Expired gases on treadmill

["guest@endurance.net" <guest@endurance.net>]

Beth Glace winorpin@aol.com
Dave wrote,
"I don't know how you derive that.  What you're basically measuring is how well the lungs absorb O2 and expel CO2.  If the blood is well-saturated with O2, you'd expect to see lower transfer of O2.  There's also some effect on the fluid dynamics of the lung from respiratory rate - lower rates would tend to reduce the surface area, which also reduces transfer. How you get from that to what is getting burned isn't clear to me."

how do we derive this?  This is pretty basic exercise physiology, and you can read about the expired gases in any basic text which describes exercise testing.  Blood gases are representative of what is happening metabolically, not only of lung function.  By comparing the O2 left in expired air to the O2 in room air you can calculate how much O2 is consumed.  As anaerobic metabolism increases, lactate is buffered by the bicarbonate buffering system, which has as its end products CO2 and H2O.  Only carbs can be used anaerobically, so when the ratio of expired CO2:O2 is equal or greater to 1.0 the substrate being used is 100% carbs.  A ratio of 0.7 CO2:O2 is accepted as representing fat as the sole source of substrate.  Formulas exist which allow you to calculate the percentage of fat and carb when the respiratory exchange ratio is any where between 0.7 and 1.0, but I am at home and do not have access to the texts.  If you are interested you could probably find the equations by searching Medline.
Dave wrote:
"No, you've just still got the blood saturated with O2 as you'd expect from a resting start."

The blood remains very saturated with O2 throughout exercise no matter how vigourous.  The lungs are not typically a limiting factor in O2 kinetics, so as much O2 as the muscles can remove for metabolism can be replaced.  If this were not so we would all be as limited as those with emphysema when we tried to exercise.  The limiting factor in prolonged exercise is the ability of the muscles to use O2, not in the respiratory delivery system.

Tom's description of what is happening in an apparently fed animal seems reasonable, except for the 0.17 ratio.  I'd guess that was a methodolgic error or equipment error.  Over a 4 hour run we would usually see ratios in the 0.75 to 0.85 range in humans, lower than Tom's observations of ratios in the 0.89 - >1.0 range.  However if they'd eaten within a few hours of the test we would see greater reliance on carbs, since blood sugar would be higher, and I'd guess that the horse was not fasting. It may be that horses use more carbs than humans during a 4 hour run. By the way, Tom, you won't see a "switch" to fat metabolism.  Not unless you thoroughly deplete glycogen, which you can do by doing series of high intensity surges, then have them continue at a moderately hard pace. Make them "hit the wall" so to speak, but then they would not be able to continue at the pace you had them at.  Other than that you will always be using a mixture of fats and carbs. I don't think the 0.89 ratio is at all significant, I'd take the average over the course of a minute and track the changes in the RER that way.
Interesting stuff Tom, and your numbers seem plausible!
Beth Glace, MS, CDN
from home in New Paltz, NY



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