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General Anesthetics By S. Bohlooli, PhD School of Medicine, Ardabil University of Medical Sciences
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General anesthesia may be defined as a state which includes A reversible loss of consciousness Inhibition of sensory and autonomic reflexes (including nociceptive reflexes) Skeletal muscle relaxation Anterograde amnesia (upon recovery) [the extent to which any individual anesthetic drug can exert these effects depend upon the drug, the dose, and the clinical circumstances}
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Chemical structures of inhaled anesthetics.
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Chemical structures of intravenous anesthetics.
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PHARMACOKINETICS OF INHALED ANESTHETICS (1) ABSORPTION AND DISTRIBUTION The concentration of a gas in an environment is proportional to its partial pressure or tension (these terms are often used interchangeably) Depth of anesthesia is determined by the concentration of the anesthetic in CNS.
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In order to reach the CNS the anesthetic must be transferred from the alveolar air to blood and from blood to brain. This transfer is influenced by: Solubility in blood (blood/gas partition coefficient) Anesthetic concentration in the inspired air Pulmonary ventilation Pulmonary blood flow Uptake of the anesthetic by the tissues
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Why induction of anesthesia is slower with more soluble anesthetic gases
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Tensions of three anesthetic gases in arterial blood as a function of time after beginning inhalation
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Ventilation rate and arterial anesthetic tensions
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INHALED ANESTHETICS: MECHANISM OF ACTION At the neurophysiological level - Early depressive effect on substantia gelatinosa - Blockade of small inhibitory neurons (e.g Golgi type II cells) in several brain areas. - Progressive depression of the ascending pathways in the reticular activating system. At the cellular level - Depression of synaptic transmission (the main cellular effect). Neurons are hyperpolarized and their threshold for firing is increased. - Depression of axonal conduction
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INHALED ANESTHETICS: MECHANISM OF ACTION At the molecular level The two main physicochemical theories of general anesthesia are: 1) The lipid theory Anesthetics would bind to hydrophobic regions of membrane lipid bilayer so increasing membrane fluidity which in turn would cause a small membrane expansion that distorts ion channels. 2) The protein theory Anesthetics would bind to hydrophobic regions of specific membrane protein channels so stabilizing the channel in its closed state. - Both theories point out that the mechanism of action of inhaled anesthetics does not involve direct interaction with specific receptors
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THE MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION - During general anesthesia the partial pressure of an anesthetic in the brain equal that in the lung when equilibrium is reached. - The Minimum Alveolar Anesthetic Concentration (MAC) is defined as the concentration of the anesthetic that results in the immobility of 50% of patients when exposed to a noxious stimulus.
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THE MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION - A MAC > 100% indicates that even when all the molecules of the inspired gas are molecules of the anesthetic, the concentration is not able to cause immobility in 50% of patients. - MAC decreases in elderly patients and in the presence of certain adjuvant drugs (opioids, benzodiazepines, barbiturates, etc.) - MACs of inhaled general anesthetic are additive
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HALOTHANE PHARMACODYNAMICS [most effects are concentration dependent, that is they increase as the partial pressure in the target tissue increases] Nervous system effects - Partial loss of nociceptive reflexes - Good postoperative amnesia Cardiac effects - Direct depression of myocardial contractility - Direct depression of cardiac rate - Increase in cardiac automaticity - 5-10% reduction of cardiac output - Sensitization of myocardium to catecholamines
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HALOTHANE Vascular effects - Decrease in brain vascular resistance (which leads to an increase in intracranial pressure) - Little changes in total peripheral resistance - Reduction of blood pressure due to: a) reduced cardiac output b) impairment of normal baroreceptor response Respiratory effects - Decrease in tidal volume - Increase in respiratory rate - Decrease in minute ventilation (the increased rate cannot compensate for the decreased tidal volume) - Ventilatory response to CO2 is decreased - Ventilatory response to hypoxia is decreased [all these effects can be overcome by assisting the ventilation] - Bronchodilation - Depression of mucociliary clearance
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HALOTHANE Urogenital effects - Decreased renal blood flow - Decreased glomerular filtration rate - Pronounced relaxation of the uterus Skeletal muscle effects - Modest relaxation of skeletal muscle - Enhancement of the action of nondepolarizing skeletal muscle relaxants Gastrointestinal effects - Postoperative nausea and vomiting (» 15%) - Decreased hepatic blood flow
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HALOTHANE HEPATITIS Occurrence: seems very low (1:35000), but the risk seems to increase after repeated exposures. Etiology: halothane is partially (» 30%) metabolized to trifluoroacetic acid, bromide and chloride ions, which have been implicated as causative factors in halothane hepatitis. Pathogenesis: the mechanism of hepatotoxicity remains obscure. Two hypotheses are : an immune response to certain fluoroacetylated liver enzymes (allergic reaction). a genetically determined defect in hepatic cell membranes that make these cells more susceptible to halothane-induced injury (idiosyncratic reaction). Pathology: the syndrome is histologically indistinguishable from viral hepatitis. Symptoms and signs: anorexia, nausea and vomiting, fever. Clinical course and prognosis: the syndrome typically starts 3-5 days after anesthesia and may progress to hepatic failure. The prognosis is poor (death occurs in » 50% of these patients).
