Nitrous Oxide

BT_GS 1.27 Describe the pharmacology of nitrous oxide


T / F   nitrous oxide does not support combustion

T / F   nitrous oxide acts synergistically with volatile agents to produce anaesthesia

T / F   nitrous oxide does not cause any peripheral vasodilation

T / F   nitrous oxide acts on GABA receptors in the brain

T / F   nitrous oxide use is associated with post-operative MI

BT_AM 1.3 Describe the effect of anaesthetic agents and other drugs on airway reflexes

Hello again my friends! It feels like ages since I have posted on the blog, but I am happy to be back…

Today, a fairly core bit of business for us.

Part of my absence involved a holiday. We visited the Science Museum in London (not my favourite to be honest) where I came across this delightful contraption.


We all know what nicotine does to the airways when inhaled, but how about when given rectally as this device was designed for ?!? It was used in patients who had drowned in the hopes of reviving them….

BT_AM 1.3 Describe the effect of anaesthetic agents and other drugs on airway reflexes

This article looks at the problem of laryngospasm, but does discuss the impact of anaesthetic agent on airway reflexes.

All iv induction agents EXCEPT ketamine depress laryngeal reflexes equally  TRUE/FALSE

Hypercapnoea protects against laryngospasm   TRUE/FALSE

Of the modern volatiles, sevoflurane causes the greatest depression of airway reflexes  TRUE/FALSE

A surgical depth of anaesthesia is in itself a protection against laryngospasm TRUE/FALSE

In addition to its irritant effect on the airways, nicotine stimulates the respiratory centre   TRUE/FALSE


SAQ 2017.2 Question 5

Outline the factors which influence the time taken for loss of consciousness with an inhalational induction of anaesthesia.

Loss of consciousness will be faster with a smaller FRC     TRUE/FALSE

Loss of consciousness will be faster in a patient who is anxious and struggling    TRUE/FALSE

Loss of consciousness will be faster with a more soluble anaesthetic agent    TRUE/FALSE

Loss of consciousness will be faster with an increased cardiac output    TRUE/FALSE

Benzodiazepine premedication may speed the process in some patients, and slow it in others    TRUE/FALSE



SAQ 2017.2 Question 3

a) Describe the immediate cardiovascular responses to the sudden loss of 30% of the blood volume in a healthy awake person
b) How are these responses different if the patient is undergoing anaesthesia with sevoflurane?

The decrease in blood volume will be detected by the high pressure baroreceptors in the atria    TRUE/FALSE

The response will be mediated by the cardiovascular centre in the medulla    TRUE/FALSE

There will be arterial but not venous constriction    TRUE/FALSE

Sevoflurane will impair contractility    TRUE/FALSE

Sevoflurane will depress baroreceptor signalling    TRUE/FALSE


Aviation 6 – Decompression Illness

Today we return to North Africa.  Flying Officer Reynolds flew 25 missions above 40,000 feet over a single month.  The German Ju 86’s flew at ever increasing altitudes as three out of the four available aircraft had been shot down by British Spitfires.  The final Ju 86 flight was at nearly 50,000 ft and Reynolds was again in pursuit.

He had been at over 45,000 feet for over an hour and he was suffering from the effects of altitude:

“his whole cockpit, instrument panel, control column and perspex were thickly coated with ice; his body was racked with pain and his arms temporarily paralysed, and his eyesight also failing with weakness.”  — John Frayn Turner.   British Aircraft of the Second World War

Reynolds was probably suffering from “The Bends” or decompression illness.  You may have encountered this condition in the context of diving but it is also well recognised in high altitude aviation.

Occasionally and ironically, tourists are diagnosed with decompression illness in Alice Springs. How is this possible you ask?

Well, they go diving in North Queensland and then fly to the Red Centre, the exposure to altitude soon after diving is enough to “Bend” them.

The relevance of decompression illness to anaesthesia may seem a little obscure.  The body in decompression illness is simply behaving like a human vapouriser where nitrogen is the volatile agent.

