The anaesthetic machine Flashcards
Components / roles of the anaesthetic machine
- Artificial atmosphere
- Anaesthesia breathing system
- Ventilator & emergency life-support system
- Monitoring & backup & safety features
Essential components of the anaesthetic machine
- Gas supply
- Measurement of gas & oxygen & vapour
- Anaesthetic vaporisers
- Breathing circuit closed / open system
- Use as a resuscitation apparatus
Role of flowmeters
used to indicate how much fresh gas flow is given to the patient
The different valves and mechanisms of the flowmeters
o The oxygen knob is a fluted valve = safety feature
o If the machine has two pipes per knob, the units are different right-sided pipe is used when the flow on the left-side is exceeded
o Fail-safe valve in-between different flowmeters that shut off the oxygen supply so that you can’t have back pressure = safety feature (to prevent giving the patient a hypoxic mixture)
Role of the Fail-safe valves
shut off or proportionally decrease the supply pressure of all other gasses (nitrous oxide, air, carbon dioxide, helium, nitrogen) as the oxygen supply pressure decreases
Total flow rate (FGF)
FGF > fresh gas flow = total flow coming out of common gas outlet of machine into attached breathing circuit
Factors influencing the amount of FGF given:
- Spontaneous breathing vs. ventilation
- Practitioner preference
- Induction vs. maintenance vs. recovery
- Age of the patient (children require higher flow for induction)
What is ventilation-perfusion mismatch?
there may be an area in the lung that is receiving blood flow but is not participating in gaseous exchange (meaning that that area is not getting its oxygen pushed up through diffusion).
Vaporisers:
delivers anaesthetic to patient in a calibrated manner & reads in %
Controls concentration of anaesthetic in gas mixture
Can only use one vaporiser at a time even though there are sometimes two on one machine
How do vaporisers work?
The total fresh gas flow (FGF) is split according to a splitting ratio where a certain percentage is going through a vaporiser (where it is fully saturated) to then be joined with the rest of the fresh gas flow to be given to the patient
As fluid is vaporised, the remaining fluid is cooled > this affects vapour pressure (vapour pressure is temperature-dependent)
Safety features in vaporisers?
- Different vaporisers are colour-coded according to the drug it works on > the specific drug / gas comes in a bottle / cylinder that matches the colour of the vaporiser to ensure that the correct drug is added to the correct vaporiser
- You also have a special nozzle on each gas cylinder that you use to fill these vaporisers that are only compatible with the specific vaporiser for that gas
Ventilators:
Ventilation implies > patient is not breathing spontaneously AND that we are doing the breathing for them via intermittent positive pressure ventilation (IPPV)
(2)
Reasons why most anaesthetised patients require ventilation:
- Anaesthetic agents depress respiration
- Muscle relaxants stop respiration
- Opioids stop respiration think of the side effect of respiratory depression in high doses of morphine
- Any open cavity surgery spontaneous ventilation is inadvisable
Formula used in ventilation
Minute volume = tidal volume x frequency
in mechanical ventilation, give a tidal volume of 8mg/kg at 9 bpm
Types of ventilators:
- Electric ventilators measures the flow electronically & calibrates itself to give that flow (bellows are on the inside & cannot be visualised from the outside)
- Pneumatic ventilators bellows are on the outside & can be visualised from the outside
Pin Index System
*Go look at a picture because they WILL ask to identify O2, Air, Entonox, NO2 pin index holes
Safety principles
- Don’t want to swop gases accidentally
- Don’t want to run out of oxygen
- Don’t want to deliver too little oxygen
Preventing swopped gases
- Extensive colour coding
- Pin index system cannot connect incorrect tubing to cylinder
- Diameter index safety system different shapes connecting
different pipes to the walls (not interchangeable)
Colour coding system for the different gases
Oxygen= White
Nitrogen= Black
Air (oxygen & nitrogen)= Black & white blocks
Nitrous oxide= Blue
Entonox (N₂O & oxygen) = Blue & white blocks
Carbon dioxide = Grey
Vacuum Yellow
Methods we use to prevent the depletion of oxygen
- gauge that reads the pressures in the pipeline to ensure that oxygen is coming out (± 3x atmospheric pressure)
- There is also a similar gauge on the backup oxygen cylinder (± 137x atmospheric pressure)
- Fail-safe valve that cuts off nitrous supply when the oxygen is too low
- Ritchie whistle = oxygen supply failure alarm
- Backup oxygen flush in the system so that you have access to oxygen when there is failure of the wall oxygen
- Flowmeters = will drop if flow of oxygen decreases
- One-way valves in the system to ensure that all oxygen is going to the patient and not backwards
- Gas measurement system in the circuit that measures oxygen
Preventing the delivery of a hypoxic mixture
- Proportioning system (nitrous oxide & oxygen) > doesn’t allow you to give the nitrous without the oxygen (as you increase the nitrous, the oxygen will also increase)
> It is a way of fixing the oxygen percentage to at least 30% > 70% nitrous is the max you can give - Oxygen measurement > analyser is attached as closely to the patient as possible & continuously measures the oxygen levels
