This is a continuos flow machine that provides a mixture of gases and vapours at a constant rate into a reservoir, usually a bag, from which the spontaneously breathing patient can inhale.
Anaesthetic machine - click to enlarge
The flow can also be delivered to a ventilator if the lungs are to be inflated mechanically. The anaesthetist will adjust the flow so that carbon dioxide does not accumulate. The expired gas is ducted away into a scavenging system.
The machine also contains pressure regulators. These are important as the gases used are pumped to the operating theatres at very high pressures that need to be regulated.
The machine also has vaporisers, as apart from nitrous oxide, most anaesthetics are liquids at room temperature but need to be administered as vapours. The flow of gases through the machine is monitored by special flow meters.
The most economical breathing system used is the 'circle system'. This is a system where expired gases are re-circulated in a circle system with carbon dioxide being removed by a reaction with soda lime. There are many different configurations of the breathing system that are classified by the Mapleson system.
Breathing system - click to enlarge
The three main gases used are:
The majority of the equipment can be divided into respiratory devices and vascular access devices.
The main types of respiratory devices are:
1. Endotracheal Tubes - These allow positive pressure ventilation and prevent contamination from contents of the pharynx and the stomach.
Endotracheal tube - click to enlarge
2. Laryngeal Mask Airway (LMA) - This is an airway intermediate between the face mask and the tracheal tube.
Laryngeal mask - click to enlarge
3. Oxygen Therapy Devices - This provides oxygen-enriched air.
4. Ventilators - There are many different types of ventilators which are classified on whether they are use negative or positive pressure, their mechanism of action and their power source.
Ventilator - click to enlarge
The main vascular devices are:
1. Intravenous Cannulae2. Intravenous Giving Sets3. Blood Warmers (for warming refrigerated blood)
Other equipment is used to monitor other organs of the body. The other equipment ranges from simple equipment such as urinary catheters for assessing kidney function to more complex devices such as bispectral index (BIS) monitors for assessing brain activity.