General anaesthesia

In modern medical practice, general anaesthesia (AmE: anesthesia) is a state of total unconsciousness resulting from general anaesthetic drugs. A variety of drugs are given to the patient that have different effects with the overall aim of ensuring unconsciousness, amnesia and analgesia. The anaesthetist (AmE: anesthesiologist) selects the optimal technique for any given patient and procedure.

Overview
General anaesthesia is a complex procedure involving:


 * Preanaesthetic assessment
 * Administration of general anaesthetic drugs
 * Cardiorespiratory monitoring
 * Analgesia
 * Airway management
 * Fluid management
 * Postoperative pain relief

Preanaesthetic evaluation
Prior to surgery, the anaesthetist interviews the patient to determine the best combination and drugs and dosages and the degree of how much monitoring is required to ensure a safe and effective procedure.

Pertinent information is the patient's age, weight, medical history, current medications, previous anaesthetics, and fasting time. Usually, the patients are required to fill out this information on a separate form during the pre-operative evaluation. Depending on the existing medical conditions reported, the anaesthetist will review this information with the patient either during his pre-operative evaluation or on the day of his surgery.

Truthful and accurate answering of the questions is important so the anaesthetist can select the proper anaesthetics. For instance, a heavy drinker or drug user who does not disclose their chemical uses could be undermedicated, which could then lead to anesthesia awareness or dangerously high blood pressure.

An important aspect of this assessment is that of the patient's airway, involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx. The condition of teeth and location of dental crowns and caps are checked, neck flexibility and head extension observed. If an endotracheal tube is indicated and airway management is deemed difficult, then alternative placement methods such as fiberoptic intubation may be used.

Premedication
Anaesthetists may give a pre-medication by injection or by mouth anywhere from a couple of hours to a couple of minutes before the onset of surgery to induce drowsiness and relaxation.

The most common drugs used for pre-medication are narcotics (opioids such as fentanyl) and sedatives (most commonly benzodiazepines such as midazolam).

Induction
The general anaesthetic is administered in either the operating theatre itself or a special ante-room.

General anaesthesia can be induced by intravenous (IV) injection, or breathing a volatile anaesthetic through a facemask (inhalational induction). Onset of anaesthesia is faster with IV injection than with inhalation, taking about 10-20 seconds to induce total unconsciousness. This has the advantage of avoiding the excitatory phase of anaesthesia (see below), and thus reduces complications related to induction of anaesthesia. An inhalational induction may be chosen by the anaesthetist where IV access is difficult to obtain, where difficulty maintaining the airway is anticipated, or due to patient preference (eg. children). Commonly used IV induction agents include propofol, sodium thiopental, etomidate, and ketamine. The most commonly-used agent for inhalational induction is sevoflurane because it causes less irritation than other inhaled gases.

Maintenance
The duration of action of IV induction agents is generally 5 to 10 minutes, after which time spontaneous recovery of consciousness will occur. In order to prolong anaesthesia for the required duration (usually the duration of surgery), anaesthesia must be maintained. Usually this is achieved by allowing the patient to breathe a carefully controlled mixture of oxygen, nitrous oxide, and a volatile anaesthetic agent. This is transferred to the patient's brain via the lungs and the bloodstream, and the patient remains unconscious. Inhaled agents are frequently supplemented by intravenous anesthetics, such as opioids (usually fentanyl or a fentanyl derivative) and sedative-hypnotics (usually propofol or midazolam). At the end of surgery the volatile anaethetic is discontinued. Recovery of consciousness occurs when the concentration of anaesthetic in the brain drops below a certain level (usually within 1 to 30 minutes depending upon the duration of surgery).

In the 1990s a novel method of maintaining anaesthesia was developed in Glasgow, UK. Called Total IntraVenous Anaesthesia (TIVA), this involves using a computer controlled syringe driver (pump) to infuse propofol throughout the duration of surgery, removing the need for a volatile anaesthetic. Purported advantages include faster recovery from anaesthesia, reduced incidence of post-operative nausea and vomiting, and absence of a trigger for malignant hyperthermia.

Other medications will occasionally be given to anesthetized patients to treat side effects or prevent complications. These medications include antihypertensives to treat high blood pressure, drugs like ephedrine and phenylephrine to treat low blood pressure, drugs like albuterol to treat asthma or laryngospasm/bronchospasm, and drugs like epinephrine or diphenhydramine to treat allergic reactions. Sometimes glucocorticoids or antibiotics are given to prevent inflammation and infection, respectively.

