Medical Gas Supplies

Question 1

1 atmosphere equates to (approximately):

bar
pounds per square inch (psi)
kPa
cmH2O

Answer 1

1 bar

15 pounds per square inch (psi)

100 kPa

1033.23 cmH2O

Question 2

What is the SI unit of pressure and what are the base units

Answer 2

Pascal (Pa)

base unit: N/m^2

Question 3

What is liquid oxygen stored in

Answer 3

Vacuum Insulated Evaporator - VIE (large vacuum flask)

Question 4

How is the Oxygen kept in liquid form?

Answer 4

Cooled to -150ºC

Pressurized to 10 atm

Question 5

Why is O2 allowed to warm to room temperature before use in the hopital

Answer 5

When oxygen is warmed to room temperature it becomes a gas and is regulated to a pressure of 4.4 x 100kPa

this allows large amounts of oxygen to be stored in a relatively small container

Question 6

Concerning the Vacuum Insulated Evaporator, what is the pressure in the:

1. Liquid oxygen chamber
2. Pressure limit for the blow-off safety valve
3. Pipeline

Answer 6

1. 10 bar
2. 15 bar
3. 4 bar

Question 7

How are the remaining contents of the VIE measured

Answer 7

Pressure difference between the top and bottom of the liquid oxygen chamber –> from which the remaining contents can be calculated

Question 8

How is Nitrous Oxide Stored and what is the pipeline pressure for N2O

Answer 8

In cylinder banks connected to a minfold which acts as the pressure regulator between the cylinders and the pipeline –> pipeline pressures are 4.4 x 100 kPa

Question 9

How is medical air supplied

Answer 9

From a compressed air plant that includes high quality drying and filtration

Blending oxygen and Nitrogen on site can also provide medical air

Pipeline pressure 4.4 x 100 kPa

Question 10

How is suction generated within the hospital

Answer 10

A large suction plant within the hospital –> multiple traps at the every point of use ensure the vacuum system does not become contaminated

Question 11

Describe the contents of a N2O cylinder

Answer 11

Mixture of liquid and vapour N2O. As vapour is used/ removed, more of the liquid evaporates and the cylinder cools as the latent heat of vaporization is used during this process.

Question 12

Does the gauge represent contents on the N2O cylinder?

Answer 12

No. It represents the saturated vapour pressure of N2O at that temperature.

this is unlike an oxygen cylinder that behaves like an ideal gas in which case the pressure on the gauge is proportional to the number of moles present at constant volume and temperature – ideal gas equation

Question 13

How is the remaining contents of N2O cylinder known

Answer 13

The cylinder is weighed and the tare weight (stamped near the top of the cylinder, and is the empty cylinder weight) is subtracted from the measured weight.

Question 14

How is entonox delivered and what is it used for

Answer 14

Demand valve - enabling patient control while reducing the risk of excessive sedation

Obstetrics, trauma, dental, minor procedures

Question 15

How is entonox produced

Answer 15

Using the Poynting effect

Question 16

What is the Poynting effect?

Answer 16

Dissolution (dissolving) of gaseous oxygen when bubbled through liquid nitrous oxide. Vaporization of this liquid mixture forms a mixed vapour containing oxygen/nitrous oxide.

Question 17

What is the pseudocritical temperature of entonox and why is it named thus. What happens if entonox goes below its pseudocritical temp

Answer 17

-6ºC

It is named pseudocritical temperature as the mixture of these substances has a different (and therefore pseudo)critical temperature to each of the substances in the mixture (O2 is -118C and N2O is 36.5ºC)

Below -6ºC lamination occurs as N2O liquifies and separates from the dissolved O2 gas.

Question 18

Describe the non-interchangeable safety system at the wall gas outled

Answer 18

Diameter indexed collar with a unique internal and external diameter

Question 19

Describe the non-interchangeable safety system at the anaesthetic machine end of the pipes

Answer 19

Non-interchangeable screw threads (NIST)

Question 20

Describe the E-cylinder pressures for O2, N2O and Medical Air

Answer 20

Oxygen

13 700 kPa

(137 bar)

Nitrous oxide

4400 kPa

(44 bar)

Air

13 700 kPa

(137 bar)

Question 21

What is an oxygen cylinder yoke?

Answer 21

High pressure in cylinders means that the fittings need to be strongly constructed.

Stainless steel cylinder fittings called yokes are designed to support the full weight of the cylinder and withstand the high pressures

Question 22

How is an air tight seal obtained between the cylinder and the yoke?

Answer 22

Specialized washer called a Bodok seal

Neoprene washer within an aluminium ring

Pressure build up pushes the neoprene washer outwards confined by the aluminium ring making the washer fatter and improving the seal

Question 23

How many Bodok seals can be used between cylinder and yoke?

Answer 23

ONLY ONE

Question 24

Describe the non-interchangeable safety system for cylinder-yoke connections

Answer 24

Pin indexed safety system (PISS)

Question 25

What are the key steps when changing an O2 E-cylinder on an anaesthetic machine

Answer 25

Flush the O2 before opening the cylinder/yoke connection

Check for Bodok seal after removing old cylinder

Question 26

How many lites of O2 does an E-cylinder hold?

Answer 26

680 L when full and is used on most anaesthetic machines

Question 27

How many litres does a D-size cylinder hold

Answer 27

340 L when full and is used for transport

Question 28

How many litres does a G-size cylinder hold

Answer 28

3400 L when full (usually has bull nose fitting rather than a pin index one and is used in the wards

Question 29

What is a Kevlar wrapped aluminium cylinder?

Answer 29

Holds 460 L
Filled to 200 x 100 kPa (versus 137 x 100 kPa)
Lightweight cylinder with integral reducing valve used for transport