[indaemed] Re: Re NMS
- From: USM Bish <bish@xxxxxxxxxxx>
- To: indaemed@xxxxxxxxxxxxx
- Date: Sun, 14 Mar 2004 20:33:34 +0530
On Sun, Mar 14, 2004 at 06:19:58PM +0500, anujc@xxxxxxxx wrote:
>
> I have been reading with interest the discussion on the
> evaluation of a case of NMS and the correct disposal. This is
> not the first time that such a discussion has taken place. For
> my part as a physiologist I would like to share my
> understanding of the head up tilt test and its relevance in
> such cases. This is important because an opinion was voiced
> suggesting that the test should have been repeated for the
> cadet in question and so on. The following points of view are
> offered. The head up tilt test is not the gold standard for
> diagnosis of vaso vagal syncope In a large number of cases the
> test may actually be normal and vice versa. The diagnostic
> yield of this test is about 65- 70% ...
>
[rest snipped]
>
The point that you make is very valid. However, the point here
is a bit different. It must be clearly understood that Acute
Hypoxia induced LOC is NOT VVS. It is a normal response of the
body on exposure to Critical levels of Hypoxia. It is a
physiological response of the body to stresses well beyond
compensatory mechanisms.
The time for total loss of consciousness would vary from person
to person and the gap between TUC (Time of Useful
Consciousness) to LOC is not well defined/ measured. TUC is the
parameter which has been recorded in various studies, and
normally under Hypoxia training flights this is the stage when
re-oxygenation is commenced. If my memory serves me right, the
50%ile value for TUC at 30,000 ft is about 125 sec or so with a
SD of about 40 or so. In which case, in about 5% of NORMAL
individuals TUC would be expected to be less than 10 sec. In
such susceptible individuals, it is my guess that the gap to
LOC is less, and therefore, LOC may be expected within 10 - 15
sec. The duration at 30,000 ft was missing in the Original Post
(OP), so things are left to conjecture.
The diagnosis of VVS would normally be based on:
a) CVS evidences like severe bradycardia, SSS, ECG abnoramali-
ties, arrythmias, episodes of asystolic spells or anything
affecting cardiac output (even transiently/ momentarily).
b) Neurological abnormalities like asynchronous epileptogenic
discharges need exclusion. I am assuming here that things
like Hysterical Reactions are ruled out by circumstantial
evidence.
c) ANS studies (of which HUT is one) or even things like cold-
pressor, vagal stimulation etc would give some indication.
A positive test would definitely indicate ANS insufficiency.
Even if a negative test is inconclusive.
d) Concurrent minor ailments like viral episodes, or major pre-
disposing factors like anaemia at the time of the LOC must
also be excluded. What was his ventilation state like just
before the LOC ?
If all these have been done (maybe even repeated), and has been
seen to be normal, the diagnosis of VVS is NOT tenable.
Shortented TUC or shortened LOC latency under acute hypoxia
episoded is NOT sufficient to label a case as VVS. Low Hypoxia
tolerance, yes. However, that would also need all physiological
parameter records within compensatory phase of hypoxia to just-
ify such a label, not at critical+ levels of 30,000 !.
The diagnosis of VVS should be established INDEPENDENT of these
hypoxic LOC episodes ... Shortened TUC or LOC on acute hypoxic
exposures beyond critical phase is NOT a ground for rejection
from flying duties.
The details furnished, based on which we are progressing with
our discussions perhaps lacks full details. It would be nice if
full details can be furnished by anybody in knowledge of the
case.
Frank as ever ...
Bish
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