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What Lies Beneath

What Lies Beneath

The Beautiful Tropics (the Suffixes, Not the Region)

Johnston, Michelle MBBS

doi: 10.1097/01.EEM.0000655016.08051.5b
    tropics. Heliotrope rash on the upper eyelid of a woman with dermatomyositis.
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    Figure

    If you have read any of my columns, you might see a common thread of a passion for all things etymological and a bit of banging on about love. Well, this month's column combines the two. The suffix -tropic is variously defined as turning or leaning toward, or having an affinity for, or, as I like to think of it, possessing a love of.

    There are some ferociously cool emergency medicine words ending in -tropic, and I would like to share a couple, if for no reason other than the delight in them (as well as to give you excellent word fodder for parties, ward rounds, and quiz nights).

    You all know, of course, agents that are inotropic (altering the force of myocardial contraction) and chronotropic (influencing the heart rate), and some of you may well have a passing acquaintance with dromotropic, which derives from the Greek word for running (as in a race) and refers to the conduction speed (or magnitude of delay) through the AV node, but were you aware of bathmotropic? Do you use it enough in everyday conversation?

    Bathmotropy refers to the excitability or irritability of the heart, and takes into account the ease of initiating an action potential. It is often used in relation to the heart's response to catecholamines. Moderate hyperkalemia, for example, is positively bathmotropic, while hypoxia is a negative bathmotrope. The fifth element in this series is the delectable lusitropic. Lusitropy is all about myocardial relaxation, and has calcium as its essence. Increased intracellular, or cytosolic, calcium causes positive inotropy but impairs myocyte relaxation, and is therefore negatively lusitropic. But the quicker the calcium can enter the sarcoplasmic reticulum, the greater the relaxation, thus the positive lusitropy, for example, with catecholamines.

    The heliotrope rash (the violaceously purple rash of dermatomyositis) was not named after the tendency of plants to grow toward the sun (heliotropic). Heliotropic is used more avidly in biology, particularly in relation to flowers that track the sun's arc from east to west, such as we see with the buds of sunflowers. The cause of this is rather remarkable, as is the case for almost all things biological when you get down to it. Below the flower part of these talented plants is a flexible segment, just like a joint, called a pulvinus, and it is full of planty motor cells that facilitate potassium movement and result in impressive turgor changes, ultimately causing the hinging of the flower.

    This knowledge is of no value to the resuscitationist but of great worth to the biological pedant. It is, however, important to know, sadly, that the heliotropic rash of dermatomyositis was, in fact, named after the heliotrope flower, which has small purple leaves, mild toxicity, and a lovely, sweet scent, not the act of turning lovingly toward the sun.

    TPA and Ketchup

    Pleiotropic molecules are fascinating in medicine. They are single agents with multiple and diverse effects. My favorite is TPA, an uber pleiotropic molecule, if you will. We're all aware of TPA's advertised function, activating plasminogen to break down fibrin, but its lesser known functions are legion, which likely account for the complexity of its effect in certain, ahem, uses. Neurologically, it plays roles in neuronal plasticity, matrix remodelling, excitotoxicity, and cerebrovascular barrier integrity, and thus may simultaneously potentiate the good and increase the deleterious effects of clot dissolution, particularly within the brain.

    The final -tropic we will meet has nothing to do with emergency medicine, but it is still fabulous and actually very important when it comes to barbecuing. Thixotropic fluids are blessed with the property of starting out thick but becoming less viscous with shear forces, or, in practical terms, shaking. A good example is ketchup. It has time-dependent viscosity and is transformed similarly in reverse, taking a fixed amount of time of letting it stand to return it to its original state. Yogurt is another one displaying non-Newtonian characteristics. There are several other physiological examples: cytoplasm, extracellular ground substance, and semen. The NASA space pen ink also.

    To finish, I give you a list of other relevant tropic-suffixed words in case you are stuck on a break playing Words With Friends: hypermetropic, psychotropic, gonadotropic, neurotropic. Who doesn't love the tropics?

    Dr. Johnstonis a board-certified emergency physician, thus the same as you but with a weird accent. She works in a trauma center situated down the unfashionable end of Perth, Western Australia. She is the author of the novel Dustfall, available on her website, http://michellejohnston.com.au/. She also contributes regularly to the blog, Life in the Fast Lane,https://lifeinthefastlane.com. Follow her on Twitter @Eleytherius, and read her past columns athttp://bit.ly/EMN-WhatLiesBeneath.

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