This contains a series of corrugated tubes, a valve through which gas can escape from the circuit, and a bag that the anaesthesiologist uses to manually ventilate the patient’s lungs when necessary. There are many different types of anaesthetic circuit, and the anaesthetist will choose the correct one according to whether the patient is going to breathe spontaneously or be ventilated, and according to the size of the patient.
The end of the circuit is attached to the mask, the laryngeal mask airway or the endotracheal tube through which the patient will breathe.
Most machines have a particular type of circuit attached to them, called a circle system. This is particular useful for preventing wastage of expensive gases and volatile agents. It consists of a tube carrying gas from the machine to the patient, and another carrying expired gas away from the patient to a device called a carbon dioxide absorber.
This is a canister containing a chemical substance (soda lime) that absorbs the carbon dioxide produced by the patient, and the remaining gas is then re-introduced to the tube carrying gas to the patient. This means that oxygen, nitrous oxide and volatile agent from the expired breath are re-used in the inspired breath.
If the circuit is absolutely free of leaks it is then possible to eventually only feed the body’s oxygen requirement (about 250 ml per minute) into the circuit from the machine when the body of the patient is saturated with the required amount of nitrous oxide and volatile agent. This results in huge cost savings. This technique is now very easy to use due to the development of monitors that precisely measure the composition of the gas in each breath in and out of the patient.
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