain dead patient is cardiogenic oscillations.
To understand why, its important to understand a little bit about the physiology of brain death:
Brain dead patients are frequently hypotensive & volume contracted from the combination of dehydration (diabetes insipidus & frequent use of mannitol in these patients). Plus, there is loss of arterial and venous sympathetic tone causing profound vasoplegia.
While there is no parasympathetic input to the heart (vagally mediated via the medulla), there is still sympathetic innervation to the heart as the sympathetic system has extra-cranial circuits. Frequently, because profound hypovolemia and this sympathetic input these patients have a hyperdynamic cardiovascular state.
☑️ Note that you can test the lack of parasympathetic innervation by giving atropine, which will not produce more than a 3% change in baseline HR. This has a good correlation with TCDs for cerebral circulatory arrest (PMID 10872134).
☑️This was actually a proposed test to be include in the criteria for brain death (PMID 3969749), but is not used as the atropine test may fail to produce a HR rise with isolated brainstem injury. This means that a lack of response to atropine does not always equate to whole brain death.
The result of the hyperdynamic precordium is a beat-to-beat change in intrathoracic volume (PMID: 22516437).
Couple these physiologic changes with the fact that the chest wall muscles are denervated (and therefore less resistance to lung expansion): the result is a beat-to-beat transpulmonary pressure gradient, which is enough to move air within the large airways (PMID: 16157923.)
Which tells the vent – “patient is trying to take a breath!”
Simplified diagram:
The outcome of this is:
“BREATH” IN THE ABSENCE OF RESPIRATORY DRIVE
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Fixing this:
Since the problem is that the vent is overly sensitive, the solution is to make it a bit harder for the vent to auto-trigger.
A generic flow trigger is set 2-3 L/min, which is a sensitive setting. One study looking at the incidence of this occurring found that it was much more common in flow trigger modes and the lower the trigger, the more breaths were generated (PMID: 10708174)
Therefore, if you adjust the flow trigger to a higher threshold (say 4-6 L/min) or change to a pressure threshold, the “over-breathing” often goes away.
But, what if the new trigger settings are just too high a bar?
It's a fair question. We’d never want to pronounce someone brain dead without complete certainty. This is NOT at all a substitution for apnea testing, which is how respiratory drive is formally assessed. This is only meant to prevent withholding formal testing for a patient that should otherwise qualify.
Is this really happening all that frequently?
Probably. It’s hard to assess the frequency. But it’s clear that this is not well taught as part of the guidelines in determining eligibility for brain death testing. Ventilator autotriggering is only mentioned in 3 of 15 official recommendations and guidelines on diagnosing brain death (PMID: 30810759) and is not mentioned at all in the Determination of Brain Death/Death by Neurologic Criteria update published recently (PMID: 32761206).
Which makes it crucial to remember.
Take Away:
Next time you are evaluating a patient for potential brain death testing, but it looks like they are “overbreathing” the vent, remember that it might just be the vent.
🌟 Change to a pressure trigger or less sensitive flow trigger and re-eval! 🌟
And remember that this is not a formal test, it is a screen to see if formal testing with the apnea test should be pursued.
The Video
More resources on Brain Death Testing
- IBCC Chapter on Brain Death
- PulmCrit – Brain Death Mimics
- Podcast 54: Organ Donation and Brain Death in the ED
A special thanks to Prem Kandiah, MD (@Capt_Ammonia) who has been a mentor and invaluable teacher in this and so many other topics.
Image from Fakurian Design From UpSplash
- NeuroEMCrit – 31 #NeuroPostItPearls - August 22, 2022
- NeuroEMCrit – IVIG 🆚 PLEX - May 3, 2022
- NeuroEMCrit – Does Vent Over-Breathing Always Disqualify Brain Death Testing? - December 27, 2021
Another awesome neuro emcrit post! Thank you !