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Principles Cont'd

Last week, we looked at positive & negative pressure, gas flows & safety mechanisms involved with the anaesthetic machine.

This week, we will explore the breathing system, scavenging & soda lime, ventilation & cleaning & checking the anaesthetic machine.

A downloadable pdf can be found here


The breathing system involves the components that allow the flow of fresh gas & volatile agents along with the escape of expired gases.

This involves

- internal tubing to allow the passage of those gases,

- valves that prevent back flow (they are called unidirectional flow valves aka the inspiratory & expiratory valves),

- a breathing circuit (which is more tubing!) &

- a system to remove expired gas

Inspiratory & Expiratory Valves can be seen under the domes that lock in place in order to preserve positive pressure


The system to remove expired gases is called scavenging. This involves a negative pressure unit (i.e. suction). If the scavenging is set too high, you may not be able to build any positive pressure on the patient’s end along with losing fresh gas flow. If the scavenging is set too low, then the patient may be rebreathing their expired gases creating a toxic mixture &/or the OT staff may end up breathing it in.

Another component to this system is a product called soda lime. Soda lime is a made up of mostly calcium hydroxide & is quite a dangerous substance which can cause eye & respiratory irritation if handled incorrectly. A chemical reaction is produced when carbon dioxide is passed through this greyish-white mixture. The reaction produces heat, water & also changes the mixture’s colour from greyish-white to purple. This helps staff to indicate when the product requires changing. The patient will also demonstrate signs that their carbon dioxide levels are rising & these include mostly signs of Sympathetic Nervous System activation - a rise (& later a fall) in Heart Rate & Blood Pressure, hyperpnoea, dysrhythmias, flushing & finally, you will see an increase in end tidal carbon dioxide on the monitor.

ABOVE: Fresh Soda Lime in a canister attached to the machine

ABOVE: Exhausted Soda Lime following emptying from the canister, ready for discarding (note the colour change)


Over the past decade, there have been major upgrades to the ventilators found on an anaesthetic machine. Anaesthesia ventilators more closely resemble those found in ICU (but without the bulkiness!). The 2 main settings discussed when learning about ventilation principles are pressure control mode & volume control mode. There are also other settings available such as those that allow for the patient to breath with the ventilator or those where the ventilator does all the work such as when the patient is paralysed.

We would need a-whole-nother week to get through discussing the principles found on the ventilator but keep your eyes peeled as Thrive has an exciting new opportunity coming up in a few months about a more in-depth review of perianaesthesia nursing fundamentals.


Each machine will vary ever so slightly so it is imperative that the manufacturer’s guidelines are adhered to. You may have that guideline attached to the machine or your policy document on checking the machine outlines, more specifically, how to check the machine.

There are different level checks that are performed by different individuals. A Level 1 Check for example is performed by the manufacturer or your hospital’s Biomedical Engineer. This check is performed when a new machine is installed or following routine servicing & involves a thorough review of the functioning, accuracy & safety of the entire system. A Level 2 Check is ultimately the responsibility of the anaesthetist but may be performed by the anaesthetic assistant at the beginning of every day or list. A Level 3 Check should be performed prior to the beginning of each case to ensure the integrity of your machine prior to the patient being anaesthetised.

The Australian & New Zealand College of Anaesthetists [ANZCA] have a guideline on the different level checks & what is required at the checks. You can find it here


As you could appreciate, when you have patient breathing through a machine, it would no doubt require cleaning every once in a while. Certain components will be cleaned or changed at the end of each day (i.e. the breathing circuit & soda lime) whilst others such as the internals of the breathing system, can hold out for a 3-6 monthly clean. It all depends on how often the machine is being used. If there is visible soiling or dust on the machine, then this can easily be cleaned with a neutral detergent wipe. The more involved cleaning procedures, whereby a machine is pulled apart, will require additional training from the manufacturer or your education team.

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