General anaesthesia
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In modern medical practice, general anaesthesia is a complex procedure involving:
- preanaesthetic assessment
- administration of general anaesthetic drugs
- cardiorespiratory monitoring
- analgesia
- airway management
- fluid management
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Preanaesthetic Evaluation
Before surgery, the anaesthesiologist or nurse anaesthetist will do a preanaesthetic evaluation to determine which drugs (including dosages), additional invasive monitors and/or analgesic therapies he or she will use. In this interview the anaesthesiologist will ask for the patient's age, weight, medical history, current medications, previous anesthetics, and other factors relevent to administering anesthesia. Often, the patient will fill in this information on a separate form when he comes to the hospital for his pre-operative evaluation. Depending on the existing medical conditions reported the anaesthesia provider will review this information with the patient either during his pre-operative evaluation or on the day of his surgery. It is extremely important that the patient answer these questions truthfully so that the anaesthesia provider can select the proper anaesthetic. For instance, a heavy drinker or drug user who does not disclose their chemical uses could be undermedicated which could then lead to awareness under anaesthesia or dangerously high blood pressures.
Monitoring
Monitoring involves the use of several technologies to allow for a controlled induction of, maintenance of and emergence from general anaesthesia.
1. Continuous Electrocardiography (ECG): The placement of electrodes which monitor heart rate and rhythm. This may also help the anesthesia provider to identify early signs of heart ischemia.
2. Continuous Pulse Oximetry (SpO2): The placement of a this device (usally on one of your fingers) allows for early detection of a fall in the patients' blood oxygen tension and warns the anaesthesia provider when the patient is not getting enough oxygen.
3. Blood Pressure Monitoring (NIBP or IBP): There are two methods of measuring the patient's blood pressure. The first, and most common, is called non-invasive blood pressure (NIBP) monitoring. This involves placing a blood pressure cuff around the patient's arm, forearm or leg. A blood pressure machine takes blood pressure readings at regular, preset intervals throughout the surgery. The second method is called invasive blood pressure (IBP) monitoring. This method is reserved for patients with significant heart or lung disease, the critically ill, major surgery such as cardiac or transplant surgery, or when large blood losses are expected. The invasive blood pressure monitoring technique involves placing a special type of plastic cannula in the patient's radial or femoral artery.
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 anaesthesia. 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 neurotransmitter substance acetylcholine at the neuromuscular junction. Thus, nerve impulses which would normally cause muscles to contract 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)it:Anestesia generale nl:Algemene anesthesie