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Date: Wed, 15 May 1996 11:40:53 -0700 (PDT)
From: Peter N.R. Heseltine <heseltin@hs*.us*.ed*>
To: Dennis Pierce <epic@so*.ha*.ed*>
Subject: Re: Your Fieno expriment
By definition, since you became hypoxic, you must have exceeded the
Fieno's fixed flow-rate of (estimated) 4 L/min during your bicycle
experiment.
I make the following assumptions:
Your vital capacity is 5 L
Your minute ventilation is 7.4 L/min @ rest
28.6 L/min light work (500 m kg/min)
43.0 L/min heavy work (800 m kg/min)
The capacity of the counter lung is 5 L (this may not be correct, maybe
it's more. Can you check in the specs?)
You are using EAN36%, which is 0.36 ATA at 0 fsw
Your total pulmonary resistance is increased by 10% due to breathing
through tubes etc. You start to get hypoxic symptoms when your PaO2
(arterial O2 pressure/conc) falls to 30 mm Hg (normal being 80-100mm Hg)
Obviously these are a lot of assumptions and there are several parts of
the equation that I have not even considered because (a) I can't estimate
them easily without measuring you (b) some probably cancel each other out
and (c) in any event are unlikely to make the situation worse, i.e., make you
more hypoxic. (The body is pretty smart about this, which is why you don't
fall over when you hold your breath - briefly)
So. Remembering that exercising at sea-level is a maximal gas test scenario
as your inspired pPO2 will never be lower than at sea-level on this
machine - I am assuming you are not diving pools on the heights Mauna
Kea :( You must be exceeding an O2 supply of .36x4 L/min = 1.44 L/min,
over at least three to four minutes. This is about how long it will take
your O2 sat or PaO2 to fall. Is this easy or hard to do? In your case
this would mean that you exceeded 20 cc/kg/min. I would estimate your
basal (resting) O2 consumption at ~200 cc/min. At moderate exercise
(light work above) this might rise to 800cc/min and heavy exercise 1.6
L/min. Now all this assumes that you are actually able to extract the O2
from the machine. Obviously the "dead space" dynamics of the machine and
your lungs are neither constant nor linear. The actual O2 available to
you under heavy exercise conditions (i.e., the efficiency of the
system: you and the machine) might fall by 20% or even 50%. Bingo. You
are out of O2, even with moderate exercise. (Despite George III's
claims, being fit does not make you use less O2, it merely increases your
efficiency of extracting it from the gas you breath. This is fine for
open circuit - which is why he appears to use less total gas - but not
relevant to rebreathers, except when you reach the machine's capacity to
deliver gas to your lungs. Your level of fitness doesn't actually make
more O2 available)
What have I learned? Well, no surprise, you can exceed the capacity of
the machine to deliver O2 and become hypoxic. You already told me that.
Now we have some (very rough) measurements of the system's limits. What
bothers me most is that you exceeded the limits over some longish period
of time - 5-10 mins. Was this because you exceeded the capacity by only a
little and this incremented over time or was it because there is some
rate that you finally exceeded that brought the system down? No doubt
those who are developing these machines could unlock some of these
secrets to us all.
It would be very useful to know which or if both of these limitations
apply to rebreathers. If it's total exercise (and at some level it has
to be this), then we could try and NOT exercise that hard. But if it is
that you cannot exceed the demand for a short period, say 2-3 mins
because you dramatically decrease the efficiency of gas delivery to your
lungs (as when you hyperventilate) this is more difficult to control,
but still useful to know.
Now the use of this machine at 1.0 ATA is the the most demanding as the
pPO2 rises with depth. Obviously stage bottles with pPO2 < 0.16 ATA don't
work at sea level either, but that requires a lot more training and a
different mid set. My (just made it up) rule is - if the system (e.g.,
rebreather) doesn't meet your O2 needs at sea-level, you shouldn't be
using it at 0-33 fsw and maybe not at all.
So let the experts respond - please!
Peter Heseltine
On Wed, 15 May 1996, Dennis Pierce wrote:
>
> i think the flow rate is 4l per but not sure, could find out for you and
> will try today,
>
> aloha,
>
> dp
>
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