General and Local Anesthetics TURNING POINT PHARM THURSDAY IMC606 Neuroscience Module

Transcription

1 General and Local Anesthetics TURNING POINT PHARM THURSDAY IMC606 Neuroscience Module Peter Bradford, PhD JSMBS December-2018

2 Disclosures NO SIGNIFICANT FINANCIAL, GENERAL, OR OBLIGATION INTERESTS TO REPORT

3 FIRST AID Neurology Anesthetics Desflurane Sevoflurane Isoflurane Nitrous oxide Propofol Ketamine Midazolam Triazolam Lorazepam Diazepam Flumazenil Tetracaine Lidocaine Articaine Bupivacaine

4 USMLE Rx Questions General Anesthetic Pharmacology

5 USMLE Rx A researcher is examining the different properties of anesthetic gases in a series of experiments. Each gas is determined to have a specific minimum alveolar concentration (MAC) and blood gas partition coefficient. The inhaled anesthetic drugs A, B, C, and D have the properties indicated in the chart. Which of the following best describes the properties of these drugs? A. Drug A is more potent than drug D B. Drug B is less soluble in blood than drug C C. Drug C will have both the highest potency and the most rapid induction D. Drug D will induce anesthesia more rapidly than drug C

6 USMLE Rx Anesthetics (general principles) Inhaled anesthetics are characterized in terms of their potency and time to induction/recovery. Minimum alveolar concentration (MAC) of an inhaled anesthetic is inversely proportional to its potency. Blood gas partition coefficient (blood solubility) determines the speed of induction and time to recovery. Low blood solubility characterizes an agent that has a rapid onset and rapid offset, whereas an agent with high blood solubility has a slow onset and slow time to recovery.

7 Minimum alveolar concentration (MAC) Measure of Potency o To control the depth of anesthesia, the anesthesiologist varies the inspired partial pressure o The alveolar pressure that results in the lightest possible anesthesia (nonresponsive to skin incision) is termed the minimum alveolar concentration (MAC) Isoflurane dose-response curves for various endpoints, Principles of Pharmacology, Golan et al NOTE: Therapeutic index (TI) TI LD50/MAC (LD50/ED50)

8 Induction of anesthesia is faster with less soluble anesthetic gases General Anesthetics, Katzung BG. Basic & Clinical Pharmacology, 14e; 2017 Solubility in blood is represented by the relative size of the blood compartment (the more soluble, the larger the compartment). Relative partial pressures of the agents in the compartments are indicated by the degree of filling of each compartment. For a given concentration or partial pressure of the two anesthetic gases in the inspired air, it will take much longer for the blood partial pressure of the more soluble gas (halothane, λ=2.3) to rise to the same partial pressure as in the alveoli. Since, CNS level of anesthetic can rise no faster than that in the blood, the onset of anesthesia will be slower with halothane than with nitrous oxide.

9 These two inhalation anesthetic agents provide great control of the depth of anesthesia and are both used for maintenance of anesthesia. However, the first agent has weak anesthetic potency and can only achieve partial sedation as a sole agent; whereas, the second agent has a greater anesthetic potency and can achieve full sedation at concentrations between 2-3% with oxygen. A. Desflurane and diethyl ether B. Nitrous oxide and sevoflurane C. Desflurane and nitrous oxide D. Nitrous oxide and diethyl ether

10 General Anesthetic Potency MAC

11 USMLE Rx A 27-year-old man is scheduled to undergo hernia repair. On administration of an inhaled anesthetic, the patient begins to experience severe muscle rigidity, and his temperature rises to 39.4 C (102.9 F). Blood pressure rapidly rises to 160/90 mmhg, and EKG monitoring shows sinus tachycardia at 125 bpm. In light of these findings, the anesthesiologist administers another medication and cools the patient with ice. Subsequently, the patient s temperature begins to normalize and his muscles relax. What is the mechanism of action of the drug that was used to treat the patient s adverse reaction? A. Blocks action potential depolarization from opening voltage-gated calcium channels B. Hyperpolarizes voltage-gated calcium channels on skeletal muscle C. Inhibits the binding of calcium to troponin C D. Inhibits the release of calcium from the sarcoplasmic reticulum

