F Br F C C H. F Cl Halothane

Transcription

1 B. 6 Inhalational anaesthetic agents a. Describe the properties of an ideal inhalational anaesthetic agent. preparation easily administered boiling point above ambient temperature low latent heat of vaporization simple apparatus chemically stable long shelf-life, compatible with soda-lime, metals and plastics not flammable cheap pharmacokinetic low solubility rapid onset, rapid offset, adjustable depth minimal metabolism predictable in all age groups pharmacodynamic high potency allows high io 2 high therapeutic index analgesic adverse actions minimal toxicity minimal unwanted effects nausea, vomiting, cardiac arrhythmogenicity no toxicity with chronic low-level exposure of staff b. Describe the structure-activity relationships of the volatile agents. C 2 H 5 O C 2 H 5 Ether CCl 2 =CHCl Trichloroethylene H C C O C H Cl Enflurane Br C C H Cl Halothane H C C O C H Cl Isoflurane Cl H H C C O C H Cl H Methoxyflurane H C C O C H Desflurane 3 C H C O C H 3 C H Sevoflurane Structure-activity relationships for the volatile anaesthetic agents apply to their physical and chemical properties and to their metabolism. physical low molecular weight and non-polar structure produce low boiling point, high vapour pressure chemical large number of hydrogen atoms increases flammability high fluorine content minimizes flammability C 2 H moiety can liberate CO in reaction with dry soda-lime Inhalational anaesthetics 2.B.6.1 James Mitchell (December 24, 2003)

2 pharmacokinetics fluorine content reduces solubility in blood and fat hydrolysis of ethers is most rapid when the adjacent carbon atoms are not halogenated hydrolysis produces a halogenated acetic acid and halogenated methanol which can release some halides fluorine on the 1-carbon of methyl-ethyl-ethers can be liberated as - pharmacodynamics chlorine and hydrogen content increases potency fluorine content reduces potency c. Provide a brief overview of the history of nitrous oxide, cyclopropane, ether and chloroform N 2 O first prepared by Priestly 1779 Humphrey Davy suggested N 2 O had anaesthetic and analgesic properties 1844 N 2 O demonstrated by Horace Wells for dental extraction 1846 ether demonstrated by Morton at Massachusetts General 1847 chloroform introduced, used by Queen Victoria 1880s ethyl chloride introduced 1930s cyclopropane and trichloroethylene introduced cyclopropane used for single-breath gas inductions (MAC 9%) trichloroethylene (Trilene) blue coloured agent with good analgesic properties 1951 halothane synthesized fluroxene enters clinical use 1956 halothane enters clinical use 1960 methoxyflurane enters clinical use 1963 enflurane synthesized 1965 isoflurane, desflurane synthesized 1966 enflurane enters clinical use 1968 sevoflurane synthesized 1971 isoflurane enters clinical use 1990 sevoflurane enters clinical use 1992 desflurane enters clinical use d. Describe the preparation of nitrous oxide and Entonox and outline their physical properties. nitrous oxide physical properties MW BP C SG 1.53 kg/l prepared by heating NH 4 NO 3 at C NH 4 NO 3 N 2 O + 2H 2 O small amounts of NH 3 and HNO 3 produced recombine to NH 4 NO 3 on cooling small amounts of NO and NO 2 are also produced can cause methaemoglobinaemia, pulmonary oedema if inspired N 2 O must be purified to remove these contaminants Entonox 50% O 2, 50% N 2 O supplied in cylinders at 138 bar maximum pseudo-critical temperature -5.5 C at 117 bar separation of constituents occurs below pseudo-critical temperature analgesic for labour and brief procedures Inhalational anaesthetics 2.B.6.2 James Mitchell (December 24, 2003)

3 e. Describe the undesirable effects of nitrous oxide. pharmaceutic contamination in manufacture NO 2, NO, HNO 3 physiological cardiovascular haematological P 50 by 1.6 mmhg inhibition of thymidylate synthetase and methionine synthetase by oxidation of cobalt ion on B 12 megaloblastic anaemia neuropathy teratogenicity CNS muscle tone, rigidity especially with opiates physical flammability not flammable, but will support combustion gas spaces partition into physiological spaces middle ear, gut? nausea other spaces expansion of pneumothorax, gas emboli, tube cuffs hypoxia low potency requires high i, potential for hypoxic gas mix rapid flow of N 2 O into alveoli on ceasing administration causes diffusional hypoxia unless supplemental oxygen is inhaled f. Describe the comparative pharmacology of halothane, enflurane, isoflurane, methoxyflurane, desflurane and sevoflurane. BP MW SVP blood: blood: MAC metab. vapor/liquid ( C) (mmhg) gas brain (%) (%) (v/v) N 2 O x desflurane sevoflurane isoflurane enflurane halothane methoxyflurane ether high trichloroethylene cyclopropane x oil:gas partition 150 MAC g. Describe the physiological effects of the volatile agents. CNS anaesthesia facts anaesthetics act at millimolar concentrations (high) lipid solubility increases potency for most agents (Meyer-Overton relationship) some stereospecificity is displayed by chiral agents Inhalational anaesthetics 2.B.6.3 James Mitchell (December 24, 2003)

