General Anesthesia: Overview & Mechanisms

# General Anaesthesia - General anaesthetics are the drugs that produce reversible loss of all sensation and consciousness. ## Classification ### 1. Inhalational - **Gas:** N₂O, Xenon - **Liquids:** Ether, Halothane, En/Iso/Des/Sevo-flurane ### 2. Intravenous - **Inducing agents:** Thio...

# General Anaesthesia - General anaesthetics are the drugs that produce reversible loss of all sensation and consciousness. ## Classification ### 1. Inhalational - **Gas:** N₂O, Xenon - **Liquids:** Ether, Halothane, En/Iso/Des/Sevo-flurane ### 2. Intravenous - **Inducing agents:** Thiopentone sodium, Propofol, Methohexitone, Etomidate - **Slow acting** - Dissociative anaesthesia: Ketamine - Benzodiazepines: Diazepam, Lorazepam, Midazolam - Opioid analgesics: Fentanyl # General Anaesthesia - Unconsciousness - Analgesia - Amnesia - Skeletal Muscle relaxation - Loss of motor reflexes # Stages of anaesthesia 1. **Stage I—Analgesia:** Loss of pain sensation results from interference with sensory transmission. The patient progresses from conscious and observes analgesia. Amnesia and reduced awareness of pain occur as Stage II is approached. 2. **Stage II—Excitement:** The patient experiences delirium and possibly combative behaviour. A rapid acting agent, such as propofol, is given intravenously before inhalation anaesthesia is administered. 3. **Stage III—Surgical anaesthesia:** There is gradual loss of muscle tone and reflexes as the CNS is further depressed. Regular respiration and relaxation of skeletal muscles with eventual loss of spontaneous movement occur in this stage. This is the ideal stage of anaesthesia for surgery. Continuous careful monitoring is required to prevent undesired progression into Stage IV. 4. **Stage IV—Medullary paralysis:** Severe depression of the respiratory and vasomotor centres occur during this stage. Death can rapidly ensue unless measures are taken to maintain circulation and respiration. # Stages of general anesthesia - The depth of anesthesia has been divided into four sequential stages: - **Stage 1: Analgesia (induction)** - **Stage 2: Excitement** - **Stage 3:Surgical anesthesia** - **Stage 4: Medullary paralysis** # Mechanism of action of General Anaethetics - Inhalational anaesthetics, barbiturates, benzodiazepines & propofol potentiate the action of inhibitory transmitter GABA to open Cl^- channels. - Action of glycine (another inhibitory transmitter which also activates Cl^- channels) in the spinal cord and medulla is augmented by barbiturates, propofol and many inhalational anaesthetics. - Ketamine selectively inhibits the excitatory NMDA type of glutamate receptor. - Certain fluorinated anaesthetics and barbiturates inhibit the neuronal cation channel gated by nicotinic cholinergic receptor. # MECHANISM OF ACTION OF GENERAL ANAESTHETICS - **GABA -A receptor:** Potentiated by Halothane, Propofol, Etomidate,Enflurane, isoflurane, Desflurane, sevoflurane - **NMDA receptors:** Inhibited by Ketamine, nitrous oxide and xenon - **Glycine receptors:** Potentiated by Halothane, Propofol, Enflurane, isoflurane, Desflurane,  # CLASSIFICATION OF GENERAL ANAESTHETICS - **General anaesthetics** - **Inhalational** - **Gas:** Nitrous oxide - **Volatile liquids:** Halothane, Desflurane, Sevoflurane, Isoflurane, Enflurane - **Intravenous** - Thiopentone sod. - Etomidate - Ketamine - Propofol - Methohexitone sod. # Inhalational Anaesthetics ## Nitrous oxide (N₂O) - It is cheap and commonly used anaesthetic. - It is low potency anaesthetic but good analgesic. - It has fast onset and recovery. - It is given as 70% N₂O + 25-30% O₂ + 0.2-2% other potent anaesthetic (Halothane) - It has little effect on respiration, heart and BP. ### Uses: - Maintain surgical anaesthesia - Obstetric practice - Emergency management of injuries - Refractory pain, etc ### Adverse Effects: - It interacts and degrades Vit B₁₂. ### Contraindications: - Pneumothorax - Air embolus - Obstructive # NITROUS OXIDE - Safest inhalational anesthetic - Noninflammable, nonirritating - Low potency anesthetic, poor muscle relaxant but a good analgesic. - No toxic effect on the heart, liver and kidney - A/E: Diffusional hypoxia, megaloblastic anemia # Halothane - It is a volatile liquid with sweet odour, non-irritating and non-explosive. - It is potent anaesthetic but poor analgesic. - It dilates bronchi so preferred in asthma patient. - For induction 2-4% and for maintenance 0.5-1% is delivered by using special vapouriser. ### Adverse effects: - Hepatitis - Hyperthermia - Hangover - Shivering ### Contraindication: - Jaundice - Raised CSF pressure # HALOTHANE - Potent anesthetic - Poor analgesic, poor muscle relaxant - Induction is pleasant - It sensitizes the heart to catecholamines. - It dilates bronchus (preferred in asthmatics) - A/E : Halothane hepatitis and malignant hyperthermia # Intravenous anaesthetics ## Inducing agents - Produce anaesthesia within 20 sec. - Reduce dose of other anaesthetics - Supplemented with analgesics and muscle relaxant ## Thiopentone sodium. - It is ultrashort acting thiobarbiturate administered i.v. - Single dose induces hypnosis and anaesthesia without analgesia - When administered i.v. it quickly enters CNS and depresses function within minutes. Injected i.v. (3-5mg/kg) as 2.5% solution. - Redistributed to skeletal muscles and adipose tissue serves as reservoir. ### Uses: - Inducing agent - Control of convulsion - Protect from cerebral ischemia ### Adverse effects: - Laryngospasm - Shivering and delirium during recovery # KETAMINE - Dissociative anesthesia - Produce - profound analgesia, immobility, amnesia with light sleep. - Heart rate and BP are elevated due to sympathetic stimulation - Respiration is not depressed and reflexes are not abolished. - Emergence delirium, hallucinations and involuntary movements occurs during recovery - Dangerous for hypertensive and IHD patients # PROPOFOL - Most commonly used IV anesthetic - Unconsciousness in 45 seconds and lasts 15 minutes - Anti-emetic - Non-irritant to airways - Suited for day care surgery (residual impairment is less marked) - A/E– Pain during injection, Fall in BP # PREANESTHETIC 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 acidity - Proton Pump Inhibitor - To prevent bradycardia and secretion - Atropine