“GENERAL ANAESTHESIA” PRPD/DN/11 Types of Anaesthesia “GENERAL ANAESTHESIA” PRPD/DN/11

Choice of Anaesthesia Selection of anaesthesia for a given surgical procedure is made by the patient, the anaesthesia provider, and the surgeon. 2

GENERAL ANAESTHESIA G.A is a reversible, unconscious state characterized by: - Amnesia (sleep, hypnosis, or basal narcosis). - Analgesia (freedom of pain). - Depression of reflexes. - Muscle relaxation. - Haemostasis or specific manipulation of physiologic systems and functions.

GENERAL ANAESTHESIA 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. The primary purpose is to: 1) induce drowsiness (sedation) 2) relaxation (reduce anxiety). - The most common drugs used are narcotics (Opioids such as Fentanyl) and sedatives (most commonly benzodiazepines e.g., Midazolam)

Premedication cont… An anticholinergic such as atropine or glycopyrrolate may be used to: prevent bradycardia in pediatric patients for controlling secretions in patients undergoing oropharyngeal procedures, or for controlling cardiac reflex that may cause bradycardia as during ophthalmic procedures.

Premedication cont … may be administered either intramuscularly, intravenously, or orally with 15 to 30 ml of water. Oral premedication is usually preferred by the patient; the absorption and uptake are more predictable than that with an intramuscular injection; and the small amount of water is readily absorbed directly across the gastric mucosa.

Premedication cont… Often premedication is not given to older patients because their anxiety levels are lower, their responses to medications are unpredictable, and sedation can be given IV in the operating room if required. 7

The Anaesthetic Machine The transport and supply of anaesthetic gases from their source in cylinders or a pipeline through the anaesthetic machine to its outlet. The most popular anaesthetic machine in common use is often referred to as Boyle’s machine, the model introduced in 1917 by Edmund Boyle, a London anaesthetist. Modern anaesthetic machines are like ‘work stations’ and include ventilators and monitoring devices built into the machine.

The Anaesthetic Machine Modern anaesthetic machine Boyle’s machine

Anaesthesia machine includes:

GENERAL ANAESTHESIA cont… Induction - G.A is induced by IV injection, or breathing a volatile anaesthetic through a face mask (inhalation induction). - Onset of anaesthesia is faster with IV injection than with inhalation, taking about 10-20 seconds to induce total unconsciousness. - Commonly used IV induction agents include propofol, sodium thiopental, etomidate and ketamine. 11

Induction cont… - The most commonly-used agent for inhalational induction is sevoflurane because it causes less irritation than other inhaled gases. An inhalational induction may be chosen by the anaesthetist where: - IV access is difficult to obtain. - difficulty maintaining the airway is anticipated. - due to patient preference (e.g. children). 12

GENERAL ANAESTHESIA cont… Maintenance - The duration of action of IV induction agents is generally 5 to 10 mins, after which time spontaneous recovery of consciousness will occur. - To prolong anaesthesia for the required surgery usually achieved by allowing the patient to breathe a carefully controlled mixture of oxygen, nitrous oxide and a volatile anaesthetic agent. - Inhaled agents are frequently supplemented by intravenous anaesthetics, such as opioids (usually fentanyl or a fentanyl derivative) and sedative-hypnotics (usually propofol or midazolam). 13

Maintenance cont….. At the end of surgery the V.A is discontinued. Recovery of consciousness occurs when the concentration of anaesthetic in the brain drops below a certain level (within 1-30mins depending upon the duration of surgery). - Total Intra Venous 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. 14

Maintenance cont….. Purported advantages include faster recovery from anaesthesia, reduced incidence of postoperative nausea and vomiting, and absence of a trigger for malignant hyperthermia. Other medications will occasionally be given to anaesthetized patients to treat side effects or prevent complications. This medications include: antihypertensive, ephedrine and phenylephrine, albuterol, epinephrine or diphenhydramine, glucocorticoids or antibiotics. 15

GENERAL ANAESTHESIA cont… Paralysis The induction of paralysis with a neuromuscular blocker is an integral part of modern anaesthesia. Paralysis allows surgery within major body cavities, e.g., abdomen and thorax without the need for very deep anaesthesia, and is also used to facilitate endotracheal intubation. 16

Paralysis cont….. 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. E.g., of skeletal muscle relaxants are pancuronium, vecuronium, atracurium, mivacurium and succinylcholine.

Paralysis cont….. Paralysis of the muscles of respiration, i.e. the diaphragm and intercoastal 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. 18

Airway management With the loss of consciousness caused by G.A, 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 anaesthetics, opoids 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. 19

The Anaesthetic Equipment

INDUCTION OF GENERAL ANAESTHESIA Loss of consciousness and Intubation Preoxygenation

INSERTION OF LMA

GENERAL ANAESTHESIA cont… Monitoring Involves the use of several technologies to allow for a controlled induction of, maintenance of and emergence from general anaesthesia. 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 ischaemia. 23

Monitoring cont…… 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 haemoglobin saturation with oxygen (hypoxaemia). Blood Pressure Monitoring: There are 2 methods of measuring the patient’s BP; (i) Most common, Non-Invasive Blood Pressure (NIBP) monitoring – placing a BP cuff around the patient’s arm, forearm or leg. A BP machine takes BP readings at regular, preset intervals throughout the surgery. 24

Monitoring cont…… (ii) 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 IBP monitoring technique involves placing a special type of plastic cannula in the patient’s artery – usually at the wrist or in the groin. 25

Monitoring cont…… Agent concentration measurement – common anaesthetic machines have meters to measure the percent of inhalation anaesthetic agent used (e.g. sevoflurane, isoflurane, desflurane, halothane etc). 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. 26

Monitoring cont… Circuit disconnect alarm – indicates failure of circuit to achieve a given pressure during mechanical ventilation. 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. 27

Monitoring cont… Temperature measurement to discern hypothermia or fever, and to aid early detection of malignant hyperthermia. 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 significant more amnesic drugs than actually necessary to do the job. 28

STAGES (DEPTH) OF GENERAL ANAESTHESIA Also known as “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 conversation at the time. 29

cont… STAGE 2 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, 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. 30

cont… STAGE 3 Also known as “surgical plane,” follows the excitement stage and is markedly by a return of regular respirations. This stage is divided into 4 planes based on changes to eye reflexes, eye movement, and pupil size. The ideal plane 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. 31

cont… STAGE 4 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 respiratory and potential cardiovascular collapse. This stage is lethal (danger stage) without cardiovascular and respiratory support. Prepare for cardiopulmonary resuscitation. 32

Emergence from GA Varies in length and depends on the patient’s state and the depth and duration of anaesthesia. Starts as the patient begins to “emerge” from anaesthesia and usually ends when the patient is ready to leave the OR. Intubation occurs during induction phase, and extubation is usually performed during emergence. 33

MORTALITY RATES Overall mortality rate for G.A is about five deaths per million anaesthetics administrations. Death in anaesthesia is most commonly related to surgical factors or pre-existing medical conditions include major haemorrhage, sepsis, and organ failure (e.g. Heart, lungs, kidneys, liver) 34

MORTALITY RATES cont…. 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. THE END 35