Yet More Reader Mail


 
 

Howard Rodenberg, MD, MPH, Dip(FM) | | Tuesday, November 27, 2007


Once again, it's time to dip into the reader mailbag to answer some pressing concerns on the future of democracy. Either that, or the questions are about my jems.com and JEMS eNews columns. It's easy to get the two mixed up.

Our first letter, referring to Of Size, Syrup & Saline: The Science Behind Fluid Flow, comes from Richard Wells, a retired attorney now working as a full-time paramedic (he saw the light). He mentions that anyone who doesn't appreciate white asparagus needs to be brought "up to snuff" on a few issues. He notes that I have incorrectly spelled the name of the esteemed Poiseuille in my efforts on breath sounds and fluid flow. He also notes that the most complete eponym for the precept would be the (and I quote) Chezy - Weisbach - Darcy - Poiseuille - Hagen - Reynolds - Fanning - Prandtl - Blasius - Von Karma - Nikuradse - Colebrook - White - Rouse - Moody equation. Wow. I'm awestruck. I don't know whether to applaud his scholarship or wish him a life.

(I have to admit to a small chill when I learned that someone from Haaavaaaad reads this column. Must be a "slumming" kind of thing, to see what the simple folk do.)

True confessions come first. In regard to my omissions of the critical "u" and "i," I will concede to Americanizing the nom de Monsieur Poiseuille (which sounds like a better excuse than admitting I screwed up). But I think I can be forgiven. I am smitten with that wonderful New World tradition of changing spelling to suit the mood, believing that the word "jail" just looks like a worse place to be than a "gaol," which is something a dyslexic soccer announcer says, loud and long, when someone accidentally puts the ball into the net during a thrilling 1-1 tie.

Quincy Chopra of Pittsburgh not only knows how to spell "Poiseuille" correctly, but also notes that the complete version of Poiseuille's Law includes a reference to the length of the tube. He asks if this conflicts with my thought that what really matters in EMS is fluid flow through the narrowest point of the tube rather than the full length of the tube itself.

I've rechecked my references, and I'll stick by my definition of Poiseuille's Law. As noted in Steadman's Medical Dictionary (23rd edition), the principle states: "The quantity of fluid flowing from a narrow tube is directly proportional to the pressure gradient, the viscosity coefficient, and the fourth power of the diameter of the tube; the mean lineal velocity of the current is proportional to the cross-sectional area of the tube, the pressure gradient, and the viscosity coefficient." Melloni's Illustrated Medical Dictionary (3rd Edition) is more succinct, defining it thusly: "Speed of fluid flowing in a tube is proportional to the cross-sectional area of the tube." However, it is true that texts focused on the basic sciences of medicine do include a place for length of the tube in the full expression of the principle.

The effect of tube length on fluid passage is linked to the concept of laminar flow. When fluid flows through a tube it travels in layers, or "streamlines," of concentric circles of equal distance from the side of the vessel (the best way to think of it is as a "bulls-eye" pattern). These circles are a function of the degree to which molecules of the fluid opt to adhere to the wall of the tube. The longer fluid is in contact with the sides of the tube, the more the molecules of the fluid may adhere to the side of the vessel, and the more sluggish the flow. Consequently, laminar flow dictates that the velocity of the fluid traveling in the center of the tube is greater than that toward the outer edges. The farther away the fluid is from the vessel wall, the more it can "slip" over the layers of adherence, and the faster the fluid will flow.

The phenomenon of laminar flow explains why the radius of the blood vessel or IV tubing plays such a critical role in blood or fluid flow. The farther away from the side of the vessel the fluid can get, the more pronounced the laminar flow, the faster flow of fluid as the tube grows in size. It also describes the effect of the length of the vessel on flow. At the molecular level, the more time the fluid has to be in contact with the vessel wall (as a function of the increased length of the tube), the slower the flow of liquid.

