Seven Medical Myths Even Doctors Believe

[rfbdorf]

Wise -
I certainly take your point about nonlinearities and thresholds - if the body can dispose of something faster than it accumulates, then it may well be expected to have little, if any effect. And I am aware that there are precious few linear relationships in biological systems (well, maybe cardiac output = stroke volume * heart rate!)
However, I did not mean common sense extrapolation in the sense of a linear extrapolation, but in the sense that I prefer to avoid exposure to smoke (to which I was referring in particular), trichloroethane, carbon tet, MEK, etc. (I imagine the latter 3 would fall into your category 6), because I have only a vague idea where my integrated exposure level lies with relation to any threshold that may exist. That's where I consider that common sense enters.
Additionally, if a substance (especially type 6) is considered harmless at a low dosage, that may be only because its toxic effect is small enough that it is lost in the noise of the many other insults to which our bodies are daily exposed.
- Richard

[Wise Young]

Wise -
I certainly take your point about nonlinearities and thresholds - if the body can dispose of something faster than it accumulates, then it may well be expected to have little, if any effect. And I am aware that there are precious few linear relationships in biological systems (well, maybe cardiac output = stroke volume * heart rate!)
However, I did not mean common sense extrapolation in the sense of a linear extrapolation, but in the sense that I prefer to avoid exposure to smoke (to which I was referring in particular), trichloroethane, carbon tet, MEK, etc. (I imagine the latter 3 would fall into your category 6), because I have only a vague idea where my integrated exposure level lies with relation to any threshold that may exist. That's where I consider that common sense enters.
Additionally, if a substance (especially type 6) is considered harmless at a low dosage, that may be only because its toxic effect is small enough that it is lost in the noise of the many other insults to which our bodies are daily exposed.
- Richard

Richard,

I thought that it was interesting that you used the example of cardiac output being equal to volume and heart rate as a linear relationship. To me, that is not a cause effect relationship so much as it is simply a definition. The variables on each side of the equation are related. I think of relationships as how two independent variables change relative to each other and tried ot think of an example of linear relationships. It is an interesting exercise because I start out with the thought that something "must be" linear and then realize on second thought that it might not be.

1. Fluid input and output. One would expect that our urine output would be linearly related to your input. It, however, may not be. First, clearly when you drink very little fluid, your urine output not only may be less than your intake (due to other outputs such as sweating, etc.) but much depends on the time at which you measure the output. If you look at urine output at 5 minutes after having drunk a liter of water, you will find relatively little urine production. However, if you measure it several hours later, you can get an amount that is probably similar to the amount that you have drunk. So, the relationship between fluid input and output depends on volume as well as time. And, of course, it also depends on absorption.

2. Drug dose response curves. Most drug dose response curves are S-shaped. In other words, they have little effect until a certain threshold where the response increases with dose. However, when the dose increases above a certain level, you may get a plateau, i.e. no further effect with increasing doses. The entire curve may be shifted upward, downward, leftward, or rightward depending on circumstances. The slope of the curve may be different.

3. Structure and function relationship. An obvious example is the relationship of the number of axons the spinal cord and walking function. One would expect a S-shaped curve. At the top end of the curve, one may find a plateau. At the bottom and left side of the curve, one may find another plateau. However, in reality, the curvilinear relationship between walking performance and number of axons is really asymptotic towards zero. With only 10% of the axons, animals such as rats still can walk. The curve tends to be asymptotic to the Y-axis. Above 10% of axons, there is a second linear relationship between walking and axon number.

I am struggling to find a linear relationship. There are so few. I am not sure that I can think on one. Yet, most doctors and scientists use linear extrapolations for biological relationships. The reason why they can get away with it is because the relationship is often linear over a limited range. But, when one is considering something like passive smoking which is perhaps several orders of magnitude less than the exposure that is associated with actually smoking. If the relationship were linear, the risk would be vanishingly small.

Wise.

[Monique]

Antiquity,

Thanks for the post. There are many other myths that doctors believe. Let me try to enumerate several additional myths that many doctors believe but what evidence there is either does not support or contradict them:
• Passive smoking causes cancer.
• Bedrest is good for patients. The evidence suggest that bedrest is at best not effective and often deleterious for most conditions.
• H1 blockers are effective for treating urticaria but H2 blockers are not. The evidence suggests that the combination of the two are better than H1 blockers alone.
• Patients allergic to penicillins are also allergic to cephalosporins.
• Prednisone must always be tapered. The evidence indicates that short course of prednisone and other glucocorticoid steroids that are 8 days in duration does need to be tapered.
• High blood pressure (such as that resulting from autonomic dysreflexia must be emergently treated with sublingual nifedine (a calcium channel blocker). Some evidence suggests that rapid reduction of hypertension may be associted complications. Sublingual nifedipine should not be used for this purpose.
• beta-blockers should not be used in congestive heart failure. Some evidence suggests that they may be beneficial for heart failure.

etc. etc.

http://www.montana.edu/wwwebm/myths.htm

wow, I thought alot of these were true. I think just two days ago my resident told me that beta-blockers should be used in MI but not CHF.

So prednisone doesn't need to be tapered if its given for less than 8 days regardless of the dose? Also, Dr. Young, reminds me of something else I was wondering but wasn't able to find an answer to -- if prednisone suppresses the immune system, why does it cause the white count to go up? Is it that prednisone suppresses the immune response at the cellular level, and the body reacts to this by increasing white cell production? (sorry for going off topic...)

p.s. Happy belated birthday Dr. Young and happy new year!

