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.
What is the anaesthetic machine like ?
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:
- Your heart rate (pulse) and rhythm
by feeling the pulse in your wrist or your neck, by using a stethoscope to listen to your heart, and by means of the electrocardiograph (ECG or EKG). The same monitor can also be used to detect if your heart is suffering any strain.
- Your (arterial) blood pressure
by attaching a traditional inflatable cuff or sometimes by inserting a small tube or cannula (or catheter) directly into an artery.
- The amount of oxygen in a small or peripheral artery
by attaching a device like a clothes-peg, known as a pulse oximeter to one of your fingers or toes or ear lobes, or to the tip of your nose. The result is known as your arterial oxygen saturation.
- The amount of carbon dioxide (CO2) you breathe out
by using a carbon dioxide detector. The result is known as your end-tidal (end of each breath) carbon dioxide concentration. This measurement helps your anaesthetist to check on the function of your lungs and on your metabolism. This monitor is also used to ensure that the breathing tube is correctly placed in your windpipe or trachea and that the breathing circuit has not become disconnected. (See below.)
- How well you are breathing (or being breathed for)
by using a stethoscope, so that your anaesthetist can ensure that your breathing tube is not inserted too deeply and that you do not have any areas of blockage in your lungs.
Other methods to measure the adequacy of your breathing include the use of a special meter to assess the size or volume of each breath you take or are given, and a gauge to measure the pressure in your lungs between breaths and the pressure that is required to inflate your lungs with each breath.
- Your nerve and muscle function
by using a peripheral nerve stimulator to check how the muscle relaxants are affecting your muscle activity and power.
- Your temperature
by using a thermometer, to ensure that you don’t get too hot or too cold, since you lose some of your ability to control your temperature while anaesthetised.
- How much urine you are producing
by inserting a catheter into your bladder during some operations and examinations. This allows your bladder to drain freely and also gives your anaesthetist an idea of how well your kidneys are functioning.
- The pressures in your large veins and your heart
by inserting a special cannula into a large vein in either your neck or arm and passing the cannula through into the large blood vessel (superior vena cava) that leads to the heart. Sometimes the cannula is actually threaded through the chambers of the heart and into your pulmonary artery (which carries blood to the lungs.) As well as pressures, the cannula can determine how much blood your heart is pumping with each beat.
- The depth of your anaesthetic, and whether or not you are unconscious (if you are having a general anaesthetic)
by using a monitor that looks at small electrical impulses or ‘brain waves’ generated by the brain. (This is similar to an ECG of the brain.)This last monitor is not available in many centres. Currently most anaesthetists do not have access to a monitor that indicates if you are adequately anaesthetised or aware of your surroundings. This kind of monitor is still under development. But current anaesthetic practice is different from that of say, ten years ago, in that your anaesthetist can now measure with an anaesthetic agent monitor exactly how much anaesthetic agent you are receiving. This measurement tells your anaesthetist that you are receiving enough anaesthetic gas to ensure that you are unconscious.Your anaesthetist also constantly observes a number of measurements from the anaesthetic machine. These include:
- the pressure of oxygen and nitrous oxide in the pipelines or tanks on the back of the machine from which these gases are delivered
- the amount of oxygen flowing to you from the machine
- the number of times each minute that the breathing machine (or ventilator) delivers a breath to you
- the amount or concentration of anaesthetic agents in the gas you breathe in and out
- that you are still connected to the breathing circuit, and that this circuit has not become disconnected.
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:
- The Australian and New Zealand College of Anaesthetists Professional documents
- The Guidelines to the Practice of Anesthesia as Recommended by the Canadian Anesthesiologists’ Society
- The Recommendations for Standards of Monitoring during Anaesthesia and Recovery, published by the Association of Anaesthetists of Great Britain and Ireland.
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.