Bohr's method: Measuring Physiological Dead Space

[brianfall]

Hello,

does anyone fancy having a go at explaining how you go about measuring physiological dead space using Bohr's method?

I know that you use the equation;

VD/VT = FACO2-FECO2/FACO2

and that

1) VD = Volume of dead space
2) VT = Tidal Volume
3) FACO2 = fractional volume of alveolar CO2
4) FECO2 = fractional volume of expired CO2

Measuring Tidal volume is easy using a spirometer, and presumably measuring fractional volume of expired CO2 is also fairly easy using some sort of fancy device.

But how do you measure fractional volume of alveolar CO2?

Thanks in advance,
Brian

[M Clayton]

Short answer, you don't.

Most researchers have taken to assuming that the alveolar fraction of CO2 is the same as the end tidal fraction.

Certainly, that's what the notes I made during a lecture at some stage in the distant past have to say!

[brianfall]

Thanks for that...

Right, so if FACO2 = FECO2 and you plug that into the equation, then that means you get;

VD/VT = 0/FACO2

or

VD/VT = 0

so VD = VT x 0

but this means that VD is always equal to zero. Am I being dim?

Thanks in advance,
Brian

[darklight]

'the last part of the expired air will have the same compostion as the alveolar air... This will giev an average for the compostion of the alveolar air. a more accurate esitmate of the CO2 content of alveolar air can be obtained by measuring the PCO2 of arterial blood' From Pocock

so the expired air conent is the first bit of expired, and the alveolar the last bit. so they wouldn't be equal. normal values are expired about 3.6% and alveolar 5.2% So with a tidal volume of 500 gives a dead space of just over 150ml

[brianfall]

Think I get you.

Do all the calculations rest on the assumption that we already know the normal values of 3.6% for FEC02 and 5.2% for FAC02?

If this is the case then it's easy to just measure tidal volume using a spirometer. Let's say I do and my tidal volume is 520ml. I can plug these values in;

VD/VT = FACO2-FECO2/FACO2

VD/520 = 5.2 - 3.6 / 5.2
so VD/520 = 0.308
and VD = 520 x 0.308
which is 160.16ml

If this is the case, then why aren't we just taught that dead space = 30.8% of tidal volume? That's an easier way of saying that on average FEC02 is 3.6% for and FAC02 is 5.2%. I can't help thinking there's more to it, like they expect us to be able to measure FAC02 in some way?

[darklight]

FACO2 is measured by the last bit of expired air.
so as 3.6 and 5.2 are the average values, then on average the dead space is 30.8%. But these are only typical values. If you were doing ti properally you would actually measure blood gases/CO2 in expired air

[I think! - someone please correct me if I'm wrong!]

[brianfall]

Feeling very dim now. Really should pay more attention in my lectures.

As you say PAC02 is measured in the last bit of expired air. Done by hooking someone up to a bit of fancy equipment and the PAC02 is simply the trough C02 value corresponding to the exhaled air.

Finally! What has happened to my brain? Has anyone seen it?