Gas and vapour controls

Question 1

Describe the function of the safety mechanism concealed behind the control knobs on the rotameter block

Answer 1

Chain mechanism between O2 and N2O control valves ensure that N2O can never be switched on without O2 being on at the same time. Furthermore, the chain ensures that if the O2 is switched off, the N2O is automatically switched off. SO O2 concentration can never fall below 25%. In addition, turning down the N2O does not affect the O2 flow.

Question 2

How is hypoxic mixture prevention achieved with fully electronic gas control systems?

Answer 2

The software prevents delivery of hypoxic gas mixtures.

Question 3

There are three separate rotameters for O2, N2O and Air. How is the total fresh gas flow calculated if they are all turned on?

How are the different gas concentrations calculated?

Answer 3

The total fresh gas flow is the sum of the flows from each rotameter

3L/min of O2 + 6L/minute of N20

3/9 = 33% O2
6/9 = 66% N2O

2L/min Air + 2L/min O2

2/4 O2 = 50%
2/4 Air = 50% But Air has 21% O2 –> 21% of 50% is also O2 = additional 10.5% O2

Overall
O2 = 60.5%

Question 4

What are the two vaporizer types in common use in anaesthetic practice

Answer 4

Plenum vaporizers - typically mechanical devices with control knob

Fuel injection vaporizers - under electronic control
- no control knob (electronic)

Question 5

How is adminstration of more than one vaporizer at a time prevented

Answer 5

Safety mechanical interlock

Question 6

What happens when a volatile agent is added to the incorrect vaporizer? How is this potential problem overcome

Answer 6

Some volatile agents do not vaporize at all in the wrong vaporizers.

A keyed filling system is used to prevent this

Question 7

What is unique about the desflurane vaporizer

Answer 7

Boiling point of Desflurane = 23.5ºC
(Boiling point is when SVP = atmospheric P)

Desflurane’s unique characteristics (high volatility/moderate potency) require the use of a special vaporizer for proper utilization of this gas. Desflurane vaporizers are heated to 39 degrees C, which increases the vapor pressure in the sump to 173 kPa preventing the possibility of boiling in warm OR rooms. Providing an external heat source compensates for the significant heat loss associated with desflurane vaporization. And unlike stand variable bypass vaporizers that pass fresh gas through the vaporizing chamber, desflurane vaporizers add agent directly to the gas stream.

Question 8

What size connectors are used in adult breathing systems

Answer 8

22mm male and female connectors at both the patient end and the machine end

Question 9

Where are 15 mm male and female connectors used within the breathing circuit?

Answer 9

endotracheal tubes and laryngeal mask airways

Question 10

How can a 22mm connector incorporate a 15mm connector

Answer 10

Many male 22mm connectors can act as the female 15 mm connector as their internal diameter is 15 mm

Question 11

What size connectors are used with Anaesthetic Gas Scavenging Systems (AGSS)

Answer 11

Usually 30 mm sized conical connectors

–> usually attaches to the expiratory APL valve of a circle breathing system

Question 12

Why corrugate breathing tubes?

Answer 12

Decrease risk of kinking and occlusion when bent

Question 13

What are the 3 functions of the Adjustable Pressure Limiting Valve (APL)

Answer 13

1. One-way exhaust valve
2. Mechanism to enable adjustable positive pressure ventilation using a reservoir bag
3. Safety blow off valve

Question 14

What are the functions of the reservoir bag

Answer 14

1. Source of gas during peak inspiration (flow rate up to 30 - 40 L/min)
2. Indicator of spontaneous ventilation
3. Allows for positive pressure ventilation by squeezing the bag
4. Provides some potection from over-pressure in the breathing system

Question 15

What determines which breathing apparatus is single use versus re-usable

Answer 15

Equipment that is patient side of the filter –> single use