General Anesthesia Mohamed A. Yaseen M.S,c
Surgery Before Anesthesia
General Anesthesia ( GA ) Drug induced absence of perception of all sensation allowing surgery or other painful procedure to be carried out.
Triad of General anaesthesia Hypnosis Analgesia Muscle relaxation
Mechanism of action of general anesthesia 1- old theory the anesthetic agent bind with lipid matrix of the nerve membrane, this lead to secondary changes in ion flux and interaction with membrane of the ligand gated ion channel. 2- facilitation GABA receptor function. 3- Antagonise the action of the excitatory neurotransmitter glutamic acid on the N-methyl-D-aspartate (NMDA) receptor.
Mechanism of action of general anesthesia 4- causes membrane hyper polarization (inhibitory action) by their activation of potassium channel. 5- anesthetic decease the duration of opening nicotinic receptor an action that decrease the excitatory effect of acetylcholine synapse
Classic Stages of Anesthesia Stage 1: Analgesia decreased awareness of pain, amnesia,without loss of consciousness Stage 2:excitation delirium & excitation, enhanced reflexes, retching, incontinence, irregular respiration Stage 3: Surgical Anesthesia unconscious, no pain reflexes, regular respiration, BP is maintained Stage 4: Medullary Depression respiratory & CV depression requiring ventilation & pharmacologic support. * Seen mainly with Ether. Not all stages are observed with modern GAs.
Pre-anesthetic medication It is the use of drugs prior to anesthesia to make it more safe and pleasant. To relieve anxiety benzodiazepines. To prevent allergic reactions antihistaminics. To prevent nausea and vomiting antiemetics. To provide analgesia opioids. To prevent Bradycardia and secretion atropine.
Phases of Anesthesia Induction : putting the patient to sleep Maintenance : keeping the patient asleep Recovery : waking the patient up
Anesthetics divide into 2 classes Inhalation Anesthetics Intravenous Anesthetics Gasses or Vapors Usually Halogenated Injections Anesthetics or induction agents
Inhalation Anesthetics Nitrous oxide Halogenated anasthetics: Halothane Isoflurane Sevoflurane Enflurane
General Anesthetics Inhalational Parenteral Gas Volatile liquids* Barbiturates (nitrous oxide) (halothane isoflurane, desflurane, sevoflurane) Enflurane (thiopental) Opioids Benzodiazepines (midazolam) (fentanyl) (etomidate, propofol) ketamin
Mechanism of Action Interaction with protein receptors Volatile increase GABA and Glycine ( inhibitory neurotransmitters)
MAC(minimum alveolar concentration) A measure of potency of inhaled anesthetics MAC is the concentration necessary to prevent responding in 50% of population.
Inhaled Anesthetics Easily vaporized liquid halogenated hydrocarbons Administered as gases (gas)
General Actions of Inhaled Anesthetics Respiration Depressed respiration and response to CO2 Kidney Depression of renal blood flow and urine output Muscle High enough concentrations will relax skeletal muscle
General Actions of Inhaled Anesthetics Cardiovascular System Generalized reduction in arterial pressure and peripheral vascular resistance. Isoflurane maintains coronary function better than other agents. Central Nervous System Increased cerebral blood flow and decreased cerebral metabolism.
Properties of Inhaled anesthetics 1- Nitrous Oxide(N 2 O) laughing gas Good analgesia Weak anesthesia Less effect on respiratory and cardiovascular system. Rapid onset & recover Used along w/ other anesthetic; fast induction & recovery
2-Halothane non-flammable 20% metabolism by P450 induction of hepatic microsomal enzymes Myocardial depressant (SA node),
Properties of Inhaled anesthetics 3- Enflurane Rapid induction and recovery Used for maintenance of anesthesia Less potent than halothane good muscle relaxation less sanitization to the myocardium Metabolize into fluoride ion Decrease BP
Parenteral Anesthetics (IV) Most commonly used drugs to induce anesthesia Benzodiazepines (Midazolam) Opioids (Morphine & Fentanyl) Propofol* Etomidate Ketamin
Parenteral Anesthetics (IV) Most exert their actions by potentiating GABA A receptor GABAergic actions may be similar to those of volatile anesthetics, but act at different sites on receptor
Organ Effects Most decrease cerebral metabolism and intracranial pressure Most cause respiratory depression May cause apnea after induction of anesthesia
Cardiovascular Effects Barbiturates, benzodiazepines and propofol cause cardiovascular depression.
1- Ketamine A dissociative anesthetic Characterized by: analgesia, amnesia, eyes open, involuntary limb movement,.(unconscious appear to be awake) Rapid onset and last for5-10 min.(short acting)
Mechanism of action It is bind to NMDA (N-methyl-D-aspartate receptor ) and also stimulate the central sympathetic outflow which cause heart stimulation, increased blood pressure and cardiac output.
Ketamine Used in children & young adults for short procedures Side Effects: 1-salivation, 2-hallucinations& vivid dreams(adult) 3-stimulation of sympathetic nervous system cause tachycardia, increase blood pressure 5- reach to stage two of anesthesia only
2- Xylazine It is used in veterinary medicine for many diagnostic and surgical procedures. It is α2 sympathomimetic agent, it act by activation of central α2 receptors which will cause reduction or depression in release of dopamine and norepinphrine.
Pharmacological effects CNS depression and sedation (30-90 min). Analgesia which last (15-30 min). Muscle relaxation. Emesis Hypotension and brady cardia Decreased respiratory rate. recovery from effect usually takes 2-4 hr.
3- Thiopental sodium rapid onset (20 sec) short-acting
Thiopental sodium Side effects Hypotension apnea airway obstruction