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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