Because anaesthetic agents depress breathing and muscle relaxants paralyse breathing, it is often necessary to use a ventilator to take over breathing for the patient. The ventilator is attached to the anaesthetic machine and connected to the breathing circuit. You will be able to recognise the ventilator on the side of the anaesthetic machine – it is usually a large clear plastic cylinder containing black bellows.
The gas to be breathed in by the patient fills the inside of the bellows, which rises in the cylinder, and to breathe in the bellows is compressed by gas outside the bellows but inside the cylinder. With each breath in, the bellows then falls, and the cycle begins again.
The anaesthesiologist can control many aspects of the ventilation, including how many breaths per minute and how much gas is used for each breath. But ventilation is a complicated science and there are many adjustments that can be made by the anaesthesiologist on the ventilator to suit the condition of the patient’s lungs.
The adequacy of ventilation is judged not as you might think by the amount of oxygen in the patient’s bloodstream, but by the amount of carbon dioxide.