General Anaesthetics PDF

General Anesthetics Presented by: Dr Karishma Jeeboo Introduction General Anesthesia: Reversible state of central nervous system depression – loss of response and perception of external stimuli Used in patients undergoing surgical and other medical procedures Provides benefit...

General Anesthetics Presented by: Dr Karishma Jeeboo Introduction General Anesthesia: Reversible state of central nervous system depression – loss of response and perception of external stimuli Used in patients undergoing surgical and other medical procedures Provides benefits such as sedation and reduced anxiety, lack of awareness and amnesia, skeletal muscle relaxation, suppression and undesirable reflexes, analgesia Low therapeutic indices – great care in administration Modern anaesthetics act more rapidly and achieve deep anaesthesia quickly Introduction No single agent will produce the desired effect, and thus, several categories of the drugs are used in a cocktail to produce optimal anaesthesia. Preanesthetic medication calms the patient, relieves pain, protects against undesirable effects. Skeletal muscle relaxants facilitate intubation of the trachea and suppress muscle tone to the degree required for surgery. Preanesthetic medications include antacids, anticholinergics, antiemetics, antihistamines, benzodiazepines, opioids. Introduction Potent general anesthetics are delivered via IV or the inhalation method. Inhaled anesthetics are all volatile, halogenated hydrocarbons. There are several patient factors that must be considered before use of anesthetics, the cardiovascular system and its function - IHD, respiratory system – asthmatics, liver and kidney function – distribution and clearance of anesthetics, nervous system and pregnancy Stages and Depth of Anesthesia Stages: Induction Maintenance Recovery from Anesthesia Induction: a period of time from the onset of administration of the anaesthetic to the development of effective surgical anaesthesia in the patient. Normally induced with an IV anesthetic like propofol, high produces unconsciousness within 30-40 secs. Stages and Depth of Anesthesia Additional inhalation or iv drugs comprising the selected anesthetic combination may be given to produce the desired depth of surgical (Stage III) anesthesia. Often includes concomitant use of other drugs like muscle relaxants, etc. inhalation induction is done to children without IV access. Inhaled general anaesthetics are used, like halothane. Stages and Depth of Anesthesia Maintenance of Anesthesia: patient is surgically anesthetized. After administering the selected anesthetic mixture, the anesthesiologist monitors the patient's vital signs and response to various stimuli throughout the surgical procedure to carefully balance the amount of drug inhaled and/or infused with the depth of anesthesia. Normally maintained with volatile anesthetics since they offer good minute to minute control over depth of anesthesia. Stages and Depth of Anesthesia Opioids, such as fentanyl, are often used for pain along with inhalation agents, because the latter are not good analgesics. Recovery: Postoperatively, the anesthesiologist withdraws the anaesthetic mixture and monitors the return of the patient to consciousness. Recovery is usually the reverse of induction, that is, redistribution from the site of action (rather than the metabolism of the anaesthetic)  Depth of Anesthesia Divided into four sequential stages: Each stage is characterized by increased central nervous system (CNS) depression, which is caused by the accumulation of the anaesthetic drug in the brain. Stage I—Analgesia - Amnesia and reduced awareness of pain occur as Stage II is approached. Consciousness may be impaired but not lost. Stage II—Disinhibition - Excitement; patient experiences delirium and possibly violent, combative behaviour; BP irregularity and increased respiratory rate; avoid this stage of anaesthesia; a short-acting barbiturate, such as thiopental, is given intravenously before inhalation anaesthesia is administered. Depth of Anesthesia Stage III—Surgical anaesthesia: Regular respiration and relaxation of the skeletal muscles occur in this stage. Eye reflexes decrease progressively until the eye movements cease and the pupil is fixed. Surgery may proceed during this stage. Stage IV—Medullary depression/paralysis: Severe depression of the respiratory and vasomotor centres occurs during