Paralysis
The induction of paralysis with a neuromuscular blocker is an integral part of modern anaesthesia. The first drug used for this purpose was curare, introduced in the 1940s, which has now been superseded by drugs with fewer side effects and generally shorter duration of action.

Paralysis allows surgery within major body cavities, eg. abdomen and thorax without the need for very deep anesthesia, and is also used to facilitate endotracheal intubation.

Acetylcholine, the natural neurotransmitter substance at the neuromuscular junction, causes muscles to contract when it is released from nerve endings. Muscle relaxants work by preventing acetylcholine from attaching to its receptor.

Paralysis of the muscles of respiration, ie. the diaphragm and intercostal muscles of the chest requires that some form of artificial respiration be implemented. As the muscles of the larynx are also paralysed, the airway usually needs to be protected by means of an endotracheal tube.

Monitoring of paralysis 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, vecuronium, atracurium, mivacurium, and succinylcholine.

Airway management
With the loss of consciousness caused by general anaesthesia, 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 tube is often used (intubation), although there are alternative devices such as face masks or laryngeal mask airways.

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 anaesthetist 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 hemoglobin saturation with oxygen (hypoxemia).

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 artery - usually at the wrist or in the groin.

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)- measures the amount of carbon dioxide expired by the patient's lungs. It allows the anaesthetist to assess the adequacy of ventilation

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

9. EEG or other system to verify depth of anaesthesia may also be used. This reduces the likelihood that a patient will be mentally awake, although unable to move because of the paralytic agents. It also reduces the likelihood of a patient receiving significantly more amnesic drugs than actually necessary to do the job.

Stage 1
Stage 1 anaesthesia, also known as the "induction," is the period between the initial administration of the induction medications and loss of consciousness. During this stage the patient progresses from analgesia without amnesia to analgesia with amnesia. Patients can carry on a conversation at the time.

Stage 2
Stage 2 anaesthesia, also known as the "excitement stage," is the period following loss of consciousness and marked by excited and delirious activity. During this stage, respirations and heart rate may become irregular. In addition, there may be uncontrolled movements, vomiting, breath holding, and pupillary dilation. Since the combination of spastic movements, vomiting, and irregular respirations may lead to airway compromise, rapidly acting drugs are used to minimize time in this stage and reach stage 3 as fast as possible.

Stage 3
Stage 3 anaesthesia, also known as "surgical plane," follows the excitement stage and is marked by a return of regular respirations. This stage is divided into 4 planes based on changes to eye reflexes, eye movements, and pupil size. The ideal plane for surgery is plane 3 where the patient has minimal use of the respiratory muscles. The main indicators of the stages of anaesthesia are the patient's respiratory and cardiovascular response to stimulation.

Stage 4
Stage 4 anaesthesia, also known as "overdose," is the stage where too much medication has been given and the patient has severe brain stem or medullary depression. This results in a cessation of respiration and potential cardiovascular collapse. This stage is lethal without cardiovascular and respiratory support.

Postoperative Analgesia
The anaesthesia concludes with a management plan for postoperative pain relief.

This may be in the form of regional analgesia, oral, transdermal or parenteral medication.

Minor surgical procedures are amenable to oral pain relief medications such as paracetamol and NSAIDS such as ibuprofen.

Moderate levels of pain require the addition of mild opiates such as codeine.

Major surgical procedures may require a combination of modalities to confer adequate pain relief. Parenteral methods include Patient Controlled Analgesia System (PCAS) involving morphine, a strong opiate. Here, the patient presses a button to activate a pump containing morphine. This administers a preset dose of the drug. As the pump is programmed not to exceed a safe amount of the drug, the patient cannot self administer a toxic dose.

Mortality rates
Overall, the mortality rate for general anaesthesia is about five deaths per million anaesthetic administrations.

Death during anaesthesia is most commonly related to surgical factors or pre-existing medical conditions. These include major haemorrhage, sepsis, and organ failure (eg. heart, lungs, kidneys, liver).

Common causes of death directly related to anaesthesia include:
 * aspiration of stomach contents
 * suffocation (due to inadequate airway management)
 * allergic reactions to anaesthesia (specifically and not limited to anti-nausea agents) and other deadly genetic predispositions
 * human error
 * equipment failure