So does length of the tube play a role in fluid flow through the vessel? Before answering, it's important to note that in Poiseuille's Law, the length of the tube is multiplied by the viscosity coefficient, a measure of how sluggish or sticky (resistant to flow) the fluid is. The fluids we use in the EMS setting (NS and 5% dextrose in water) have very low viscosity coefficients. So while my statement that the length of the tubing makes no difference at all was probably misleading from the theoretical standpoint, I still stand by the operational concept. If we were talsking about infusing cooled fluids (containing ice crystals), blood products or maple syrup, there would certainly be a practical impact. But given the nature of typical IV fluids and medications used in prehospital care, I think the length of the tubing is of negligible import. The radius of the tube multiplied to the fourth power remains the dominant factor in fluid flow.

Before leaving this topic, I would be remiss if I did not address the vegetable issue raised by Mr. Wells. Here's the scoop on white asparagus. To start with, it looks wrong, like sickly wheat with a glandular problem. As to taste, asparagus is God's way of supporting children's arguments that vegetables are disgusting and properly eaten only by grownups and other assorted non-juvenile creatures (even my dog, which could be billed at the sideshow as The Amazing Furry Walking Garbage Disposal, won't touch the stuff). What's more, it's not even a vegetable you can play with. At least with broccoli you can build a forest of trees, and brussel sprouts easily become soccer balls (Note to self: Stop shooting at the wall. The green color never comes off). But asparagus? It's too flimsy to use as a spear, not aerodynamic enough for a javelin. It's no wonder that the annoying, overbearing superego character in the VeggieTales children's videos is an asparagus. But my Ivy League colleague would have you believe that just because the asparagus is pale, that makes it all better. A Midwestern harrumph to you.

Since we've discounted the role of asparagus in the heroism of the Belge (I know the word is really "Belgians," but "Belge," or even "Belgish," just sounds more fun), what else could the brave soldiers of the Armee fight for? We've already mentioned chocolate although, in my limited experience with chocaholics, it may have been the women of Belgium who coerced their manfolk to fight for the confection. "Hon, the Germans want our chocolate again. Be a dear and go fight them, won't you? And when you get back, we'll have a roll in the asparagus patch. Toss me that box of bonbons on your way out, won't you?"

Which leads us to the whole topic of Belgium, which is probably worth some commentary (and yes, I'm well aware that all of this will come back to haunt me when I become Secretary of State and address the European Parliament in that's right Brussels). My knowledge of Belgium comes from an earlier epoch in life, when en route to Europe I would always fly via Brussels because it was the cheapest way to go. I have to confess a certain fondness for the old Brussels airport. There was one concourse that was so deserted you could sleep there and never be disturbed.

The city fathers would undoubtedly like us to recall that Brussels is the cosmopolitan capital of the European Community. However, the major tourist attraction of this Belgian metropolis is undoubtedly the Manneken Pis. As you may have surmised, this is a statue of a small boy relieving himself. It is not the prototype of those playful and charming statues of a boy being irrigated in the loins by a spitting frog. It is simply a two-foot high statue of a small boy grasping his manhood and relieving himself, without any particular expression of ecstasy or regret. You may think I'm making this up, but millions of people visit the statue each year. You can also visit the City Museum, where you might gaze upon the many costumes the good folk of Brussels have created for their boy. ("Hon, on your way back from the war, could you pick up some gingham? I want to make the little guy some pants.")

Brussels does have some wonderful sights, but they are unexpected surprises. In the center of town, you turn a series of nondescript corners and find yourself in the midst of the Grand Place, a magnificent public square straight out of medieval times. A bit of a walk takes you to the Parc Leopold, where pushcart vendors sell French fries, the real kind, hot and crispy on the outside, cloud-soft on the inside, with your choice of mustards and curries (Ketchup? How very colonial). Nearby is the Royal Belgium Institute of Natural Sciences, where one finds the greatest display of mounted iguanodons in the world. Discovered in a coal mine in 1878, there are fourteen in all. They stand together in their prehistoric herd, the century-old ochre shellac on the bones makes them shine as jewels of antiquity. But for me, the true gem of the city is Le Sablon, a blissful spot of green on the Rue de la Regence between the Palais Royal and the Palais de Justice. Here is where one can settle in for a beer and baguette in the company of the rustle of trees, the scent of flowers, and a statue of two doomed noblemen embracing on their way to the executioner. I just hope they weren't dying for the asparagus.


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