Exam Intelligence

EI has been asked about this concept, and spent some time trying to explain it. However, although EI understood the concept, it actually became quite difficult to explain. So we’ve thought about it some more, and here is our attempt at trying to explain it.

Firstly, an adiabatic change is one in which NO HEAT is TRANSFERRED TO or FROM a fluid (gas/liquid) doing work, or having work done on it. Normally this occurs when there is a change in pressure in a gas.

In other words, as a gas is compressed, it’s temperature will increase. Have you ever pumped up your bicycle tyre, and the nozzle or barrel of the pump has got really hot, almost too hot to touch by the time you’ve finished pumping the last bit of air in? That’s because of adiabatic HEATING. Diesel engines work on the same process of compression generating enough heat to cause ignition. There is no external source of heat, but the temperature has still increased. This must all have come from the act of pumping, i.e. pressurising, the air. The energy of the pumping has been converted to heat energy (internal energy) of the compressed gas.

Conversely, if a gas is suddenly allowed to expand, it will cool. A CO2 fire extinguisher (They used to be solid black, didn’t they? Now we’ve just got those EU compliant red things with different labels on. How the hell are you supposed to recognise the difference in a hurry now?? – Ed.)…when it is used, or any gas cylinder opened and allowed to vent suddenly will rapidly cool. In fact, if you are using a CO2 extinguisher, don’t put your hand on the funnel, because it might freeze to it. Why does this happen? As the gas expands, it does work on the surrounding air, pushing it out of the way. Since energy cannot be created or destroyed, merely converted from one form to another, the energy has to come from somewhere, and it comes from the internal energy of the gas doing the expanding, which we conveniently refer to as temperature.

So, as a gas is compressed or expands rapidly, it’s temperature changes, but no HEAT energy has been transferred into or out of the system. If heat is added to or lost from the surroundings, this is NOT a-diabatic. So for example a gas expanding as a result of being heated is not adiabatic, and a gas contracting as a result of being cooled is not adiabatic either. These processes involve a transfer of heat energy.

Eventually, after the sudden compression or expansion, there will be a transfer of heat energy, but at the time of the expansion or compression, there is not.

And that, in a nutshell, is adiabatic changes.

(It also happens with magnets, apparently, and they’re not fluids…- Ed.) Okay, yes it does happen with magnets, but don’t try and complicate the issue. (If you want to know more, see Adiabatic Demagnetisation on Wikipedia, but make sure you have your maths head on. You have been warned!)

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)

This is the final day of Final FRCA vivas, and EI hopes
many people have passed the exam. For those of you who
have yet to sit the exam this coming Autumn, all the best
of luck.

The last question we can tell you about, as intelligence
received has run thin in the last 24 hours, was a
pharmacology question which came up about the use of
NSAIDs, including the pathway and enzymes they act on and
the implications of their use and the controversy of COX-2
selective inhibitors and why increased cardiovascular
deaths occurred.

EI is going to take a (quite frankly well deserved) break
for a few days, before starting to home in on further
tips, tricks and techniques for studying, learning,
remembering and most importantly passing the Final
Examination.

Please keep visiting, as there is something new on the
horizon which is going to be developed behind the scenes,
and will initially be released in bits and pieces before
coming together in one fell swoop.

Congratulations to all those who passed, and our
comiserations for those of you that didn’t. Stick with EI
and we will try to bring you information to maximise your
chances of success.

EI has heard about some more exam questions that have come up in the vivas this week, so here we go:

-Describe the pathophysiology of ARDS
-Describe your management of a patient with ARDS
-How do you optimise PEEP?
-How do you optimise PEEP if you don’t have fancy ventilators(!)?

-Draw a saggital section of the eye.
-Describe the anatomy.
-Mark the insertion of the conjunctiva into the sclera.
-Why is the anatomy of the eye important to anaesthetists?

A physics/measurement question on CPX and examining a CPX test result came up.

Future Sounds…
Keep an eye on this blog for some well researched answers to the questions that have come up in this last Final FRCA Exam.
Also, as the next sitting approaches, EI will bring together more resources, and simplified explanations of topics that might come up. Hopefully we can help others achieve the same success we have, by sharing some of our revision tips and tricks, and some of the cunning ploys we adopted.
We welcome any suggestions and questions, please feel free to comment or contact EI on the email link in the right-hand column.

If you have a topic you struggle with, ask us, and we will try to help.

If you are still waiting to take your viva tomorrow: GOOD LUCK!

Firstly, E.I. hears that the questions today included THAT
kyphoscoliotic lady for cholecystectomy, a head injured
child with fractured tib and fib, a question about
categorisation of Emergency LSCS, and a question about
heart blocks. More detailed information than that has not
really yet come this way.

Secondly, for those doing the Final FRCA in the
future….the grapevine has told us that The Clinical
Anaesthesia Viva book is going to reach us in a second
incarnation sometime soon, so keep your eyes peeled for
that one….

If you have any information you want to share, please pass
it on to examintelligence”AT”googlemail.com .

Good luck to anyone still awaiting their viva!