Limits to Nature’s Wisdom II (Emphysema)

 

When we have a portion of lung tissue that is not aerated for any reason, of which pneumonia is a common example, nature in her wisdom constricts the small arteries to that portion of the lung.  Because the normally aerated blood from the normal lung is not diluted with as much of this low oxygen blood, the rest of the body suffers less oxygen lack.  This is nature’s wisdom.  The limit to it is that when the entire lung lack oxygen due to high altitude, the same mechanism is invoked but to all of both lungs.  This constriction of small arteries in all of both lungs raises the main pulmonary artery pressure and makes the right ventricle work harder just when more blood is needed to provide enough oxygen, greatly reducing exercise tolerance.  Incidentally, air at 18,000 feet altitude has only one-half the sea level amount of oxygen per unit volume.  Air at 18,000 feet is still 21% oxygen, but a lung full of it contains only one-half the weight of oxygen compared to sea level.

Even for normal people, continuous survival above 16,000 to 18,000 feet is impossible.  This means that those of us with chronic lung disease on the edge of survival at sea level breathing air can be restored to near normal blood oxygen by breathing 40% oxygen.  These people don’t want to be dependent on supplementary oxygen.  They especially rebel about using oxygen at night because they are fairly comfortable at rest.  However, the increased burden of pumping blood through the lungs at the previously described increased pressure leads to congestive heart failure on top of their lung disability.  Breathing 40% oxygen at night restores this problem to normal for 8 hours per day. I have found this explanation permits intelligent patients to accept oxygen therapy at night and at all other times when it is the least  inconvenient to use it. In fact the more they will wear it the better they will do.  They may also find out that oxygen therapy permits them to work out athletically and feel better by staying fit.  Lung patients are prevented from undertaking excessive exercise by the same instincts that prevent athletes from dropping dead from excessive effort.  Unfortunately, this is not always true for heart patients.

Tibetans have lived at altitude for more generations than Andeans in South America.  The Tibetans greater tolerance to altitude probably depends on less constriction of their small pulmonary arteries with an oxygen deficit.  They probably pay a price for this with less tolerance to part of their lungs being out of action when they have pneumonia, but this has never been demonstrated.  There are several other mechanisms of adjustment to high altitude that also occur in lung patients mostly with the same benefits that occur in normal people.  However, excessive blood viscosity from too many red blood cells is much more likely to occur in emphysema patients than in normal people at high altitude.  A previous column entitled “Second Wind” has some of this discussion.

Birds have a more powerful method of tolerating high altitude that is not available to us presumably because they have been going there much more regularly and for many more eons than we have.  This is out of my field of expertise, but I understand that birds have a counter-current blood-gas exchange arrangement that permits them to “top off” the best oxygenated blood with the freshest air.  When engineers use a similar strategy for the most efficient heat exchangers, they are called “counter-current heat exchangers”.

The bottom line:  Expert patients do better than those who merely try to follow orders because they are better equipped with knowledge to “adjust” the orders intelligently.

John A. Frantz, M.D. June 8, 2002, Chairman, Monroe City Board of Health

           See also 1) Smoking under Recreatioal Drugs, 2) Why All of Us Don’t Get  Tuberculosis under Medical Basic Science, and 3) An Innovative Treatment for Emphysema under Other Science and Technology.