2.1 Of all the agents we use, only Oxygen is stored in gaseous form at room temperature. This is because the critical temperature of Oxygen is -118 degrees Celsius. Consequently, cylinders containing Oxygen obey the gas laws, and as I mentioned in tutorial 1, the pressure gauge on the cylinder directly represents the amount of gas inside as PV.

2.2 The main supply of oxygen in a hospital usually comes for a liquid oxygen tank. Huge quantities of the gas can be stored in this way, with the result that it is very economical. The container is a giant sized thermos flask, insulated from the outside. In the liquid container the temperature is kept at -160 degrees Celsius, the vapour pressure at this temperature being 7 bar. The oxygen is heated using a coil, and as the liquid evaporates, that which is left behind is cooled. This is because heat is required for vaporisation, the phase shift from liquid and gas: this is known as latent heat, it is borrowed by the molecules which are evaporation, and donated by those left behind. Although the cooling of the liquid reduces its saturated vapour pressure, and this is a problem in anaesthetic vaporisers (we will discuss this later), it is an advantage in the liquid oxygen container: as long as there is continuous usage of the oxygen supply, the liquid cools itself, making its storage very energy efficient.

2.3 Nitrous Oxide has a boiling point of -88 ⁰C and a critical temperature of +36 degrees Celsius. This means that, in room air (20 ⁰Celsius), N20 is a gas, but because it is below it’s critical temperature, it can be stored under pressure as a liquid. Consequently, cylinders of Nitrous Oxide contain liquid and vapour. In the UK and Ireland, 65% of the N20 cylinder is filled with liquid, vapour fills the rest (the filling ratio is 0.65) - this allows for liquid expansion if the ambient temperature rises: if there is too much liquid in the cylinder, expansion could cause explosion.

2.4 The pressure gauge on a cylinder of Nitrous Oxide measures only the vapour pressure. As the liquid in the cylinder evaporates, it constantly replenishes the gas this is used up, and the vapour pressure remains the same until there is only a small amount of liquid left. So by looking at the gauge you have absolutely no idea how much Nitrous Oxide is in the cylinder.

How do you estimate how much is in a cylinder of N20?

We apply what we learnt about Avogadro’s number in the thprevious tutorial: you need to know two things e empty. - the weight of cylinder now, and the weight when it is Subtract the N20 two and you have the weight of the liquid inside. We know that 1 mole of N20 occupies 22.4 litres at standard temperature and pressure. The molecular weight of N20 is 44. If the cylinder contains 2.5 kg (2500 g) of N20, then you can calculate the volume of gas by dividing the weight by the molecular weight and multiplying it by 22.4: 2500/44 x 22.4 = 1272 Litres.

People get confused about the physical properties of anaesthetic vapours, because each has a different relevance. This is a simple overview:

Solubility Illustration 1.

Imagine two cups of warm water: into one you put a spoon of sugar and into the other a spoon of sand. Which will be in higher concentration in the bottom of the cup? The sand is insoluble, the sugar dissolves, so very little reaches the bottom. For bottom of cup read brain.

Solubility Illustration 2.

If one breathes in a highly soluble anaesthetic vapour, such as Ether, as soon as it arrives in the alveoli it dissolves off into the bloodstream and ends up in muscle, fat, bone - everywhere, and so, as you can imagine, very little ether arrives where one wants it - in the brain. Consequently it takes "all day" to go to sleep and wake up. Desflurane, on the other hand, is extremely insoluble; it stays in the alveoli and only a small amount of agent passes, undissolved, in the blood; the tension thus becomes high in the brain (if the concentration of drug is high in the alveoli, it’s high in the brain!). Because Desflurane is so insoluble, its alveolar concentration (and consequently its brain concentration) rises and falls very quickly - and the patient goes asleep and wakes up very quickly.

BGPC Illustration

The concentration effect and the second gas effect are terms describing different aspects of the same phenomenon: the reduction in the size of the alveolus due to the marginally greater solubility of Nitrous Oxide over Nitrogen.

What people forget about the MAC is that MAC does not equal anaesthesia, as 50% of patients will move at this concentration. MAC follows a "normal distribution"; consequently other measures such as MAC/ED 95, MAC aware and MAC awake are used. Movement does not mean awareness, this occurs at 0.3MAC.

MAC is not static:

1. MAC is higher in infants and lower in the elderly.
2. Premedication, opioids, hypothermia, hypothyroidism and pregnancy decrease it.
3. MAC is increased with anxiety and thyrotoxicosis.
4. MAC amongst anaesthetic agents is additive: 0.7 MAC of Isoflurane and 0.3 MAC of nitrous oxide is equivalent to 1 MA.C of Isoflurane. Likewise two volatile agents e.g. Sevo and Iso will have additive effects.
5. N20 is considerably less expensive than other inhalational agents, but is also much less potent: MAC (104%) cannot be achieved at normal atmospheric pressure with nitrous oxide.

1. Vapour sparing effect: the amount of inhalational agent required is reduced (decreased MAC), this is more economical.

2. Reduced inspired concentration of inhalational agents: reduction in the cardiovascular and respiratory depressant effects of the volatile agents. N20 smoothens the anaesthetic.

3. Nitrous Oxide has mild analgesic properties.

An ideal anaesthetic would act extremely quickly (be highly insoluble in blood: have a low blood gas partition coefficient), and achieve anaesthesia at very low concentrations (be very potent- highly soluble in fat [brain]: have a high oil gas partition coefficient. Desflurane is highly insoluble (BGPC of 0.42), but not potent (OGPC of 18.7, MAC of 6.0). Methoxyflurane is extremely potent (OGPC 950, MAC 0.2), but is also very soluble in blood (BGPC 13). If you find this difficult to grasp, consider Desflurane to be a 6.0 litre Porsche, which does 0 - 60 mph in 4 seconds, but guzzles fuel, and Methoxyflurane to be a 1.0 litre city car, does 60 miles per gallon, but takes all week to get from 0 to 60 mph.

2.13 The blood gas partition coefficient is important to a certain extent, as once the drug is in the blood, the lower the BGPC, the quicker the drug is excreted through the lungs. However, every tissue in the body has its own tissue-blood partition coefficient and as the tissues act as a reservoir for anaesthetic gas, the rate at which this diffuses back from the tissues ultimately determines how quickly the patient wakes up.

The combination of the tissue-blood and blood-gas partition coefficients determines how quickly the patient wakes up. As you would imagine patients wake very quickly from Desflurane, and quite slowly from Halothane. Sevo is quicker than Iso because of its much lower BGPC. Finally, as you can see, volatile agents dissolve much better in fat than in any other tissue. The more fat, the longer the wash out time: obese patients wake up much more slowly!