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ISOFLURANE PHARMACODYNAMICS [most effects are concentration dependent, that is they increase as the partial pressure in the target tissue increases] Nervous system effects - Partial loss of nociceptive reflexes - Good postoperative amnesia Cardiovascular effects [direct effects are partially counteracted by a centrally mediated sympathetic activation; therefore the final effects are usually the following] - Small depression of myocardial contractility - Increased cardiac rate - Cardiac automaticity is not affected - Cardiac output is well maintained - Decrease in brain vascular resistance (which leads to an increase in intracranial pressure) - Decreased in total peripheral resistance - Reduction of blood pressure
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ISOFLURANE Respiratory effects - Decrease in tidal volume - No change respiratory rate - Minute ventilation is decreased - Ventilatory response to CO2 is decreased - Ventilatory response to hypoxia is decreased [all these effects are lessened by surgical stimulation and can be overcome by assisting the ventilation] - Bronchodilation - Depression of mucociliary clearance Urogenital effects - Decreased renal blood flow - Decreased glomerular filtration rate - Pronounced relaxation of the uterus Skeletal muscle effects - Good relaxation of skeletal muscle - Enhancement of the action of nondepolarizing skeletal muscle relaxants Gastrointestinal effects - Postoperative nausea and vomiting (» 15%) - Decreased hepatic blood flow - No evidence of direct hepatic toxicity
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PHARMACOLOGY OF DESFLURANE AND SEVOFLURANE - Desflurane and Sevoflurane resemble Isoflurane in most of their pharmacological properties. Main differences are: Desflurane - Rapid induction and recovery - Coughing and sometimes laryngospasm (due to its irritant properties) - No changes in renal blood flow Sevoflurane - Rapid induction and recovery - No changes in heart rate - Metabolism by the liver may release fluoride ions that could be nephrotoxic - Chemically unstable when exposed to CO2 absorbents
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MALIGNANT HYPERTHERMIA Occurrence: is very low (1:20000) Etiology: general anesthesia with all halogenated anesthetics, especially when supplemented with depolarizing muscle relaxants seems to be the causative factor. Pathogenesis: malignant hyperthermia is an autosomal dominant disorder which arises from a stimulus-elicited excessive release of Ca++ from the sarcoplasmic reticulum. Symptoms and signs:, hyperthermia, muscular rigidity, acidosis, tachycardia and shock. Hyperkalemia, hypercalcemia and myoglobinuria usually occur. Creatinine kinase levels are hugely elevated. Clinical course and prognosis: the syndrome can start during surgery or few days later, progresses rapidly and can be fatal (death occurs in 10-20% of these patients). Therapy: dantrolene is the drug of choice
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NITROUS OXIDE PHARMACOLOGY Chemistry and physicochemical properties - A gas (N2O) without odor or taste. - Blood/gas partition ratio = 0.47 Central nervous system effects -The MAC is >100%, therefore surgical anesthesia can be reached only when is administered under hyperbaric conditions - Analgesia is very good with 20% of N2O - Induction and recovery are very rapid - Postsurgical amnesia is incomplete Cardiovascular system - Slight direct depression of contractility which is completely counteracted by sympathetic stimulation. - No change or increase in heart rate. - No effect on cardiac automaticity. - Negligible effects on blood pressure
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NITROUS OXIDE Respiratory system - Respiration is well maintained - Ventilatory response to CO2 is not affected - Ventilatory response to hypoxia is reduced - Respiratory depressant effects of other anesthetics are enhanced Urogenital effects - Little effects on renal blood flow - No effect on uterine tone Skeletal muscle effects - No effect on skeletal muscle tone - No enhancement of the action of nondepolarizing skeletal muscle relaxants Gastrointestinal effects - Postoperative nausea and vomiting (» 15%) Blood effects - Prolonged exposure to N2O may cause megaloblastic anemia due to oxidation of the cobalt atom in Vit.B12 Clinical uses - As a sole agent to provide analgesia for dental procedures and for parturition - In combination with other drug for general anesthesia
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INTRAVENOUS ANESTHETICS: BARBITURATES Drugs Thiopental sodium is the agents most commonly used. Mechanism of action - Neurophysiological: Progressive depression of the ascending pathways in the reticular system. - Molecular: Enhancement of GABA-mediated inhibition (the opening of Cl- channels is prolonged by facilitating GABA action) or, at high doses, direct opening of Cl- channels. Central nervous system effects - Following a standard IV dose unconsciousness occurs in 10-20 seconds and returns in 15-20 minutes (due to the redistribution process) - Inhibition of sensory and autonomic reflexes (including nociceptive reflexes) is negligible and so movements, vocalization and sympathetic responses can occur in response to surgery.
