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Pulmonary Function 101 ...page 2
Understanding Your Lungs
Picture a balloon
that is inflated to 3 liters as representing the lungs after one takes in a
normal breath. You let out about 500 ml, representing a normal exhalation and
blow a fresh 500 ml back into the balloon. Repeating this pattern depicts a
normal breathing pattern. The VT changes very little as COPD develops and
worsens. But the resting volumes (FRC, ERV and RV) increase significantly as
more air becomes trapped. As COPD worsens, tidal breathing is not able to
‘dilute’ the gas within the lungs as much as it could when the lungs were
normal, because of the ratio of the tidal volume to the total gas volume in the
lungs that it is trying to dilute. Again, picture that balloon, only now, to
represent the increase in resting volumes, you have it blown up to 3500 ml or so
AND, as with a balloon that has been blown up too many times and becomes less
elastic, the lungs become less elastic, as well, like the increasingly flimsy
balloon. Additionally, when you breathe your normal tidal volume of 500 ml, the
amount of dilution it is able to accomplish is less because the same 500 ml
tidal volume is now trying to dilute a much greater total lung volume. As a
result, resting CO2 levels rise and oxygen levels fall, though not in the same
proportion as the rise in CO2 that occurs. But, the end result is that as COPD
reaches the more severe stages, both CO2 and O2 exchange become worse such that
we generally see CO2-retention most significantly in those who ALSO have
worsened oxygen exchange. Resting pO2’s drop and pCo2’s rise on blood gas
measurement (p=”pressure of” the gas measured).
Anatomically, as
lung damage from COPD worsens, it is the conducting airways – a.k.a. the
bronchial tubes – especially the smaller ones out further in the lungs,
that
become damaged and flimsy because of loss of supportive tissues in the process.
When you breathe out with more severe COPD, those remaining airways tend to
collapse, like the opening of a whoopee cushion, trapping air behind them and
decreasing the amount of air that can be expelled on exhalation. Using PLB,
‘splints’ those airways and allows more of the air that is behind them to be
expelled during exhalation. NOW, the reason that CO2 does not go down much or
permanently is because while PLB allows more air to be expelled, relative to the
total amount of air in the lungs, especially in relation to the amount that is
trapped, subsequent inhalations cannot significantly increase the resulting
dilution of the abnormally increased volume that remains within the lungs. In
this situation or condition, the new ranges of CO2 from breath to breath can
rise to between 45 and 55, resulting in a pCO2 of 50 on a blood gas
measurement. Similarly, oxygen may drop to between 45 and 65 for a resulting
pO2 of 55 on blood gas measurement. B UT, another benefit of PLB is that the
splinting action raises pressure within the lungs and helps to “push” more
oxygen molecules through the membranes from the alveoli to the blood, and can
result in a small increase in oxygen levels.
Another matter to
consider is the basic fact that oxygen, is a ‘physically’ larger
molecule
in
comparison to the CO2 molecule. Owing also
to the nature of the membrane between the alveolus and the blood vessels passing
by them, CO2 diffuses across that membrane 20-times faster or more readily than
does O2. That is why there is such a greater swing in the range of O2 to CO2
when we look at the pressures of each between the gas in the alveoli and that in
the blood. This is also why folks who have severe COPD can have low oxygen
levels and NOT necessarily have CO2-retention, or ‘as severe’ a degree of
CO2-retention as they have hypoxia (low oxygen levels).
If you have
followed me this far, you deserve a medal!
Hopefully, this has provided insight and understanding to what can be a daunting and confusing
process and physiology that, when you get down to the brass tacks, really isn’t
all that difficult to understand, IF explained sufficiently and properly. I
hope I have done that with this effort.
If you still have further questions or
need clarification on certain points, please post your questions and I’ll try to
explain further.
Ask the RT
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This page was last updated January
19th, 2011
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