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59 Cards in this Set
- Front
- Back
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what are the two types of nerve fibers and when do local anesthetics act on each?
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they act on the mantle (outer fibers) first then the core (inner fibers) later
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describe the mechanism of action of local anesthetics.
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bind to the transmembrane domain of Na channels near the intracellular side resulting in a slow rate of depol, reduce height and rate of rise of AP, slow axonal conductance that prevents the propagation of AP's, does not alter resting membrane potential, does increase the threshold potential.
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what are the effects of Ca and K on local anesthetics?
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Ca attenuates their action while K enhances it
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what are two toxins that bind Na channels near the extracellular portion and cause it to be blocked?
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tetrodotoxin from the puffer and saxitoxin from shell fish
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what are 3 hypothesized mechanisms of action for local anesthetics as opposed to the traditional thought and what drug is thought to act via one of these mechanisms?
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the membrane expansion theory (expands region of membrane to squeeze the channel shut - thought to be benzocaine's mechanism of action bc it has a positive charge and is not lipophillic), surface charge theory (lipophilic part of drug binds to membrane and changes surface charge), antagonist at nicotinic Ach receptor
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during infection, the extracellular pH may be lowered thus affecting the efficacy of local anesthetics, how can you correct for this?
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add bicarbonate to bring the pH back up
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what factors affect local anesthetic action and how?
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pH (can change efficacy), lipophilicity (affects potency), protein binding (more protein binding means longer duration of action), diffusability and vasodilator action (can make these not so local, often epi is given as well to constrict the vessels and keep everything local)
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what fibers are blocked by local anesthetics and what sensations are affected (in temporal order)?
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usually small fibers are effected more than large. Motors are not affected. Pain fibers (a delta and C's are affected always). Pain affected first then cold then warm then touch then deep pressure
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what are systemic side effects of lacal anesthetics?
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decreased myocardial excitability and change conduction rate and force of contraction, arteriolar dilation (except cocaine which is a vasodilator), depress inhibitory neurons stimulating the CNS resulting in convulsions and respiratory depression at high, lethal doses, NMJ affected due to large number of Na channels, smooth muscle relaxation
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describe the pharmacokinetics and metabolism of the local anesthetics.
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absorbed well in topical and IV administrations and this depends on vascularity of region and vasoactivity of the drug (epi is often used to vasoconstrict to confine the drug to the area of injection and decrease systemic toxicity, widely distributed if it can be (crosses BBB and placental barrier), ester linkages are metabolized mainly by plasma pseudocholinesterase to p aminobenzoic acid (PABA) except cocain which is metabolized in the liver. Amide linked drugs are metabolized in the liver by N dealkylation and hydrolysis. Excretion is via the kidneys
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describe cocaine and its use as a local anesthetic.
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topical usually, especially muscosal surfaces, ester linked but metabolized in liver, vasoconstrictor, small dose decreases HR, large dose increases HR, CNS stimulant with euphoria, blocks reuptake of NE, DA, and sero, affects sympathetic NS.
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describe three ester linked local anesthetics besides cocaine.
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Note that all are used in spinal anesthesia. Procaine has slow onset of action and not effective topically, chloroprocaine is the safest of all LA's, more potent than procaine, used for continuous epidural anesthesia during surgery. Tetracaine more potent than procaine and still slow, toxicity risk, one of few LA's useful for the eye
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describe lidocaine.
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most widely used LA. Amide linked. Used with epi. More potent than procaine with toxicity, very effective topically.
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describe mepivacaine.
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simillar to lidocaine, somewhat longer duration of action, toxic to neonate, not used in obstetrics
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describe bupivacaine.
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amide linked LA. Long duration of action, can cause cardiotoxicity
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describe articaine.
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recently approved in US, rapid onset and short duration of action, used in dentistry
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describe benzocaine.
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surface anesthesua iof skin and mucus membranes, safe, works by membrane expansion theory, minor burn use, can produce methemoglobinemia.
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describe dibucaine.
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long acting surface anesthesia. Can be injected but is toxic and not allowed in the US
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describe dyclonine.
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topically used prior to endoscopy
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describe proparacaine.
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useful in the eye, not an ester so no cross reactivity with ester LA's
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what drugs are useful for infiltration anesthesia and what is the chief disadvantage?
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this is injection directly into the tissue. Lidocaine, procaine, and bupivacaine. Disadvantage is that a lot must be injected to anesthetize relatively small areas
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what is field block anesthesia and what drugs may be used for this?
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sc injection that anesthetizes region distal to the injection site and thus can use less amount of drug than with infiltration. Lidocaine, procaine, and bupivacaine.
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describe nerve block anesthesia and what drugs are used with it?
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injection perineurally proximal to operative site, do not hit nerve! Causes better anesthesia than field block and infiltration anesthesia. Usually blocks peripheral nerves and plexuses, resulting in somatic relaxation and skeletal muscle relaxation. lidocaine, mepivacine, and bupivacaine (bup provides longest lasting anesthesia).
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describe IV or bier block anasthesia and what drugs are used with it?
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iv injection into vein of traumatic site, must have proximal tourniquet so it does not go systemic. Lidocaine and prilocaine (used by some bc greater therapeutic index).
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describe epidural anesthesia and the drugs used for that.
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injected into epidural space to get the spinal nerve roots. Chloroprocaine for short duration, lidocaine for intermediate duration, and bupivacaine for long duration. Bupivicaine with fentanyl is used for obstetrical analgesia/anesthesia
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describe spinal anesthesia and the drugs used.