this stage. Death can rapidly ensue unless measures are taken to maintain circulation and respiration. General Anesthesia Terms to Learn Balanced Anesthesia – Anesthesia produced by a mixture of drugs, often including both inhaled and intravenous agents Inhalation Anesthesia – Anesthesia induced by inhalation of a drug Minimal Alveolar Anesthetic Concentration (MAC) – The alveolar concentration of an inhaled anaesthetic required to prevent a response to a standardised painful stimulus in 50% of patients. Analgesia – A state of decreased awareness of pain, sometimes with amnesia General Anesthesia – A state of unconsciousness, analgesia, and amnesia, with skeletal muscle relaxation and loss of reflexes. Overview The administration of general anaesthesia is driven by three general objectives: Minimizing the potentially deleterious direct and indirect effects of anesthetic agents and techniques Sustaining physiologic homeostasis during surgical procedures that may involve major blood loss, tissue ischemia, reperfusion of ischemic tissue, fluid shifts, exposure to a cold environment, and impaired coagulation Improving postoperative outcomes by choosing techniques that block or treat components of the surgical stress response, which may lead to short- or long-term sequelae Overview Depress the central nervous system IV and inhalational administration is preferred Molecular Actions There is a common view that most IV anaesthetics act predominantly through GABAA NO, Ketamine and Xenon are likely to produce unconsciousness via inhibition of the NMDA receptor and glutamate receptors (ketamine). Can either be inhalation or parenteral anaesthetics MOA They are CNS depressants Increase the threshold for firing of CNS neurons by acting on a variety of ion channels and receptor targets Inhaled anaesthetics, barbiturates, benzodiazepines, etomidate, and propofol facilitate GABA-mediated action at GABAA receptors to increase chlorine conductance and reduce cell firing. General anaesthetics can also target potassium channels, glycine receptors, and serotonin receptors. General Effects of Anesthesia CNS Effects – decreased vascular resistance and increased cerebral blood flow – can cause increased intracranial pressure. High concentrations of enflurane – cause muscle twitching. Hemodynamic effects – decrease in systemic arterial BP; direct vasodilation, myocardial depression Respiratory depression/effects Malignant hyperthermia, especially when used with succinylcholine. Nausea and vomiting Postoperative pneumonia? Inhalation Anesthetics Mainstay of anesthesia Used primarily for maintaining anaesthesia after administering an intravenous agent. Depth of anaesthesia can be rapidly altered by changing the concentration of the drug. Reversible because most are rapidly eliminated from the body by exhalation. They have steep dose-response curves and narrow therapeutic indices (2-4), so there is a small difference in no effect, surgical anaesthesia, and respiratory depression. Low safety margin and among the most dangerous drugs in clinical use. Potency is roughly proportional to lipid solubility Current Inhalation Anesthetics Nitrous Oxide (a gaseous anesthetic) Vapourised liquid halogenated hydrocarbons – halothane, desflurane, enflurane, isoflurane and sevoflurane (volatile anesthetics) Speed of induction depends on several factors – solubility, inspired gas partial pressure, ventilation rate, pulmonary blood flow, and arteriovenous concentration gradient. Elimination – inhaled agents are redistributed from the brain to the blood; from blood to alveolar air, and elimination through the lungs. Recovery Rate of recovery from anaesthesia using agents with low blood: gas partition coefficients are faster than anaesthetics with high blood solubility. This vital property has led to the introduction of several inhaled anaesthetics – desflurane and sevoflurane – low blood solubility – shorter recovery time compared to older agents.  Intravenous Anaesthetics Propofol - produces anaesthesia and recovery rapidly. Has antiemetic action; recovery is not delayed after prolonged infusion. Very commonly used as a component of balanced anaesthesia. Can cause marked hypotension during induction due to decreased peripheral resistance. Barbiturates – Thiopental and methohexital – high lipid solubility – promote rapid entry into the brain and result in surgical anaesthesia in one circulation time (