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In modern medical practice, general anaesthesia is a complex procedure involving:
Preaneasthetic evaluation
=Before your surgery, the anesthesiologist must do a preanesthetic evaluation to determine which drugs to use, how to use them, and in what dosages. Anesthesia is commonly referred to as both a science and art. The art of the process is this crucial preoperative interview. In this interview, the anesthesiologist will look at the patient's weight, age, medical history, current medications, etc. It's extremely important that you answer every single question truthfully. For instance, if a person is a heavy drinker or drug user and doesn't admit to it, it could lead to problems for them during the anesthesia, because the anesthesiologist won't know that they're on these drugs.
Monitoring=
Monitoring involves the use of several technologies to allow for a controlled induction of anaesthesia, maintenance and emergence from anaesthesia.
There are different guidelines concerning monitoring during anaesthesia, illustrated by the Australian and New Zealand College of Anaesthetists (http://www.anzca.edu.au) recommendations:
1. Continuous electrocardiography (ECG) - placement of electrodes which monitor heart rate and rhythm, as well as showing evidence of other cardiac pathologies (e.g. ischaemic heart disease).
2. Continuous pulse oximetry (SpO2) - Allows early detection of cyanosis and a fall in patients' blood oxygen tension.
3. Blood pressure Monitoring (NIBP or IBP) - Generally non-invasive methods of measuring blood pressure such as a dinamap(TM) or machine which continuously checks blood pressure non-invasively. Alternatively, for major surgery such as cardiac surgery, anaesthetists may use invasive monitoring with an arterial cannula.
4. Agent concentration measurement - Common anaesthetic machines have meters to measure the percent of inhalational anaesthetic agent used (e.g. sevoflurane, isoflurane, desflurane, halothane etc).
5. Low oxygen alarm - Almost all circuits have a backup alarm in case the oxygen delivery to the patient becomes compromised. This warns if the fraction of inspired oxygen drops lower than room air (21%) and allows the anaesthetist to take immediate remedial action.
6. Circuit disconnect alarm - indicates failure of circuit to achieve a given pressure during mechanical ventilation.
7. Carbon dioxide measurement (capnography)
Muscle Relaxation
Muscle relaxation with skeletal muscle relaxants is an integral part of modern anesthesia. The first drug used for this purpose was curare, introduced in the 1940's and now superseded with drugs with fewer side effects, and generally shorter duration.
Muscle relaxation, also known as neuro-muscular blockade, allows surgery within major body cavities, eg. abdomen and thorax without the need for very deep planes of anesthesia, and is also used to facilitate endotracheal intubation.
Muscle relaxation causes paralysis of the muscles of respiration, ie. the diaphragm and intercostal muscles of the chest, and therefore requires that some form of artificial respiration be implemented, usually by connection of the patient to a mechanical ventilator. The muscles of the larynx are also paralysed so that the airway usually needs to be protected by means of an endo-tracheal tube.
Muscle relaxants work by antagonising the natural transmitter substance acetylcholine at the neuromuscular junction. Thus, nerve impulses which would normally cause muscles to contact are prevented from reaching their supplied muscles, causing the muscles to relax.
Monitoring of muscle relaxation is most easily provided by means of a peripheral nerve stimulator. This device intermittently sends short electrical pulses through the skin over a peripheral nerve while the contraction of a muscle supplied by that nerve is observed. The effects of muscle relaxants are commonly reversed at the termination of surgery by anticholinesterase drugs.
Examples of skeletal muscle relaxants in use today are pancuronium, rocuronium atracurium and succinylcholine.
Airway management
External Links:
Australian & New Zealand College of Anaesthetists Monitoring Standard (http://www.anzca.edu.au/publications/profdocs/profstandards/PS18_2000.htm)
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