12 USMLE Rx A 27-year-old man is scheduled to undergo hernia repair. On administration of an inhaled anesthetic, the patient begins to experience severe muscle rigidity, and his temperature rises to 39.4 C (102.9 F). Blood pressure rapidly rises to 160/90 mmhg, and EKG monitoring shows sinus tachycardia at 125 bpm. In light of these findings, the anesthesiologist administers another medication and cools the patient with ice. Subsequently, the patient s temperature begins to normalize and his muscles relax. What is the mechanism of action of the drug that was used to treat the patient s adverse reaction? A. Blocks action potential depolarization from opening voltage-gated calcium channels B. Hyperpolarizes voltage-gated calcium channels on skeletal muscle C. Inhibits the binding of calcium to troponin C D. Inhibits the release of calcium from the sarcoplasmic reticulum

13 USMLE Rx Treatment of Malignant Hyperthermia This patient with abrupt onset of muscular rigidity and hyperthermia following the administration of general anesthesia demonstrates the classic symptoms and signs for malignant hyperthermia. Malignant hyperthermia caused by rapid consumption of ATP is an autosomal-dominant genetic disorder that affects skeletal muscles. A ryanodine receptor mediated increase in the free-calcium concentration in the skeletal muscle is responsible for the rigidity that characterizes malignant hyperthermia. Dantrolene acts by inhibiting the ryanodine receptor calcium-release channels in the sarcoplasmic reticulum, thus lowering the intracellular calcium concentration and alleviating the ensuing muscle rigidity

14 Low solubility of desflurane and sevoflurane facilitates smooth, rapid loss of consciousness during inhalation induction. Ventilation is most important for drug elimination and time for 50% decrease of desflurane and sevoflurane is < 5 minutes. A. Both statements are true. B. The first statement is false. The second statement is true. C. The first statement is true. The second statement is false. D. Both statements are false

15 USMLE Rx A 35-year-old man is brought to the recovery area after undergoing an open surgical procedure on his abdomen. He is groggy and slow to wake. He continues to have muscle stiffness accompanied by severe contractions. His temperature is 38.8 C (101.8 F), pulse is 152/min, respiratory rate is 30/min, and blood pressure is 160/95 mm Hg. The surgical wound covering appears dry and intact. His extremities are rigid and the skin is developing a fine reticular pattern. A staff member rapidly administers a medication to treat the patient s condition. Which of the following is the mechanism of action of the medication most likely given? A. Inhibits fibrinolysis B. Antagonizes γ-aminobutyric acid receptors C. Prevents calcium release D. Stimulates adrenergic receptors E. Inhibits acetylcholinesterase

16 USMLE Rx A 35-year-old man is brought to the recovery area after undergoing an open surgical procedure on his abdomen. He is groggy and slow to wake. He continues to have muscle stiffness accompanied by severe contractions. His temperature is 38.8 C (101.8 F), pulse is 152/min, respiratory rate is 30/min, and blood pressure is 160/95 mm Hg. The surgical wound covering appears dry and intact. His extremities are rigid and the skin is developing a fine reticular pattern. A staff member rapidly administers a medication to treat the patient s condition. Which of the following is the mechanism of action of the medication most likely given? A. Inhibits fibrinolysis B. Antagonizes γ-aminobutyric acid receptors C. Prevents calcium release D. Stimulates adrenergic receptors E. Inhibits acetylcholinesterase

17 USMLE Rx Malignant hyperthermia treatment This patient presents with prolonged muscle rigidity and vital sign instability shortly after surgery. Muscle stiffness, mottled skin, tachycardia, tachypnea, hypertension, and elevated temperature (pyrexia) in the setting of anesthesia is worrisome for malignant hyperthermia. MH can occur at the onset of anesthesia induction, during surgery, or shortly after withdrawal of anesthetic agents. The trigger is often an inhaled anesthetic and/or succinylcholine (a depolarizing neuromuscular drug). Dantrolene is the only known therapy for malignant hyperthermia. It is a drug that inhibits the release of calcium from the sarcoplasmic reticulum of skeletal muscle.

