Usually drink, usually dance, usually bubble

EI has relations with a Street Style blog over at Stitsh.com, and recently a little vid caught our attention over there. Click here and scroll down to 28.06.08.

It reminded us of a little law that the examiners sometimes like to question, that is Henry's Law:

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Okay, so what does that mean.

Most of the time we refer to Henry's law by the formula p=kc (that's one way of looking at it - Ed).

Another way is to say:

where:

is approximately 2.718, the base of the natural logarithm

is the partial pressure of the solute (the gas being dissolved) above the liquid in which is being dissolved.

is the concentration of the solute in the solution

is the Henry's Law constant, which has units such as L·atm/mol, atm/(mol fraction) or Pa·m³/mol (this is so that the dimensions all work out correctly - the funny thing about constants is that they usually can be expressed in many different units, depending on what units the rest of the equation is being calculated in....more on that another time).

(In other words, most of the time, we take the natural logarithms of both sides).

The pressure above a solution dictates how many collisions occur between the gas and the liquid. So if you increase the pressure above the solution, the partial pressure of the gas increases, the number of collisions increases, and more gas is dissolved. What will then happen is that an equillibrium will be achieved, where the number of molecules of gas crashing into the surface of the liquid will be the same as the number of molecules leaving the surface of the liquid.

The more observant amongst you will have realised that temperature hasn't been mentioned yet except in the definition....

So what effect does temperature have?

Well, think of a can of "fizzy pop" (you're showing your age there - Ed). When it comes out of the fridge, it's not that fizzy, is it? However, the longer you leave it standing around, the closer it's temperature comes to room temperature, and then when you go back to the can, first it will seem quite gassy, and then eventually it will go flat. This is because the gas in the drink is coming out of solution. The gas solubility relationship with temperature is very similar to the reason that vapor pressure increases with temperature. (This is Gay-Lussac's Law: The pressure of a given number of moles (given amount) of gas, is directly proportional to its temperature in Kelvin (absolute temperature scale), when the volume is kept constant. Better known as P/T=k).

Increased temperature causes an increase in kinetic energy, which in a gas causes either expansion or an increase in pressure, or in this instance, more movement of the molecules, which break free of the surface of the solution! (The surface could be the gas side of a small bubble of gas trapped within the solution, which is one reason we get bubbles!)

If you want to see another demonstration of Henry's law in action, look at a pan of water. As you warm the pan, small bubbles start to form, well before the pan reaches 100°C (373K). Those bubbles are air coming out of solution.

So why do the examiners like this concept: the Bends.

Decompression Sickness occurs when gas (specifically nitrogen) is breathed at higher than atmospheric pressure, and the diver then returns to atmospheric pressure without allowing the gas to come out of solution slowly, resulting in gas bubble formation, and hence, "the bends" (gas in the joints) and "the staggers" (gas bubbles in the brain causing confusion and ataxia) and "the chokes" (probably PE).

It is also a concept that comes into play when talking about Ostwald and Bunsen coefficients....(more on that another time).



(Equations courtesy of Wikipedia)

Practice makes perfect

Do you waffle when telling stories?
Does your partner tell you to get to the point when you're explaining about something that happened at work?
Do your family hold their heads and groan when you start off by saying "A funny thing happened the other day....", or something similar?

Then you may be a waffler, and I'm not talking about someone who bakes light crisp battercakes in a waffle iron, rather, I mean the other type of waffler, who speaks or writes in a vague and wordy manner (The Free Dictionary).

You may not even realise you are doing it, unless some particularly harsh person in your vicinity tells you about it, or you do one of the things I will come to shortly.

To realise you waffle will take some getting used to. There will have to be acceptance on your part that you are using empty "filler" words as previously discussed, which simply waste your time, as you are not scoring points. Remember back to the SAQ. The key was to transmit as much information as possible in the most succinct, legible manner possible (A remarkable achievement you got a viva then - Ed.) Yes, thanks, I know my handwriting is terrible... Anyway, BACK TO THE POINT: in the viva, you have to do the same, but in the spoken word, so to speak (ahem).

To help you on your way out of denial, try doing one of the following:
1. Pop down to your nearest Lidl and buy yourself a £15 dictaphone with 15minute blank tape, or failing that, blow all your money on one of these dinky gadgets, and practice talking about, say, "What are the changes in physiology in a runner's body from 30 minutes prior a marathon race, until some time after the race?"
2. Better yet, borrow a video camera and do the same.
3. Sit down with two really harsh consultants from your department and practice being viva'd by them on the above topic, whilst recording the whole thing with one of the above devices, or just get them to feed back to you whether you waffle or not.

If it is the case that you harp on without going anywhere, then you only have a few days in which to hone your technique to eliminate waffling. Nil desperandum, as they say. It is all perfectly feasible.

The key is in practicing with yourself, in front of a mirror, with your wife/husband/boyfriend/girlfriend, with a dictaphone or video camera, or in front of pairs of consultants (or even one will do, at a pinch) in your department.

• Try using the Five Word Viva Game to cut out absolutely everything extraneous, then flesh out your answers a little bit to build up to a sensible answer.
• Don't repeat yourself: "The main concerns are residual nerve block, excess opioid and residual narcotisation, incomplete recovery of neuromuscular function, hypoxia and metabolic/endocrine derangement, are the main concerns."
• Try to cut out saying "Er, um, ar, ah, aer" etc. Try a pause instead.
• Elongate vowels in starts of sentences slightly "Weeelll", "Theeeere aaaarre" and "Iiiiii wooouuullld" for example (you get the idea, I hope?).
• Be confident about your knowledge when you are sure about it, and make the examiners feel you are confident. Remember that you are not about to become a consultant (this isn't an exit exam, as one of my consultants put it), but they are looking for someone who is a good Registrar (Specialist, Specialty or otherwise) to whom they could entrust a case in the middle of the night whilst they cosy up back to sleep, and not worry about it (unless worrying is really necessary, in which case they'll probably come in anyway).

Keep at it: one way or another it'll all be over bar the drinking in a 10 days time...

Congratulations!

Congratulations to all those who passed the Primary MCQ, results of which came out on Thursday!

As anyone who has done this exam will be aware, it is a bugger, but only the first step on the way to becoming FRCA.

Next up is the OSCE and viva, which are also a bugger in a different way. If you want a useful book for this (it has many little tidbits and lots of diagrams you could be expected to reproduce in the exam) consider this one The Anaesthesia Viva: Volume 1, Physiology and Pharmacology and this one: The Anaesthesia Viva: Volume 2, Physics, Clinical Measurement, Safety and Clinical Anaesthesia. Although not perfect, they have helped many people in their preparation for the exam, and I've even looked back at it in revising for the Final (though there are better books for this).

For the OSCE examinations, the most popular book is probably this one: Anaesthesia OSCE. I am not sure that there is actually anything else out there at the moment.

Other books I bought and used to revise for the Primary Structured Orals, and useful for the Finals as well were The Clinical Anaesthesia Viva Book and The Anaesthesia Science Viva Book. The latter is probably more use going into Primary, to be honest, and contains a reasonable amount of detail. I didn't come to these until very late in the day, but I reckon that if you can reproduce some of the stuff in the Science Viva book in the Primary, you'll be well away.

Anyhow, best of luck!