[indaemed] Re: Syncope yet again
- From: Sanjiv Sharma <sanjivshrma@xxxxxxxx>
- To: indaemed@xxxxxxxxxxxxx
- Date: Sun, 14 Mar 2004 21:04:54 +0530
> Dear Bish Sir and all, I fully agree of what Bish sir has said. Diagnosis of
> VVS should have been corrobrated by Tilt Table studies or any other objective
> criteria apart from LOC at 30,000 feet. I am in no position of objecting to
> the disposal but few points
> (a) scientific deliberations on this subject(if at all they were done)
> should be made public to the aeromedical as well as aircrew community for the
> benefit of one and all.
Dear List
thanks for all the valuable inputs about the case under discussion. I had
floated this query on Aeromed List as well. Here is the complete feedback,
starting with my query. The red script and bold is to emphasise the points. I
haven't edited any matter, except deleting the names.
My question now is, do we need to rethink about this case as far as the present
diagnosis is concerned?
May also request the Seniors, if they would like to offer their comments, for
the benefits of learning for all.
with warm wishes
Sanjiv
PS: Pl forgive me if the mail is too long - but I felt it imperative to get the
indaemed list on board the other discussions.
Those already on aeromed list may please forgive my indulgence.
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(1) a trainee military pilot, with an old history of solitary episode of
Vasovagal Syncope, has a brief episode of loss of consciousness at 30,000 ft
during hypoxia indoctrination in an altitude simulator. Regained
consciousness shortly after 100% oxygen was switched on. Subsequent test,
the next day, at 30,000ft was uneventful.
However while undergoing medical evaluation at another Centre, had another
brief episode of loss of consciousness at 30,000 ft during hypoxia
indoctrination in an altitude simulator. His previous investigations,
including tilt-table test ruled out any cardio-neurological pathology.
My questions are:
Must Hypoxic syncope at 30,000 ft be viewed in isolation, or continuum of
neurally mediated syncope? After all, at 30,000 ft, he is in the Critical
Stage of Hypoxia, where loss of consciousness is known to occur, with
instant recovery with breathing of 100% oxygen.
If all his investigations remain inconclusive, can he continue his flying
training? After all, even if he is trained in an unpressurised propeller
aircraft, he does not exceed 8,000 ft. And the aerobatic manoeuvres are
short duration of the magnitude of 4-5G. Can he be risked solo sorties to
complete basic flying training for a military aviation career.
I would welcome the comments and advise of the List.
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(2) It is my understanding that solitary vasovagal syncope is relatively common
(in the order of 10 - 15%) in the males of our population. I do not know
whether the same applies to India but will assume so.
Loss of consciousness is a perfectly normal response to 30kft altitude. The
Times of Useful Consciousness we study and quote are based on population
data and there are always going to be folk who are more, or less,
susceptible to hypoxic exposure.
The important question then becomes were these recurrent vasovagal syncopes,
acute hypoxic incapacitation, or incapacitation from other cause. The
response to oxygen briefly described suggests you do not think vasovagal
recurrence is likely.
Then, in the absence of other pathologies likely to lead to syncope in this
population (such as primary or secondary convulsion, arrhythmia,
recreational drug use, reasons for emboli, cerebrovascular disease,
significant pulmonary pathology messing up gas transfer etc etc etc), what
would be the logic behind precluding this fellow from military aviation? I
suppose there could be a tentative argument that he's failing in his hypoxia
training ... not able to identify premonitory symptoms early enough to
implement remedial action. There could also be the argument that he is
unacceptably susceptible to hypoxia.
I think that either of these arguments would be rather difficult to support
with terribly robust argument (and evidence). While they may be true are
they really significant hazards to flight safety?
You could do all sorts of altitude chamber experimental work on the fellow
but I can't think of what that'd achieve that would be reasonably useful.
Based on the information you've provided I'd be looking towards excluding
pathology and then returning him to unrestricted flight duties ... perhaps
with a briefing that he appears to be more susceptible than most to hypoxia
and should manage his flying career with the highest regard for altitude.
My answer to the next question is "Yes". If you exclude the pathologies I'd
be happy to fly with him :-)
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(3) Dr Sharma is asking about an aircrew trainee who lost consciousness,
exposed to 30,000 ft in an hypoxia training session.
I would have been worried if he had NOT lost consciousness !
I cannot help but wonder what the aircrew student has to gain from such
a training procedure. Exposure to 25,000 ft. gives a progressive
incapacitation which the trainee may or may not recognise in himself but
can certainly recognise in his peers.
The classical rationale of this training is to make the aircrew aware of
the dangers and give him some idea of what to watch for in himself.
At 30,000 ft. the onset is too quick to appreciate symptoms or follow
complicated emergency procedures. Perhaps it will stress to transport
crew with an emergency loss of cabin pressure, the need for Oxygen ON
and GET DOWN.
Fast jet crew will already be on oxygen at this altitude and it would
require a double failure case, cabin pressure AND oxygen to produce a
real emergency.
The biggest risk to the aircrew from this procedure is that some Doctor
will come along and say you are unfit for flying.
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(4) During my service as a USAF Aerospace Physiologist, we took students to
30,000 feet (and 35,000 feet) in the altitude chamber during their initial
altitude physiology training. The objectives (as I saw them) were three fold.
