Anesthetics - Medical Information

Anesthetics Terms ○ Anesthesia → loss of all sensation ○ Analgesia → loss of pain sensation ○ Local anesthetics → block conduction of nerve impulses in peripheral nerves or spinal cord ○ General anesthetics → block cortical neuronal a...

Anesthetics Terms ○ Anesthesia → loss of all sensation ○ Analgesia → loss of pain sensation ○ Local anesthetics → block conduction of nerve impulses in peripheral nerves or spinal cord ○ General anesthetics → block cortical neuronal activity underlying consciousness and all sensation General anesthesia ○ Used to prevent consciousness during major surgical procedures ○ Produce loss of consciousness and amnesia (patient doesn’t recall the surgical procedure) Drug properties ○ Local anesthetics are divided into two main groups… Ester type drugs (e.g., cocaine, benzocaine) Amide-type drugs (e.g., lidocaine, bupivacaine) Duration of action ○ Can be short, medium or long ○ Determined primarily by rate of diffusion and absorption away from site of administration Chemical properties of the anesthetics pH Blood flow Epinephrine ○ Sometimes added to prolong an anesthetic’s duration of action ○ Produces vasoconstriction, slowing the anesthetic’s rate of absorption Metabolism Ester-type local anesthetics get metabolized to PABA derivatives Amide-type local anesthetics get metabolized by CYP450 enzymes In both cases, the metabolites get excreted in urine Allergic reactions ○ Reactions are fairly common ○ Those who get repeated administration of topical anesthetics are usually like most susceptible to sensitization ○ Ester-type anesthetics cause hypersensitivity reactions more frequently than amide type anesthetics Nerve block anesthesia ○ Local anesthetic injected into or adjacent to a peripheral nerve or nerve plexus Radial nerve block used to anesthetize structures innervated by radial nerve (i.e., forearm, hand) Infraorbital block often used for orbital surgery Brachial plexus and cervical plexus blockade  Spinal intrathecal anesthesia ○ Used to block somatosensory and motor fibers during procedures like surgery on lower limb or pelvic structures ○ Anesthetic injected into subarachnoid intrathecal space below level at which spinal cord terminates ○ Spread of anesthetic controlled by horizontal tilt of patient and spgr of local anesthetic solution ○ Can cause HA associated with CSF leakage from LP and respiratory depression if anesthetic ascends too high ○ Small risk of meningitis and infection Lidocaine ○ Produces local anesthesia after topical or parenteral administration ○ Most widely-used local anesthetics ○ Available in many forms (e.g., topical solutions/ointment, oral sprays, viscous gels, and parenteral formulations) ○ Part of EMLA (along with prilocaine), can be used to anesthetize intact skin to a depth of 5 mm Used in kids for local anesthesia before venipuncture, IV placement, or circumcision Induction rate ○ Three major factors determine induction rate of anesthesia 1. Alveolar partial pressure of the anesthetic is inspired air 2. Ventilation rate 3. Rate at which the anesthetic’s partial pressure in blood increases as its administered Largely dependent on the blood:gas partition coefficient Pharmacologic effects ○ Induction characterized by four major stages … Stage 1 – neurons in spinal cord prevented from firing so analgesia and conscious sedation occur Stage 2– inhibition of firing in small inhibitory neurons causes paradoxical execution (not usually observed with modern anesthesia) Stage 3– goal of surgical anesthesia, causes suppression of reticular-activating system, LOC, and inhibition of spinal reflexes Stage 4– can cause cardiovascular collapse, noted by depression of respiratory and vasomotor nuclei in brainstem Thiopental and Proprofol ○ Both drugs have rapid onset of action that cause unconsciousness in about 20 seconds ○ Duration of action is short, usually 5-10 minutes because they redistribute from brain to peripheral tissues as blood concentration falls ○ Propofol is rapidly metabolized and eliminated causing little hangover ○ Thiopental accumulates in fats and muscles, gets eliminated slowly so some hangover can occur  ○ Either drug can depress cardiac and respiratory function Fentanyl ○ Strong opioid agonist used to treat moderate-to-severe pain ○ Given IV or epidurally in combo with other drugs for surgical or obstetric analgesia and anesthesia ○ Used to provide anesthesia during cardiac surgery (e.g., CABG) because it doesn’t cause CV toxicity ○ Doesn’t produce amnesia or complete LOC so usually combined with a benzo (e.g., midazolam) to produce amnesia and increased sedation Ketamine ○ When given IV, produces dissociative anesthesia ○ Mental state where individual appears dissociated from environment without complete LOC Characterized by analgesia, reduced sensory perception, immobility and amnesia Usually increases blood pressure but has little effect on respiration ○ Major drawback is tendency to cause unpleasant effects during recovery like delirium, hallucinations, irrational behavior ○ Often used in kids because they are less likely to experience these adverse effects (usually combined with a benzo)