General anaesthesia

In modern medical practice, general anaesthesia is a state of total unconsciousness resulting from anesthetic drugs. A variety of drugs are given to the patient that have different effects with the overall aim of ensuring unconsciousness, amnesiaand analgesia. The anesthesiologistselects the optimal technique for any given patient and procedure.

General anaesthesia is a complex procedure involving:

• Preanaesthetic assessment
• Administration of general anaestheticdrugs
• Cardiorespiratory monitoring
• Analgesia
• Airway management
• Fluid management

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 relevant 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. Hospital staff will take note of the last meal ingested (including gum chewed!), and any fluids consumed, and a history of acid-indigestion or regurgitation is also sought.

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 pressure.

An important aspect of this assessment is that of the patients airway, involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx (tongue, uvular, soft palate and tonsils) - graded by the "Mallampati test". The condition of teeth and location of dental crowns and caps are checked, as well as the ability to adopt the ideal position for intubation (known as "sniffing the morning air"), tested by observing neck flexibility and head extension. If an endotracheal tubeis indicated and airway management is deemed difficult, then alternative placement methods such as fiberoptic intubation may be used.

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 this device (usually on one of the fingers) allows for early detection of a fall in a patient's hemoglobinpercent saturation of oxygen which warns the anaesthesia provider when the patient is hypoxemic, low blood levels of 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 concentrationmeasurement - Common anaesthetic machineshave meters to measure the percent of inhalational anaestheticagent used (e.g. sevoflurane, isoflurane, desflurane, halothaneetc).

5. Low oxygenalarm - 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 anaesthetistto take immediate remedial action.

6. Circuit disconnect alarm - indicates failure of circuit to achieve a given pressure during mechanical ventilation.

7. Carbon dioxidemeasurement (capnography)

8. Temperature measurement to discern hypothermiaor fever, and to aid early detection of malignant hyperthermia.

Administration of General Anaesthetic

Anaesthetists may give a pre-medication ('pre-med') by injection or tablets a couple of hours before surgery to induce drowsiness and relaxation. The general anaesthetic will then be administered in either the operating theatre itself or a special ante-room. General anaesthetic can be given by IV injection, or inhaled by mask, or by both. IV injection works quicker than inhalation, it taking ten seconds or less to induce total unconsciousness. The mask is used more for children, but is sometimes used on adults along with IV injection.

Muscle Relaxation

Muscle relaxationwith skeletal muscle relaxantsis 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 surgerywithin major body cavities, eg. abdomenand thoraxwithout the need for very deep planes of anesthesia, and is also used to facilitate endotracheal intubation.

Muscle relaxation causes paralysisof the muscles of respiration, ie. the diaphragmand intercostal muscles of the chest, and therefore requires that some form of artificial respiration, usually by connection of the patient to a mechanical ventilator. The muscles of the larynxare also paralysed so that the airway usually needs to be protected by means of an endotracheal tube.

Muscle relaxants work by antagonising the natural neurotransmittersubstance acetylcholineat the neuromuscular junction. Thus, nerve impulseswhich would normally cause muscles to contract are prevented from reaching their supplied muscles, causing them to relax.

Monitoring of muscle relaxation is most easily provided by means of a peripheral nervestimulator. This device intermittently sends short electrical pulses through the skinover 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 anticholinesterasedrugs.

Examples of skeletal muscle relaxants in use today are pancuronium, rocuronium, vecuronium, atracurium, mivacurium, and succinylcholine.

Airway management

With the loss of consciousness caused by general anesthesia, there is loss of protective airway reflexes, such as coughing, loss of airway patency and sometimes loss of a regular breathing pattern due to the effect of anesthetics, opioids, or muscle relaxants. To maintain an open airway and regulate breathing within acceptable parameters, some form of "breathing tube" is inserted in the airway after the patient is unconscious. To enable mechanical ventilation, an endotracheal tubeis often used (intubation), although there are alternative devices such as face masks or laryngeal mask airways.

External links

• Australian & New Zealand College of Anaesthetists Monitoring Standardda:Universel anæstesi

it:Anestesia generale nl:Algemene anesthesie

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/General+anaesthesia Wikipedia article General anaesthesia.