Allow the students to: 1) experience and deal with problems associated with
high altitude exposure (trapped gas, etc...) so the first time they dealt with
these issues was not when the aircraft decompressed, 2) experience pressure
breathing under realistic conditions (and prove that it worked), 3) observe a
controlled demonstration (one student off O2 and closely monitored by the
inside observers while the others watched) of how quickly incapacitation due to
hypoxia occurred at 30,000 feet (1 - 2 minutes) versus 25,000 feet (3 - 5
minutes). Of course, hypoxia occurs even more quickly at 35,000 feet (0.5 - 1
minute). I emphasize the "controlled demonstration" description above. The
student was either instructed to recover or recovered by an instructor when he
/ she appeared to have exceeded his / her time of useful consciousness.
Unconsciousness was (and remains) undesirable and was carefully avoided.
I believe this demonstration was valuable because the students could see first
hand that the progression of symptoms (while still orderly) was alarmingly
faster at 30,000 feet that at 25,000 feet, even though the altitude change was
only 5,000 feet (this certainly reinforced that portion of my classroom
instruction). This realization emphasized the need to expeditiously execute
the hypoxia recovery checklist whenever hypoxia was suspected by the aircrew.
I personally, experienced onset of hypoxia and the "demonstration subject
volunteer" and it was an eye opener for me.
Finally, I believe each aerospace physiology training professional should
carefully weigh the risks of high altitude exposure with the training benefits
to be gained by such exposure, then make their decision (fortunately, there is
a great deal of information available). As with many things in our business,
this area is not necessarily black and white.
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(5) Unconsciousness during a hypoxia demonstration at 30,000 feet in the
altitude chamber with immediate and uneventful recovery upon administration of
100% Oxygen and no other complications is, what I would consider, normal.
Personally, I wouldn't view this (with the absence of any other medical
problems) as an indication of a fitness for flying problem. However, I would
defer the ultimate decision to an aerospace medical practitioner (flight
surgeon or equivalent).
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(6) As a practicing neurologist, I agree with the comments of others that this
is a benign situation which should not have implications for this fellow's
military flying. At worst there might be an unusually abrupt response to
hypoxia at that altitude in this individual as compared to others. Ross, in
his monograph on syncope, in describing benign syncope stated "Physiologic
systems are not diseased, but momentarily defeated." I feel that statement is
appropriate to this situation.
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(7) If we subject airmen to altitude chambers to enlighten them to the effects
of hypoxia, and knowing LOC is a result in a specific time frame why the alarm
when an airman with otherwise normal physiology reacts as would be predicted?
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(8) When there is a clearly demonstrated link between the event and the
cause (temporal) and a recovery with O2 (arguably showing causation) AND
there is no other pathology, one would be justified in saying that the
cause was hypoxia. If so, we must return him to the cockpit.
That seems to be what the list thinks, and I think that if this had
happened to a 747 captain during a rapid decompression, we would have
accepted that as the problem, and reflighted him.
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(9) I have just read comments on the 30,000 ft. hypoxia run with great interest.
I still feel that it depends on what stage of training you are
completing, as to whether it is worth going to 30,000 ft. rather than
25,000.
For initial students and personnel flying transport aircraft, I believe
that 25,000 ft.is quite high enough to demonstrate the effects and
pre-oxygenation is not necessary (my experience dates from 1956 to
1988). This does not expose the aircrew to unnecessary risks of pressure
change. ( I chose the phrase carefully !).
For fast jet aircrew, , if you need to demonstrate pressure breathing, a
rapid decompression to 35,000 ft. (using an American regulator) is
advisable at least once in a flying career. Pre-oxygenation should be
used for this.
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(10) While I have to agree that this trainee has proved to be normal in all
respects, and there are perfectly understandable and justifiable reasons
for his loss of consciousness, doubts still plague my mind. I will share
these doubts with you and would seek your opinions.
He has lost consciousness on three occasions in the past three months -
albeit under stress.
His course mates undergoing the same stresses have not had a single
episode of loss of consciousness in the same period.
Moreover, the training academy has been giving these hypoxia runs at
30,000' for many years and the protocol is well established. LOC during
this hypoxia run is rare. A quick chat with one of the operators indicated
that in the 180 or so runs that he has given this is the only case of LOC
(0.5%). Two such events in the same person are rarer still.
Keeping the above in mind, are the others 'more normal' in an Orwellian
sense, than he?
Also, is there a way to ensure that he would not have a similar episode of
LOC during a stressful event in the future? Consider the following
scenarios:-
He is undergoing combat training - in summers in a tropical country - the
ambient temperatures are 40 deg C - the G is high and the combat is at low
levels - what happens if he loses consciousness.
He is flying a transport aircraft at 33 kft. There is a fire in the
cockpit - relative hypoxia due to the fire and some hypercapnea plus the
stress of the emergency - what happens if he loses consciousness. Would it
turn a potentially retrievable emergency into an irretrievable one?
I agree that such situations would be rare - but would we be justified in
"taking that risk on his behalf"? Maybe I am being overcautious -
piggishly so - but as I said these are doubts plaguing me - what would you
say??
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(11) I would just say that this guy is at the lower extreme of Gaussian
distribution in terms of tolerance to hypoxia (somewhere between 1st and 5th
percentile). The MEAN time of useful consciousness at 30,000 feet is
described as being equal to 145 sec with an SD of 45 sec (Dave Gradwell et
al in Ernsting). The lower end of the RANGE for 95% population would be mean
minus 2SD, which brings it down to 55 sec. For 99% population, it would be
mean minus 3SD, equals10 sec. Please remember that we are still in the
"normal" range. The point is that TUC at 30kft can very very short even for
some healthy people. I suggest we should not be surprised if we see a a
subject fall over quickly (at 30kft) once in a while.
We conduct hypoxia training at 25kft, and still see people fall over
sometimes. Not often, but it does happen. We accept that as a "normal"
occurrence.
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