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injection into subarachnoid space. Lasts a while in CSF, can get postural headache and hypotension. Lidocaine used for short duration, bupivacaine for intermediate to long duration, tetracaine for long duration, sometimes procaine
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what are good topical anesthetics?
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tetracaine, lidocaine, cocaine, dibucaine, pramoxine, lidocaine +prilocaine AKA EMLA, dyclonine (throat lozenges), proparacaine and tetracaine for eye
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what are the effects of general anesthesia with increasing dose and which levels are in the surgical plane of anesthesia?
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loss of motor function, alteration of consciousness, analgesia, loss of temp regulation, unconsciousness, the following three are in the surgical plane : effects on eye motion, pupil size, and light reflex; loss of protective reflexes; loss of muscle tone. respiratory failure, circulatory failure, then death.
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define MAC
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minimum alveolar anesthetic concentration. Alveolar concentration needed to prevent a gross skeletal response to a standard painful stimulus like a skin incision in 50% of patients at atmospheric pressure. Very simillar to ED 50. accepted estimate of potency is 1/MAC.
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describe lambda
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blood solubility over gas solubility, high lambda means gas goes to the blood (leaks) thus steady state is reached slowly, but the agent is more potent. Low lambda means steady state is reached quickly.
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list agents in terms of decreasing MAC and thus increasing lambda.
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NO, desflurane, sevoflurane, enflurane, isoflurane, halothan, methoxyflurane
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in the rise of alveolar concentration of GA over concentration inspired increases with time and then reaches a steady state. How do you determine the height of the "knee" when the slope changes, as it is approaching steady state?
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1/ (cardiac output X lambda)
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how is steady state possible if cardiac output and leak into the blood are constant?
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leak is not constant bc eventually venous blood will have some GA in it so the leak into the blood will be less.
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what is the third phase of GA and how do you figure out its slope on the curve?
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uptake by the bpdy and recirculation. This is the flatter part of the curve… (lambda X cardiac output X [Pinspired-P aeriolar])/(tissue solubilities Xmass)
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describe the slopes of substances with high lambda in the third phase of GA.
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it will have the most positive slope and will thus change the most over administration
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when is decreased cardiac output and increased respiratory rate important in GA?
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when dealing with high lambda drugs bc they have a higher initial rate.
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with obese patients, what phase of GA is important?
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tail phase (deals with mass), the cardiac output goes mainly to vital organs which are not part of the fat, thus induction is about the same for everyone
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describe halothane.
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GA that is well tolerated with low blood:gas coefficient. Problem is only produces sleep and no analgesia. Hepatic toxicity is also possible
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describe enflurane.
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less nausea than halothan, skeletal muscle relaxation and analgesic properties, rapid and smoot adjustments in depth. Major disadvantage is seizures
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describe isoflurane.
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smooth and rapid induction, cardiac output maintained, potentiates muscle relaxants, more pungent than halothane
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describe sevoflurane.
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minimal odor and rapid. Low lambda close to NO. Very popular now, some toxicity and does not dilate coronary arteries.
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describe desflurane
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commonly used, outpatients, rapid induction and elimination, more irritating to airway, and pungent
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describe methoxyflurane.
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high lambda and very potent, non irritating, does not relax uterus thus is good for obstetrics, takes forever to get to steady state, renal failure, extensive metabolism, cannot be used for induction
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describe NO
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lowers MAC of the second gas as well as side effects, used as adjuvant, powerful analgesic, must be in hyperbaric situation for complete anesthesia to occur.
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as anesthetic concentration is increased what happens to BP and cardiac output?
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both lowered
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what two drugs maintain cardiac output in GA?
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desflurane and isoflurane
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what GA usually increases HR?
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halothane
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what are the respiratory effects of all GA's?
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decrease minute vol and increase CO2
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what are the exceptions to the trends of CNS effects by the GA's?
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NO increases metabolism and enflurane can cause seizures.
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what are the primary and secondary goals of preanesthetic medication?
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primary is to reduce anxiety, get amnesia, and analgesia. Secondary (which may require multiple drugs) is to decrease the dose of inhalational agents, decrease gastric acidity and volume, and decrease intraoperative stress
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what are the rapid acting preanesthetic meds? Slow onset?
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barbituates, etomidate, propofol. Ketamine, benzos, opioids, neuroleptics
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describe pbarbituates in terms of preanesthesia and what is the prototypical one?
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rarely cause nausea or vomiting, may cause disorientation rather than sedation if pt is in pain. Pentobarbital is the classic/prototypical barbituate
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what is the effect of barbituates on respiration, CNS function, and HR?
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decrease all but HR increases
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what are the preanesthetic effects of benzos? What is the drug of choice for this purpose?
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non-muscle relaxant, nonanalgesic, rare nausea and vomiting, amnesia. Midazolam
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describe etomidate in terms of preanesthetic meds.
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rapid induction and emergence, least coronary side effects, used in trauma, and CHF, can get nausea and vomiting, myoclonus, adrenal suppression
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describe propofol as a preanesthetic drug.
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rapid induction, main advantage is quick recovery. Not as safe as etomidate
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describe ketamine as a preanesthitic drug
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dissociative anesthetic (causes delirium in adults NOT in children), slow onset, good in asthma pts bc bronchodilator
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these neuroeptics potentiate the actions of opioids.
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butyrophenones (droperidol)
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what is the purpose of opioids in general anesthesia?
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decreases the amount of GA needed
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