18 USMLE Rx Malignant hyperthermia treatment Distractor Responses Agents that inhibit fibrinolysis are used for acute bleeding or prevention of excessive bleeding. One such drug is aminocaproic acid which inhibits plasmin, the enzyme responsible for fibrinolysis. Stimulation of adrenergic receptors - Epinephrine has beneficial effects in the treatment of anaphylactic shock. Although this patient s skin changes could be mistaken for a rash signaling an anaphylactic reaction, a characteristic feature of shock is hypotension, while this patient s blood pressure is elevated. Acetylcholinesterase inhibitors are used to treat toxicity from anesthetics as well, but as a reversal agent for nondepolarizing neuromuscular drugs (tubocurarine, vecuronium). They are not proven to work for MH.

19 USMLE Rx A 43-year-old woman presents to the emergency department following a motor vehicle collision. She sustained no serious physical injuries, but she appears drowsy. Her pupils are 4 mm in size. Emergency medical staff collected several open medication bottles from the automobile. Although the contents of two of the bottles could not be determined, the patient s recently filled bottle of diazepam is almost empty. The patient quickly becomes unresponsive and stops breathing. What is the mechanism of action of the agent that would reverse her symptoms? A. Competitive inhibitor of glycine receptors B. Positive modulator of GABA at the GABA-A receptor C. Positive modulator of GABA at the GABA-B receptor D. Competitive inhibitor of the modulator actions of benzodiazepines at the GABA-A receptors

20 USMLE Rx Reversal of diazepam overdose GABA is a major inhibitory neurotransmitter in the CNS, and on activation of GABA A receptors, Cl enters the cell, resulting in hyperpolarization and the subsequent inhibition of new action potentials. Benzodiazepines and barbiturates are modulators of GABA-A receptors that act at allosteric binding sites to enhance GABAergic neurotransmission. Flumazenil antagonizes the effects of benzodiazepine agonists by competing for occupancy of the high-affinity benzodiazepine sites on GABA-A receptors. Glycine receptors also have inhibitory effects on the nervous system. Competitive inhibitors of glycine receptors (e.g. strychnine) would cause convulsions rather than reversal of this patient's symptoms, making this choice incorrect.

21 USMLE Rx Reversal of diazepam overdose Flumazenil is the reversal agent for benzodiazepine intoxication, which is characterized by somnolence, respiratory depression, amnesia, and ataxia. Flumazenil is a competitive antagonist of benzodiazepine binding to its allosteric activator site at GABA-A receptor.

22 This is the major phytocannabinoid identified in cannabis, and in 2018 the FDA has approved its safety and effectiveness in the treatment of specific rare forms of intractable epilepsy. A. Capsaicin B. Arachidonyl glycerol C. Cannabidiol D. Anandamide E. Tetrahydrocannabinol

23 This anesthetic agent is said to produce a state of dissociative anesthesia in which the patient seems to be awake but is actually in an analgesic and amnesiac state. A. Halothane B. Ketamine C. Methohexital D. Propofol E. Fentanyl

24 The primary mechanism of action of local anesthetics is: A. Activation of ligand-gated chloride channels B. Stimulation of voltage-gated N-type calcium channels C. Blockade of voltage-gated sodium channels D. Blockade of voltage-gated potassium channels

25 Classes of agents used for epidural analgesia include opioids such as fentanyl and local anesthetics. Most current labor analgesia regimens employ low concentrations of this long-acting local anesthetic. A. Tetracaine B. Bupivacaine C. Lidocaine D. Procaine