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BARBITURATES Respiratory effects - Dose-dependent depression of the respiratory center which causes a decrease in: a) minute ventilation; b)ventilatory response to CO2; c) ventilatory response to hypoxia - Coughing, bronchospasm and laryngospasm can occur (the basis of these reactions is unknown) Cardiovascular effects - Dose-dependent decrease in cardiac contractility - Increased venous capacitance - Negligible change in total peripheral resistance - Decrease in cardiac output and blood pressure - Brain blood flow is decreased and intracranial pressure is markedly reduced - Baroreceptor reflex is not affected Skeletal muscle effects - Negligible effects on skeletal muscle tone. Gastrointestinal effects - Postanesthetic nausea and vomiting is » 15% - Induction of P450 system in the liver Use in anesthesia - As a sole agent in case of short surgery - For induction of anesthesia, in combination with inhaled anesthetics
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INTRAVENOUS ANESTHETICS: PROPOFOL Central nervous system effects - Following a standard IV dose unconsciousness occurs in 20-40 seconds and returns in 4-8 minutes (due to the redistribution process) - Analgesic effect is negligible Respiratory effects - Dose-dependent depression of the respiration with a marked decrease in minute ventilation, ventilatory response to CO2 and to hypoxia. Cardiovascular effects - Decrease in myocardial contractility - Decrease in total peripheral resistance which leads to a marked dose-dependent decrease of blood pressure - Brain blood flow is decreased and intracranial pressure is reduced - Baroreceptor reflex is not affected
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PROPOFOL Skeletal muscle effects - Negligible effects on skeletal muscle tone - Tremors are sometimes seen on induction Gastrointestinal effects - The drug has an antiemetic activity which prevent postanesthetic nausea and vomiting Use in anesthesia - Because of its rapid recovery and antiemetic properties propofol is mostly used in ambulatory anesthesia - Sometimes used to obtain prolonged sedation in critically ill patients
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INTRAVENOUS ANESTHETICS: ETOMIDATE Central nervous system effects - Following a standard IV dose unconsciousness occurs in 10-20 seconds and returns in 3-5 minutes (due to the redistribution process) - Analgesic effect is negligible Respiratory effects - Respiration is well maintained and respiratory depression usually dos not occur Cardiovascular effects - Cardiac output is well maintained - Blood pressure is normal or slightly reduced - Brain blood flow is decreased and intracranial pressure is reduced - Baroreceptor reflex is not affected
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ETOMIDATE Skeletal muscle effects - Negligible effects on skeletal muscle tone - Involuntary movements (myoclonus) occur in »40% of patients Gastrointestinal effects - Postoperative nausea and vomiting are frequent (> 30%) Other effects - Inhibition of steroidogenesis occurs and plasma levels of cortisol are reduced after a single dose Use in anesthesia - As an induction agent in patients with serious cardiovascular disease
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INTRAVENOUS ANESTHETICS: KETAMINE Mechanism of action - Neurophysiological: impairment of neuronal pathways in the cortex and limbic structures. - Molecular: blockade NMDA-type glutamate receptors (the mechanism is quite similar to that of the psychedelic drug phencyclidine) Central nervous system effects - The drug induces dissociative anesthesia which can be defined as a state of sedation, immobility, analgesia, anterograde amnesia and a strong feeling of dissociation from the environment without actual loss of consciousness. - Following a standard IV dose dissociation occurs in 10-15 seconds and last 10-15 minutes (due to the redistribution process), but complete recovery often required several hours - Analgesic effect is very pronounced Respiratory effects - Respiration is well maintained - Pharyngeal and laryngeal reflexes are retained - Bronchodilation
