11-GENERAL ANAESTHETICS.pptx

General Anesthetics 1 Learning Objectives • Explain General anesthesia – Properties of an ideal anesthetic – Stages of anesthesia • Describe common general anesthetics and their : – Classification – Mechanism of action – Uses and Adverse effects General anesthetics • It is reversible loss of s...

General Anesthetics 1 Learning Objectives • Explain General anesthesia – Properties of an ideal anesthetic – Stages of anesthesia • Describe common general anesthetics and their : – Classification – Mechanism of action – Uses and Adverse effects General anesthetics • It is reversible loss of sensation and consciousness • The cardinal features of GA: 5 (LUAAM) – – – – – Loss of all sensation, specially pain Unconsciousness Amnesia Abolition of reflexes Muscle relaxation Alcohol, Opium, Cannabis, N20 , Ether, Chloroform, Thiopentone 3 Properties of an ideal anesthetic • For the patient: – should be pleasant to inhale, non-irritating – Induction and recovery fast and smooth – no nausea or vomiting • For the surgeon: – adequate muscular relaxation, – good analgesic property • For the anesthetist – ready administration and easily controllable – Margin of safety: wide – Should not cause respiratory or circulatory depression – Readily eliminated from the body 4 Stages of general anesthesia 6 Classification: 2 classes Inhalational Gas Liquids N2 O Ether Halothane Enflurane Isoflurane Desflurane Sevofluran e Intravenous Inducing Slower acting agents Thiopentone Benzodiazepines sod. Diazepam Methohexitone Lorazepam sod. Midazolam Propofol Etomidate Dissociative Anaes. Ketamine Opioid analgesia Fentanyl Mechanism of general anesthesia • Lipid theory: Meyer and Overton • Dissolving in the membrane lipids by causing gelliquid transition (fluidization) → closes of ion channels • Protein theory – The GABA-A receptor gated Cl- channel: • Barbiturates, benzodiazepines and propofol potentiate GABA – Some inhibit neuronal cation channel gated by nicotinic cholinergic receptor: • Flourinated anesthetics – Excitatory NMDA type glutamate receptor • N2O and ketamine selectively inhibits 9 Factors affecting the PP of Inhalational anaesthetic attained in brain • Solubility of anaesthetic in blood: – Blood: gas partition coefficient: index of solubility in blood • PP of anesthetic in the inspired gas • Pulmonary ventilation and blood flow • Solubility of anesthetic in brain tissue (Anaesthetics Easily Cross BBB) • Cerebral blood flow 10 Induction and recovery 1. Highly lipid soluble means , time to reach equilibrium will be more…eg. 2. Low lipid ETHER soluble means , time to reach equilibrium will be faster…eg. N2O Inhalational Anesthetics • The main agents: – Nitrous oxide – Halothane – Enflurane, methoxyflurane – Isoflurane, desflurane and sevoflurane • Ether is largely obsolete 12 Nitrous Oxide (Laughing gas) • No complete anesthesia • Good analgesic properties • Used: – 60-80% N2O + 40-20% O2 – Lower doses (30-40%): not applicable in aspiration pneumonia – Dentistry and obstetric, short painful cases • Adverse effects: – bone marrow depression: on long term use – accumulates in gaseous cavities 13 Halothane • Induction and recovery are relatively faster • Poor analgesic and muscle relaxant • Hangover: likely, due to high lipid solubility • Adverse effects: – Hepatotoxocity, Malignant hyperthermia – Sensitizes heart to catecholamines – Hypotension and Dysrhythmias 14 Other inhalational agents • Methoxyflurane (Penthrane): – Not as induction anesthetic – Renal tubular dysfunction (nephrotoxicity) • Enflurane: – less renal toxicity than methoxyflurane – Frank tonic – clonic seizures 15 Other inhalational agents_2 • Isoflurane: – but lacks epileptogenic property – Irritant to respiratory tract • Desflurane: – Similar to isoflurane – but with faster onset and recovery • Sevoflurane: – Similar to desflurane – with lack of respiratory irritation 16 Intravenous Anesthetics • Induction of anaesthesia: – Thiopental, etomidate , propofol • Conscious sedation: – (BDZs + propofol) + short acting opioids • Neurolept analgesia – neuroleptic drug + opioids(DROPERIDOL+FENTANYL) • Neurolept anesthesia: – nitrous oxide is added for complete anesthesia 17 Barbiturates • Thiopental sodium, Methohexital • Features: – – – – Rapid and pleasant induction Unconsciousness with amnesia analgesia and SKM relaxation (inadequate) Rapid recovery by redistribution • Adverse effects: • CVS and Respiratory depression • Hypersensitivity: asthma, urticaria, angioedema Redistribution Etomidate • Similar to thiopental – But Greater margin of safety (CVS and Respiratory) • Short elimination half-life: 2.9 hours • Adverse Effects – Injection is painful – Myoclonic muscle (involuntary) movements: 40% cases – Suppress the adrenocortical response to stress 20 Propofol • Features: – Antiemetic properties • Used: – Conscious sedation – with opioids for the maintenance in CV surgery • Adverse effects – Significant reduction in BP (vasodilatation and decreased contractility) – Inhibits baroreflexes so no tachycardia 21 Ketamine • Block activation of NMDA-type glutamate receptors • Dissociative anesthesia (Complete analgesia combined with amnesiacharacteristic of Ketamine): – profound analgesia, amnesia, paralysis of movement – without actual loss of consciousness • Main drawback: – High incidence of dysphoria, hallucinations • Used mainly: – minor procedures in CHILDREN 22 Benzodiazepines • Features: – Unconsciousness without analgesia, inadequate Skeletal muscle relaxation – Midazolam, diazepam, and lorazepam – Used for premedications and conscious sedation rather than hypnosis • Midazolam: – good aqueous solubility – short DOA, not converted to active metabolites • Lorazepam: – longer acting – more potent than either midazolam or diazepam 23 Notes 1. What are all Preanaesthetic medications? And what is the use related thereof? 2. What are all the drugs given during anaesthesia…. along with anaesthetic agent ? Notes