Unit 06: inhalant Anesthetics Flashcards
what is the ideal inhalent anesthetic
chemically stable without preservatives, compatible with anesthetic machine materials, non-flammable, potent, does not cause cardiopulmonary depression, has a non-pungent odour, and is not metabolized so that 100% of the drug is exhaled intact.
what inhalane tanesthetics are currently used
- halogenated hydrocarbons including halogenated aiphatic hydrocarbons or ethers
- replacemnt of H with halogens reduces flammability and enhances stability - more F- groups usually increases stability as well
main inhalants are : halothane, isoflurane and sevoflurane
what is the mechansim fo action of inhalants
- facilitate GABA at low concentrations and directly activte GABA receptors at higher concentrations
- opens chloride channels and inhibits action potentials
- toehr actions may occur include: opening certain K+ channels and inhibiting excitatroy channels such as nicotinic acetylcholine receptors (nAChRs) or Ca2+ channels
how do you make inhalants
- volatile liquids palced in vapouriser that is connected to a gas flow system of an anesthetic machine
- anesthetic vapour is delivered with oxygen and absorbed from lungs and into the blood where it is distributed to the tissues
- elimination occurs by exhalation - constant delivery of fresh gas with controlled percentrage of anesthetic maintains anesthesia
- veiliver from alveolar gas to the blood and brains depends on several factors
what factors influence the delivery form alveolar gas tot eh blood and brain
- Solubility of the drug in tissues vs alveolar gas
- involved blood: gas partiticion coefficient (BGPC)
- indicates relative affinity of the drug for tissues compared to air, can range drastically depending on inhlanat
- the onset (if no induction used) and recovery are gast with poorly soluble drugs and slow with highly solble drugs
- Concentration of anesthetic in inspired air affects the delivery of anesthetic
- highly volatile drugs can reach high concentrations in air and can be used for induction when injectibles too dangerous
- ex: to use inhalent for induction may use a vapourizer setting of 3-4% of drug conc then reduce it formaintenance
- Ventilation rate or rate taht patient breaths
- faster patient breaths the faster the drug is delivered and abosrbed in lungs
- an be controlled if necessary
- Pulmoney blood flow or cardiac output
* higher bloow flow moves drug to and from tissues faster, genrall this is not altered
describe the BGPC levels of sevoflurance, isoflurane, halothane and methoxyflurane
Sevoflurane: 0.69 poor solubility in tissues
Isoflurane: 1.4, moderate solubility in tissues
Halothane: 2.3, moderately soluble in tissues
Methocyflurane: 12, highly soluble in issues
*recovery is fast with poorly soluble drugs and low with highly soluble
what is MAC and MAC50
MAC = minimum alveolar concetration (used to compare the potency of inhalant )
- MAC50 refers to the drug concetration in inspired air that will prevent 50% of patients from feeling painful stimuli
*very potent drugs requrie low conc to have desired effect
- use MAC value to determine vapourizer setting alowing the anestheiologist to control the depth of anesthesia
ex: Halothane has MAC of approx 0.75 which means vapourizer should be set at 1.2-1.5 x the MAC value at start then adjustied to response
what is related to inalent drug toxiicty
extent of inhalent drug metabolism
- Halothane is 25% metabolized, sevoflurane is 3% and isoflurane is 0.25%
- metabolites that are produced are very harmful to the liver, usually only problem with repeated or long procedures with haloethane
CYP2E1 is the major P450 enzyme that catalyzes the defluorination of volatile anesthetics
*Isoflurane and sevoflurane are much safer for patients with hepatic dysfunction.
how are inhalants eliminated?
mainly by exhalation
- greatly influences by tissue solubiltiy
- poorly soluble drugs leave tissues faster = faster recovery\mothoxyglurane is highly soluble (BGPC =12) so ery slow recovery, Isoflurane is relatively poorly soluble (BGPC =1.4) so rapid recovery
what is Halothane
- mainly used in pediatirc mask inductions
- approx 25% is metabolized can result in some lvier damage, probesm with repeated exposure and post anesthetic depatic necrosis
- widely distributed and goes everywhere- including developing fetus
- can cause hypoxia and icnreased CO2 in cardiovasular system causing epinephrine release
- uuually no change in HR but cardiac output can decrease, having direct effect on heart and reducing blood pressue
- Halothane also increases the sensitivity of myocardium to epinephrine and can cause arrhythmias
Halothane and thermoregulation
- can depress thermoregulation, patients experience some gedree of hypothermia
- can also cause Malignant hyperthermia - susceptibilty is based on genetic mutation resulting in defective calcium release channels in skeletal muscle
- channels open upon exposure to any halogenated hydrocarbon anesthetic and cause uncontrolled muscle contraction, inc body temp and potential brain damage
is patient is suspected to be experiencing maligant hyperthermia what should eb done
* potential effect of Halothane- due to genetic muation
- give patient skeletal muscle relaxants like dantrolene for treatment
describe the toxicity assocaited with halothane
- assocaited with the drugs metabolites: trifluoroacetic acid, Br-, Cl- but little F-
these are hepatotoxic, adverse effects are worsened with hypoxia and repeated exposure
descirbe Isoflurane
- most common anesthetic
- alsmot no toxicity since very little is metabolized
- low solubiltiy in tissues so moves in and out of bdoy quickly
- causes less depression of CO than halothane but does ccase vasodilation which can result in dose dependent drop in blood pressure
- post anesthetic hepatic necrosis has been reported in 1/500,000 pateints
- less potent than halothane and has pungent odour (only real drawback)
Compare the BGPC, MAC%, metabolism of Halothane, isoflurane, sevoflurane and N2)
