Anaesthetic Machines

Question 1

Five elements of all anaesthetic machines:

Answer 1

1. High-pressure gas supply
2. Gas flow measurement and adjustment (Pressure reducing valve & Flowmeters)
3. Vaporisers
4. Breathing system (Ventilator, circuit etc)
5. Scavenging

Question 2

Elements of wall gas supply to anaesthetic machine:

Answer 2

i. O2 from liquid supply in large tanks, which evaporate to form gas at ≈4 atm
ii. N2O from large cylinder manifold
iii. Schrader socket, self closing valve with protruding indexing collar to prevent misconnection
iv. Flexible colour-coded hose-pipe
v. NIST (Non-Interchangeable Screw Thread) connection to anaesthetic

Question 3

Elements of cylinder backup supply to anaesthetic machine:

Answer 3

i. Mounted on yokes at the back of the machine
ii. Pressure in a full O2 cylinder is 137 bar
iii. Safety systems:
1. Molybdenum steel alloy construction
2. Colour-coding
3. Pin-index
4. Bodok seals (Bonded disk) – to prevent leakage at the cylinder head and yoke
iv. Pass through primary regulator to have pressure reduced to just under the wall supply pressure

Question 4

Elements of gas-flow measurement and adjustment in anaesthetic machine:

Answer 4

a. Flow control valves mark the transition from high-pressure to low-pressure system. Reduce the pressure to just above atmospheric
b. Conventional rotameters have accuracy maintained by:
i. Calibration
ii. Leakproof O-rings
iii. Antistatic coating
iv. Visible bobbin
c. O2 flowmeter should be the most downstream to prevent hypoxic gas delivery in the event of a downstream leak.
d. O2 and N2O flowmeters are interlocked to prevent hypoxic gas delivery.
i. Mechanical interlocks (eg Ohmeda) have an intentional leak through the O2 valve so 175-250mL of O2 is constantly delivered

Question 5

Elements of vaporisers and backbar:

Answer 5

a. Sit on the backbar, downstream from the flowmeter
b. Drager uses Interlock, Ohmeda uses Selectatec
c. All are variable bypass, each calibrated for a specific agent
d. Safety features on the backbar with the vaporisers:
i. Oxygen failure warning device – pressure activated at <200kPa (??)
ii. Spring-loaded non-return valve (ie a unidirectional valve)
iii. Pressure relief valve set at 30-40kPa
iv. Emergency oxygen flush (from the circuit upstream of the vapourisers)

Question 6

Components of breathing system in anaesthetic machine:

Answer 6

a. Circle system or T-piece
b. Circle:
i. CO2 absorber
ii. APL valve – not calibrated, adjusted empirically. Should open at 60cm.H2O
iii. Reservoir bag
iv. Circuit pressure gauge
v. Bag/ Vent switch

Question 7

Four components of scavenging system in anaesthetic machine

Answer 7

i. Collecting system – surrounds the APL valve and the ventilator expiratory port
ii. Transfer system of tubing
iii. Receiving system that prevents excessive negative and positive pressures.
iv. Disposal (exhaust) system to atmosphere.