[rfbdorf]

...
I thought that it was interesting that you used the example of cardiac output being equal to volume and heart rate as a linear relationship. To me, that is not a cause effect relationship so much as it is simply a definition. The variables on each side of the equation are related. I think of relationships as how two independent variables change relative to each other and tried ot think of an example of linear relationships. It is an interesting exercise because I start out with the thought that something "must be" linear and then realize on second thought that it might not be.
...
I am struggling to find a linear relationship. There are so few. I am not sure that I can think on one. Yet, most doctors and scientists use linear extrapolations for biological relationships. The reason why they can get away with it is because the relationship is often linear over a limited range. But, when one is considering something like passive smoking which is perhaps several orders of magnitude less than the exposure that is associated with actually smoking. If the relationship were linear, the risk would be vanishingly small.

I generally think of cardiac output as a measure of the volume of blood flowing through the aorta per unit time - somewhat more than a simple definition by CO=SV*HR. But linearity in that falls apart if you look at a short enough time interval (e.g., less than one pulse interval, where heart rate begins to lose its meaning), or when you consider leakages, for example due to mitral regurgitation.

For just about any purpose, one should indeed be very cautious about applying a linear extrapolation. However, linear interpolation is generally pretty safe even for biological purposes, when applied over a small enough interval, but I bet you're right that over a large range, there are no true linearities in the body. Come to think of it, maybe in the universe, considering quantum and relativistic effects!

- Richard

[Wise Young]

wow, I thought alot of these were true. I think just two days ago my resident told me that beta-blockers should be used in MI but not CHF.

So prednisone doesn't need to be tapered if its given for less than 8 days regardless of the dose? Also, Dr. Young, reminds me of something else I was wondering but wasn't able to find an answer to -- if prednisone suppresses the immune system, why does it cause the white count to go up? Is it that prednisone suppresses the immune response at the cellular level, and the body reacts to this by increasing white cell production? (sorry for going off topic...)

p.s. Happy belated birthday Dr. Young and happy new year!

Hi, Monique. Happy New Year to you.

You were always the best student who asked the best questions. It is an interesting and important question that shed light on the mechanism by which glucocorticoids work as anti-inflammatory drugs. Glucocorticoids decrease inflammation by suppressing NF-kappa-B induced genes. These genes include IL-1, IL-6, and TNF-alpha, which you know all about.

The mechanism by which glucocorticoids affect NF-kappa B activity is not well-understood. Recent work suggest that glucocorticoids do not directly interfere with DNA binding capabilities of NF-kappa-B. Nor does it seem to affect the inhibitor of NF-kappa-B (iKb). Most textbook suggest that glucocorticoids bind to cytoplasmic glucocorticoid receptors (GR) that act in combination with CREB-binding protein (CBP), p300, and SRC-1 to prevent NF-kappa-B activation. One study suggested that glucocorticoids suppress NF-kappa-B induced gene expression regardless of the levels of CBP, p300, SRC-1. It may work by interfering with p65 interaction with transcription machinery (see attached Bosscher, et al. 2000). Others (Novac, et al. 2006) have reported that the GR directly regulates the human FasL promoter. As you know, Fax ligand is a proinflammatory messenger. In any case, much data now suggests the glucocorticoids antagonize NF-kappa-B effects through multiple mechanisms (See Almawi and Melemedjian, 2002).

So, why does glucocorticoid therapy increase white blood cell counts? I am not sure. I think that it does so by increasing the migration of white blood cells from tissues into the bloodstream, as opposed to increasing proliferation of such cells. In a normal person, glucocorticoids will increase leukocytosis and may mask the onset of an infection.

While the mechanism of glucocorticoid induced leukocytosis is not clear, the increased in white blood cell count is understandable considering the role of glucocorticoids. This is a stress-induced hormone that prepares the body for infection and injury. If given or naturally induced for too long, i.e. more than a week, glucocorticoids can suppress immune function.

You can usually tell a glucocorticoid-induced leukocytosis by the profile of the white cell counts. Glucocorticoids generally cause lymphocytes to undergo apoptosis and thereby reduce lymphocyte counts. However, it can increase neutrophils. The effect of steroids is also dose dependent.

Wise.

[Monique]

Thanks Dr. Young. I can always rely on you to answer all my questions in the most clear manner.

Haven't seen you in such a long time. I miss you! Hope everything is going well. I am currently doing my medicine clerkship, and enjoying it very much.

Monique

[Wise Young]

Thanks Dr. Young. I can always rely on you to answer all my questions in the most clear manner.

Haven't seen you in such a long time. I miss you! Hope everything is going well. I am currently doing my medicine clerkship, and enjoying it very much.

Monique

Monique, around New Year's time, some of the old students are coming back to visit. I am amazed that my first crop of undergraduate students from Rutgers are now in the last year of medical schools. You were really the first of the students who trained in the W. M. Keck Center.

Yes, medicine clerkships are fantastic. Enjoy it while you can. As I think that I have told you, medical school was the best time for me because it was license for me to learn all about the human body without the responsibilities that come after graduation.

As I think that I have told the students, the responsibility was initially terrifying. But, the responsibility becomes enjoyable when you realize that you are responsible for making people better and that you are good at it. It is then when you realize that you were born to do this.

Wise.

[mommylilangel]

I liked believing the myth that we only use 10% of our brain