Nitrous Oxide

Question 1

What is the blood gas coefficient?

Answer 1

0.47 (very low)

Question 2

What is the MAC?

Why is this significant?

Answer 2

104

At 1 atm adequate anaesthesia can never be provided by nitrous alone

Question 3

Is nitrous metabolised?

Answer 3

Less than 0.01% is metabolised to N2

Question 4

What are the toxic effects?

Answer 4

Oxidation of the cobalt ion in Vitamin B12 (cofactor for methionine synthetase)
Dec synthesis of methionine and DNA -> megaloblastic change in bone marrow
Teratogen in rats
Inc respiratory complications, post-op fever, wound infections, PONV

Question 5

How is it manufactured?

Answer 5

Heating of ammonium nitrate which breaks down to N2O and water vapour. Contaminants are actively removed

Question 6

Outline the storage

Answer 6

Stored as a liquid in blue shoulder and white body cylinders at 51bar and 20 deg

Question 7

What is the critical temperature?

Answer 7

36.5degC

Question 8

What is diffusion hypoxia?

Answer 8

When inhalation of nitrous is discontinued abruptly, the reversal of partial pressure gradients means nitrous leaves the blood to enter alveoli in high volumes. This dilutes the PAO2 to such an extent that the PaO2 drops. Provided a reservoir of O2 in the lungs at the end of anaesthesia can counteract this effect

Question 9

What is the oil:gas coefficient

Answer 9

1.4

Question 10

Why is it important that the blood:gas coefficient of nitrous oxide is ~34x greater than that of nitrogen?

Answer 10

The differential solubility means that nitrous can leave the blood to enter air-filled cavities 34x more rapidly than nitrogen can leave the cavity to enter blood. This can cause an increase in volume or pressure of an air-filled cavity depending on the compliance of the wall of the cavity. This can have devastating effects eg in the eye post retinal tamponade