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KETAMINE Cardiovascular effects - Increase in heart rate, cardiac output and blood pressure (up to 25%) (all these effects are due to central sympathetic stimulation) - Brain blood flow and intracranial pressure are markedly increased Skeletal muscle effects - Muscle tone is increased (catatonia can occur) Gastrointestinal effects - Postoperative nausea and vomiting (» 15%) Adverse effects - Hallucination and delirium (» 30% of adult patients) that can recur weeks later (flash-backs) Use in anesthesia - In poor-risk patients or patients with shock - In children
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BENZODIAZEPINES AS GENERAL ANESTHETIC DRUGS Drugs -Certain benzodiazepines (e.g. diazepam, lorazepam, midazolam) are used in anesthetic procedures Mechanism of action - Neurophysiological: increased activity of small inhibitory GABAergic neurons in several brain areas - Molecular: Enhancement of GABA-mediated inhibition (the frequency of opening of Cl- channels is increased by facilitating GABA action) Central nervous system effects - Following a 70 mg IV dose of diazepam drowsiness occurs in 2-3 minutes, but a complete unconsciousness is not achieved - Recovery from drowsiness is slow - Anterograde amnesia occurs in > 50% of patients - Analgesic effect is negligible
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BENZODIAZEPINES Effects on other organs - Respiration and circulation are only moderately depressed, but cardiovascular and respiratory depression produced by of other drugs are enhanced - Skeletal muscle tone is moderately reduced by a centrally mediated action. The effect of curare-like drugs is not modified. - Renal and hepatic function are not affected. -Postoperative nausea and vomiting are rare Use in anesthesia - As a sole agent in case of procedures that do not require analgesia (radiodiagnostic procedures, etc.) - For induction of anesthesia, in combination with inhaled anesthetics (but thiopental is largely preferred) - In the preanesthetic medication.
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OPIOIDS AS GENERAL ANESTHETIC DRUGS Drugs - Morphine, fentanyl, sulfentanil, alfentanil Central nervous system effects In some situations very large dose of opioids may be infused to obtain anesthesia - Large IV doses of morphine or fentanyl administered slowly induce unconsciousness and profound analgesia - With fentanyl unconsciousness occurs in 10-20 seconds and returns in about 30 minutes (due to a redistribution process). - Anterograde amnesia is negligible and postoperative recall of events may occur. Respiratory effects - Respiration is severely depressed and ventilation must be mechanically controlled
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OPIOIDS Cardiovascular effects - Cardiovascular system is moderately depressed with morphine (due to histamine release) and unaffected by fentanyl and congeners. Skeletal muscle effects - Rigidity of respiratory muscle may be prominent and administration of a muscle relaxant may be necessary to permit artificial respiration. Gastrointestinal effects - Postoperative nausea and vomiting are frequent Use in anesthesia - They are used (often together with nitrous oxide) in cardiac surgery or in case of surgery in patients with very serious cardiac disease. - They are used widely to provide relief from pain during general anesthesia of all types. - They are frequently used as preanesthetic medication in order to decrease pain-anticipatory anxiety.
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NEUROLEPTANALGESIA AND NEUROLEPTANESTHESIA NEUROLEPTANALGESIA - When a potent opioid (fentanyl) is combined with a potent neuroleptic compound (droperidol) a state of neuroleptanalgesia is established - This is a state of quiescence, with reduced motor activity reduced anxiety, indifference to the surrounding. - Analgesia is profound, but consciousness is retained. - Cardiovascular effects generally are not marked. - Respiratory depression is severe but predictable. Neuroleptanalgesia can be used for minor surgical procedures like endoscopy, burn dressing etc. NEUROLEPTANESTHESIA - Neuroleptanalgesia can be converted to neuroleptanesthesia by the concurrent administration of 65-70% of nitrous oxide. Adverse effects of the procedures - Postoperative respiratory depression (which can be reversed by naloxone) - Extrapyramidal muscle movements (which can be controlled by benztropine)
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