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Basic Principles of the Anaesthetic Machine

You can download your pdf version of this with some additional pictures here

The anaesthetic machine is a literal life saver – it helps to maintain oxygenation, provides ventilation & can induce & maintain anaesthesia via the delivery of fresh & volatile gas flows throughout a case (we will learn what volatile agents are in May).

There are lots of different types of anaesthetic machines out there – ones that look like space stations & others that have looks of shiny bells & whistles. Modern machines are thankfully all fairly similar – they work off of the same principles so remembering these can sometimes get you out of a jam.


In order to delivery these gases, positive pressure is required. When we breath normally, we suck air into our lungs – this is an example of negative pressure. When a person can no longer take that breath on our own, we need to force the gas into the lungs – which is an example of positive pressure. You’ve probably heard of these 2 concepts before when looking at wall suction & ventilation techniques in intensive care patients. The anaesthetic machine has its own high flow suction (just like wall suction) along with a system that helps to remove any gases breathed out by the patient. This is called scavenging & we take a more detailed look at it next week.

There are also other parts of the machine that can build positive pressure. If you lose pressure in the machine there is likely to be a leak somewhere & thus the patient cannot be adequately ventilated or oxygenated. We will explore where these leaks may occur next week when we review the machine components.


The machine requires a supply of fresh gas with oxygen, nitrous oxide & medical gas outlets provided. These supplies come from a central source within the hospital. You may have seen a big white reservoir labelled Oxygen on it or even the multiple large cylinders of nitrous & medical gas. No doubt you’ve also seen the smaller cylinders – newer machines contain these cylinders on the back of the machine as an emergency reserve (i.e. if your main wall supply fails). It’s important to remember that these gases are highly flammable & thus are very dangerous is not handled correctly.


Colour coding

Colour coded systems are used throughout healthcare so it is no surprise to see this safety option used in the operating theatre. You will find the gas hoses are different colours, the reserve cylinders have different coloured markings & those volatile agents also have distinctly coloured markings.

Hose fittings

In addition to the colour coding, the gas hoses also have a particular fitting in each hose making it very difficult to connect the incorrect hose.

Cylinder Seals

As with the hose fittings, gas cylinders also have a fitting mechanism that prevents for example an Oxygen cylinder being placed where a Carbon Dioxide cylinder is meant to be placed. These are referred to the as the Pin Index System. There are also O-rings between the fittings & the cylinder that avoid gas leaking & they are called Bodok seals.


When the main supply of gas is disconnected or lost, a very distinct whistling noise will be heard. This is checked before the start of each operating theatre list to ensure this safety mechanism is functioning. Modern machines also have alarms within the self-checking component of their daily checks where leaks are noted & reported in their system. If the malfunction is serious the computer will not allow you to proceed to have an operational machine thus rendering it useless!

It’s also important to note here that even with all of these shiny objects to keep patients safe, nothing is as vigilant as the anaesthetist & the assistant. So not only are they monitoring the patient, they also assess their equipment on an ongoing basis. The alarms are however extremely helpful during busy times of the case – on induction, during a tricky part of the procedure, on emergence or during an emergency

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