The Beautiful Tropics

If you have read any of my columns, although, quite frankly, I’m not sure I’d recommend it, you might see a common thread of a) a passion for all things etymological and b) a bit of banging on about love. Well this month’s column will combine the two. The suffix –tropic, is variously defined as ‘turning, or leaning towards,’ 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 today I would like to share a couple, if for no other reason than the delight of them (as well as, and I hope you appreciate that this is one of my missions, to give you excellent word fodder for parties, ward rounds, quiz nights, other).

You all know, of course, agents which 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: derived from the Greek word running (as in a race), and refers to the conduction speed (or magnitude of delay) through the A-V 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. For example, moderate hyperkalemia is positively bathmotropic, whereas 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 towards the sun (heliotropic). That term, 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 for this is rather remarkable, as is the case for almost all things biological when you get down to it. Below the flower part in these talented plants is a flexible segment, just like a joint, called a pulvinus, and it is full of planty motor cells which 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, however, it is also 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, and not the act of turning lovingly towards the sun.

Pleiotropic molecules are fascinating in medicine. They are single agents with multiple and diverse effects. My very favourite is TPA. An uberpleiotropic 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 accounts 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 have effects both potentiating the good as well as increasing the deleterious effects of clot dissolution, particularly within the brain.

The final tropic we will meet has nothing to do with emergency medicine, but is 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. This has time dependent viscosity, and is transformed similarly in reverse, taking a fixed time of letting it stand in order to return 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 to you a list of other relevant tropical-suffixed words, in case you are stuck on a break playing Words With Friends: Hypermetropic, psychotropic, gonadotropic, neurotropic. Who doesn’t love the tropics.

About the author

Dr Michelle Johnston is a consultant Emergency Physician who works at an inner city hospital. Mostly her days consist of trauma and mess. Also, she writes.

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