3 - 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, analgesia and paralysis. The anaesthetist (AmE: "anesthesiologist," if a medical doctor, "nurse anesthetist" if an advanced clinical practitioner who does not hold a medical degree; CRNA=certified registered nurse anesthetist) 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 monitoring 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 or her 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. Commonly used medications such as Viagra can interact with anaesthesia drugs; failure to disclose such usage can endanger the patient.

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.

Stages of anaesthesia

The progression of stages described here was devised for anaesthesia using diethyl ether and is largely replaced by the 3 stage classification.

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 anesthesia, 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 Three: Surgical Anesthesia. During this stage, the skeletal muscles relax, and the patient's breathing becomes regular. The gag reflex and corneal reflex are lost. Eye movements slow, then stop, and surgery can begin.

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.^[1] 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

In the US, up until about 1980 anesthesia was a significant risk, with at least one death per 10,000 times administered. After becoming something of a public scandal, a careful effort was made to understand the causes and improve the results. It is generally believed that anesthesia is now at least ten times safer than it was then.However, there is some controversy about this. In the US, the data is not made public (in fact, the data is not even collected), so the truth is uncertain.The rate for dental anesthesia is reported to be one out of 350,000.