BT_SQ 1.12 Describe the principles and safe operation of vaporisers

TRUE/FALSE Henry’s Law is relevant to vaporiser functioning 

TRUE/FALSE The Aladin cassette vaporiser is an example of an injection vaporiser system

TRUE/FALSE Modern vaporisers use an electrical heating coil to compensate for the cooling caused by latent heat of vaporisation

TRUE/FALSE A plenum vaporiser is designed so that the gas leaving the bypass is fully saturated under normal conditions

TRUE/FALSE The Tec Mark 5 vaporiser is designed to be ‘tip resistant’

As an extra exercise, see if you can find or work out the properties of nitrogen and the circumstances that relate to it “vapourising” in the body at altitude (in the same the way you would think about an inhalational agent).

BT_GS 1.53 Describe the synergism between anaesthetic agents, opioids and regional blockade and how this is used clinically

TRUE/FALSE  Moderate doses of opioids can reduce MAC of volatile agents by as much as 75%.

TRUE/FALSE  50% reduction in doses is expected when propofol and midazolam are used together for hypnosis.

TRUE/FALSE  The exact degree of drug synergism can be calculated from pharmacokinetic data of individual agents.

TRUE/FALSE  The bispectral index is additive when propofol and remifentanil are used in total intravenous anaesthesia.

TRUE/FALSE  Midazolam has no effect on the ketamine dose required to suppress movement to a noxious stimulus.

BT_GS 1.50 Describe the concept and clinical application of MAC in relation to inhaled anaesthetic agents

TRUE/FALSE  MAC-AWAKE for Nitrous Oxide is approximately 30% of its MAC-Value

TRUE/FALSE  Sevoflurane is metabolised by the cytochrome p450 enzyme 2E1

TRUE/FALSE  The MAC of sevoflurane in a 6 month child is approximately 2%

TRUE/FALSE  The higher the inspired anesthetic concentration, the less it then diminishes because of uptake

TRUE/FALSE  Administration of 50% Nitrous oxide will double the volume of air-filled spaces

Pharmacoeconomics of volatile anaesthesia

BT_GS 1.23 Describe the physical properties of inhalational agents, including the principles of vaporisation

Following on from yesterday’s post is another about volatile agents.

While pharmacoeconomics is not part of the primary exam it does have an LO in the curriculum (AR_MG 3.2) but the answers to most of the following questions require primary exam knowledge.

  1. Demonstrate that liquid volatile usage in ml per hr, for agents used in 2017, is approximately 3 x fresh gas flow (l/min) x inspired concentration (%)
  2. Estimate the amount of sevoflurane used the next time it is part of your anaesthetic.
  3. Some anaesthetic machines will provide you with an exact usage.  See how close you can get with your calculations.
  4. How much did the sevoflurane cost?

Schrödinger’s Registrar – Part 2

BT_GS 1.23 Describe the physical properties of inhalational agents, including the principles of vaporisation

Yesterday the following was asserted:

“You will be anaesthetised if you smash a bottle of sevoflurane in an anaesthetic room (assume the air-conditioning is broken and the doors are sealed)”

If you had some trouble coming to a conclusion then here are a few hints:

Assume the sevoflurane completely vaporises.

Assume the room is a certain size – e.g. 5m x 4m x 3m

How much gaseous sevoflurane is produced from the liquid sevoflurane?

Avogadro is important here (PV = nRT)

What concentration of sevoflurane will cause anaesthesia?

Tomorrow, we will be looking at the reverse situation – i.e. how much liquid sevoflurane is consumed from a certain amount of gaseous sevoflurane.





Schrödinger’s Registrar – Part 1

BT_GS 1.23 Describe the physical properties of inhalational agents, including the principles of vaporisation

This LO has been the subject of previous posts, but volatiles and vaporisation are core knowledge and a detailed understanding is important.

Today there is only one question.  You can treat it as a True/False statement or consider a longer answer by treating it as a discussion point.  You decide which will lead you to greater understanding.

This question will require you to make a few assumptions in addition to the two provided. See if you can varying those assumptions to change your conclusion.

“You will be anaesthetised if you smash a bottle of sevoflurane in an anaesthetic room (assume the air-conditioning is broken and the doors are sealed)”

If you are having trouble with this question, stay tuned for hints tomorrow.