Some of the drugs given by the anaesthetist are injected, but others are inhaled. To deliver these inhaled drugs, as well as oxygen, your anaesthetist uses an anaesthetic machine.
The anaesthetic machine is not a machine that makes anaesthetics, but a complex collection of equipment. It has three major components: a gas mixing and delivery system; an anaesthetic breathing system (circuit) and a ventilator; and an array of monitors. Some recently developed machines have highly complex integrated electronic systems and are usually called anaesthesia workstations.
The gas mixing and delivery system
The anaesthetic machine is connected to a supply of purified gases. These gases usually include oxygen and nitrous oxide, and many machines also have a supply of compressed air. All the gases are mixed in a special device, which ensures accurate concentrations and limits the minimum amount of oxygen which can be used. To this gas mixture, the anaesthetist can add one of a range of additional, more powerful anaesthetic agents, known as inhalational agents. These come as a liquid and are placed in a device called a vaporiser, which converts them into a gas and adds them in carefully controlled concentrations to the gas mixture.
The anaesthetic breathing system (circuit) and ventilator
The anaesthetist determines the flow rate of the final mixture of gases supplied to the breathing system. This is a series of hoses about three centimetres in diameter, which connects to either the mask or the endotracheal tube, but also to a ventilator. The breathing circuit is often attached to a container of ‘soda lime’ granules: these absorb carbon dioxide that the patient exhales with each breath.
The ventilator is an automatic breathing device, which takes over the rhythmic inflating and deflating of the patient’s lungs in a programmed manner. The anaesthetist sets the gas flow, the oxygen concentration, the anaesthetic agent concentration, the amount of gas in each breath, and the number of breaths per minute.
Some people think that anaesthetists do not do anything during an operation, once the anaesthetic has started. In fact, anaesthetists are very busy, watching and evaluating their patients, the progress of the operation, the surgeon, and all the other members of the operating room team. By watching and evaluating – or processing all this information – your anaesthetist is able, if necessary, to make moment-by-moment adjustments to the drugs and fluids that you need during your anaesthetic and operation. Your anaesthetist is also able to consider the plan for the next phases of your care, such as in the recovery room.
Some of the information that your anaesthetist evaluates comes from special monitors. Two kinds of monitors are used to make continuous checks. One kind tells your anaesthetist all about you, including include your heart rate, blood pressure, and temperature. The other kind shows how the anaesthetic machine is functioning.
Measurements of how your body is reacting to the anaesthetic and operation or examination include:
This last observation is vital, because you may have been given drugs which stop you from breathing. If so, then you must remain connected to the breathing circuit to make sure that you continue to receive oxygen. If your breathing circuit becomes disconnected, you could suffer brain damage or death if the disconnection is not detected in time.
These two lists give an idea of the large number of monitors used. Specialty societies and regulatory bodies in anaesthesia have published guidelines describing the equipment and monitors necessary to provide anaesthetic care. Examples are:
One function of these guidelines is to provide details of the absolute minimum type and number of monitors that should be present, functioning, and used before an anaesthetic is given. This is similar to the ‘Minimum Equipment List’ required in aviation before a pilot can take off.
By now you should also have an idea of the amount and complexity of information that your anaesthetist must constantly observe and monitor. Currently there is no ‘black box’ like the one used in aviation that can integrate all the measurements and provide a ‘flight profile’ for your anaesthetic and operation. Some new anaesthetic machines do incorporate automated record-keeping systems, which help to document and integrate some of this information. Automated systems offer advantages in that a record can be kept during emergencies when the anaesthetist may be very busy (as described later).
Another similarity with aeroplanes is that the anaesthetic machine also has self-checking and monitoring capabilities, so that problems can be easily identified. There is also always a back-up system, so that if the system fails, or there is a power blackout, the anaesthetist can safely carry the patient through such a crisis. In many ways the anaesthetist is indeed like a pilot, flying the plane, watching the instruments, and looking out the window (at the surgeon), always ready to take control if a problem occurs. This concept is also emphasised in the equipment guidelines mentioned above. In addition to listing pieces of equipment, these documents also recommend that the most important monitor of the anaesthetised patient is the continuous presence of an anaesthetist.