4 lipid fluidity changes induced by agents are very small inhibition of intracellular Ca 2+ release occurs several theories unitary vs degenerate agents act at a single site or at multiple sites lipid vs protein agents act by altering lipid fluidity or binding lipophilic regions of proteins likely explanation binding to lipophilic protein regions (differing slightly for different agents) alters ligand-gated ion channel activity, altering some or all of ACh, GABA, NMDA, AMPA and KA transmission analgesia uncertain mechanism of action, probably related to anaesthetic actions EEG frequency voltage from 0.4MAC (asleep) I, S, D burst suppression at 1.5MAC, silence at 2MAC H silence at 3.5MAC (impractical) E seizure activity ( by low PCO 2 ) evoked potentials amplitude latency CS volume E I CB loss of autoregulation vasodilation H from 0.6MAC E > I from 1.0MAC CVS contractility H, E > I, S, D RAP H, E > I SVR I, S, D > E > H (I can cause coronary steal, no tachycardia with S) catecholamine sensitization H > I, S, D > E respiratory hypoxic response markedly from 0.1MAC hypercapnic response: E, S > I > H TV RR up to 1MAC, then I H, E bronchodilation ( vagal tone, smooth muscle relaxation) GIT hepatotoxicity all portal flow hepatic artery flow H I renal - ion toxicity (below) E RB and GR I, H, D maintain RB and GR muscle relaxant due to central outflow, blood flow, post junctional sensitivity, Ca 2+ flux E, I > H trigger for malignant hyperpyrexia H > E > I obstetric contractility, vasodilation ( blood loss), depression in fetus immune, haematological H platelet aggregation impair neutrophil activity N 2 O > volatiles teratogenic Inhalational anaesthetics 2.B.6.4 James Mitchell (December 24, 2003)

5 no proven toxicity from volatiles h. Describe the metabolism of the volatile agents and the role of their metabolites in toxicity. halothane oxidation (most metabolism if hepatic oxygen delivery is adequate) trifluoroacetic acid, Br -, Cl - conjugates of TA inhibited by cimetidine, isoflurane, ischaemia reduction ( %) Br - + C 3 CH 2 Cl H + 2 C=CHCl conjugates toxicity alkane volatiles are more arrhythmogenic than ethers Br - direct sedative - nephrotoxicity (but very little liberated) dose- and hepatic blood flow-related hepatotoxicity mild ALT, AST incidence 20% associated with increased reductive metabolism autoimmune fulminant hepatic necrosis 1:30000 accompanied by eosinophilia, rash associated with oxidative metabolites modifying hepatic proteins may also be associated with other volatiles more rarely fluoride ion liberation most severe with α-carbon fluorinated ethers M >> S > E >> I > D systemic [ - ] > 50µmol/l associated with high output renal failure sevoflurane is metabolized by cytochrome p450 E1 in liver methoxyflurane and enflurane are metabolized by p450 in kidney, producing higher local - concentrations sevoflurane may also alter renal handling of amino-acids and glucose i. Describe the interaction of soda-lime with trichloroethylene, halothane and sevoflurane. sevoflurane forms compound A (PIE) on reaction with warm soda-lime sevoflurane H + H 2 C-O-C(C 3 )=C 2 (a vinyl ether) other compounds formed in very small quantities compound A causes nephrotoxicity in rats at ppm (LD ppm) normal levels in human anaesthesia don't exceed 30 ppm reduced by gas flow (>2 l/min recommended) halothane forms a vinyl compound in soda-lime halothane H + 2 C=CBrCl greater toxicity in rats than compound A (LD ppm) normal levels in high flow anaesthesia 4-6 ppm trichloroethylene forms toxic compounds with soda-lime trichloroethylene dichloroacetylene phosgene (Cl 2 C=O) + CO dichloroacetylene causes neurotoxicity esp. cranial nerves V, VII, VIII phosgene causes pulmonary toxicity phosgene + H 2 O 2HCl + CO 2 (never to be used with soda-lime; rarely used anyway) Inhalational anaesthetics 2.B.6.5 James Mitchell (December 24, 2003)