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>It is well documented in aviation
medicine that night vision is detrimentally affected by hypoxia due to the
retinas high oxygen consumption. This occurs even at altitudes of
5000'. I wonder if there is a similar reduction in the ability to pick up
day-time traffic.
My understanding is that peripheral vision is affected at about
the same altitude as night vision.
>Here endeth the lesson.
Dave
Long
Amen - and thanks for a useful and thought provoking
contribution:-)
--
Brian Wade
Personal Computer Concepts
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----- Original Message -----
Sent: Monday, August 23, 2004 10:52
PM
Subject: Re: [Aus-soaring] Oxygen and
Pulse Oximetry
> A pulse oximeter is a device that measures blood oxygen
saturation. Sensor
> clips to earlobe or finger.
> Then you know
your saturation level and can adjust oxy flow to keep it
good
If
anyone is using a pulse oximeter it is important that they have
some
understanding of the physiology behind it.
Pulse oximetry is a
useful, non-invasive indirect measure of the partial
pressure
of
arterial oxygen, but it has limitations. It utilizes the
difference in
infra-red absorption of saturated (oxygenated) and
de-saturated
(de-oxygenated) arterial and venous blood.
The sensor has
to pick up a good arterial trace to read accurately. This is
very
dramatically influenced by movement and cold (and we know it can be
cold up
there!) Indeed, at times it can be wildly inaccurate.
The sensor
reads carboxy haemoglobin the same as oxy-haemoglobin, so you
smokers out
there full of carbon monoxide will artificially read a percent
or two
higher, while
actually carrying less oxygen.
Oxygen saturation is
only an indirect measure of the all-important factor of
arterial oxygen
tension. The normal healthy range is 96-99%. Decreases
below
this represent a very significant reduction in arterial
oxygen
carriage. One medical definition of respiratory failure is a
PaO2 of less
than 60 mmHg. This is reached at an altitude of about
10,000'. This
usually corresponds to a saturation level of about
90%. 90% sounds pretty
good compared to 96%, but actually represents
an alarmingly low arterial
oxygen level. At around 90 %, small
changes in saturation also reflect a
very large change in oxygen
level. Even one or two percent represents a big
difference, and that
is well and truly within the error limit of the
machine.
It is well
documented in aviation medicine that night vision is
detrimentally affected
by hypoxia due to the retinas high oxygen
consumption. This occurs
even at altitudes of 5000'. I wonder if there is
a similar reduction
in the ability to pick up day-time traffic.
Interestingly, the "oxygen
above 10,000' " rule does not take into account
density altitude.
This is based on an ISA of 10,000', ie 15 deg C at sea
level. On a
hot summer day of 40 deg at ground level, this is ISA+25, which
equates to
an extra 3000' of altitude. So at 10,000' you're actually
already at
a density altitude of 13,000, and starting to get quite hypoxic.
Remember
it's the number of molecules of oxygen (the density) that is
all
important.
One of the most common symptoms of hypoxia is lack of
insight and euphoria.
After a particularly good day's flying when it was 40
degrees on the ground
with thermals up to 14,000' one pilot without
supplemental oxygen remarked
"I felt a bit short of breath to start with,
but after a while I felt fine!"
Here endeth the lesson.
Dave
Long
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