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22 Cards in this Set
- Front
- Back
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Local anesthesia
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loss of sensetion in circumscribed area..
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What are some methods of inducing local anesthesia
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1. mechanical trauma
2. low temp 3. anoxia 4. chemical irritants 5. neurolytic agents such as alcohol and phenol |
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Properties for a good local anesthetic
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1. non irritant
2. no permanent alteration of nerve structure. 3. low systemic toxicity 4. time of onset should be as short as possible 5. long enough duration to complete procedure 6. free of allergic reaction 7. sterile, or capable of being sterilized |
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What are fcns of the axon membrane?
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1. block diffusion of water sol. molecules
2. selectively perm. 3. transduce info by protein receptors fcn: metabolic support of membrane FINISH |
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What is saltatory conduction?
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conduction between nodes in a stepwise fashion, but you need 8-10 mm of nerve that is covered with anesthetic to ensure blockage
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What can't you do an absolute refractory period?
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You cannot have the signal conduc in the opposite direction; this is what prevents it from going backwards.
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Membrane Expansion theory
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Distorting the membrane by the anesthetic (stimulus) allows the blockage of sodium channels, and therefore the blockage of signal conduction.
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Specific receptor theory
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certain areas in the sodium channels where the anesthetic can go through, but may not go through all of them. Each anesthetic has a different target receptor. These are classified based on biological site and mode of action
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What class are most clinically useful anesthetics part of?
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Class D - agents act by combo of receptor and receptor independent mechanisms
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Non depolarizing nerve block
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Decrease the permeability of the ion channels to sodium ions. Anesthetic has to make its way through the membrane, and fix itself into the channels.
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What's the first step in the mechanisms of local anesthetic action?
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Need to displace the Ca ions from the sodium channel receptor sites. This eventually blocks the Na receptor site with the anesthesia...
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What type of molecules are local anesthetics?
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amphipathic either as esters or amides.
- need a noncharged molecule to pass through the membrane - once its passed that, it can become charged in order to block the channel. - pKa of the anesthetic determines the dissociation (pKa = measure of molecule's affinity for hydrogen ions) - pKa = pH means half base form and half acid form |
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Why may you not get 100 uptake into the nerve?
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local anesthetics are vasodilators, so much of the anesthetic leaves the local site and goes to other areas.
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What if your pH in blood is 5 and your anesthetic is 7.4?
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In order for the anesthetic to pass through the membrane, it cannot be charged. in an acidic environment, much of it will be converted into cation so it wont be able to penetrate the membrane. May need to administer more anesthetic.
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It is possible for a local anesthetic to interfere with the excitation of the nerve in one or more of the following ways...
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1. altering resting potential
2. altering threshold 3. decrease depolarization rate 4. increase the depolarization rate many LAs work by decreasing the depolarization rate and this doesn't affect the other steps |
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Describe the classification of local anesthetics
1. Class A 2. Class B 3. Class C 4. Class D |
1. act on external surface of receptor site; biotoxins
2. act on internal receptor sites; quaternary ammonium 3. act by receptor independent mechanism; benzocaine 4. act by combination of receptor independent and dependent mechanisms; most LAs |
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Where is the only site that LA has access to on the nerve?
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Nodes of Ranvier where sodium channels are found in abundance.
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What is the rate of onset of anesthetic action related to?
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The pka of the anesthetic. A pka close to 7.4, the normal pKa of human tissues ensures the fastest onset. Lower pka possess faster time of onset than those of higher pKa.
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What physical aspect of the nerve determines how fast the local anesthetic penetrates it?
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The perineurium binds individual nerve fibers to each other in bundles called fasciculi. The thicker the perineurium, the slower the onset of LA.
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What property in the LA determines the potency ? What property determines the duration of anesthetic activity?
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Lipid solubility - potency
Degree of protein binding - duration |
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What is tachyphylaxis?
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Increased tolerance to a drug after its been administered several times.
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What are factors that influence the rate of drug removal from an action site?
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Protein binding, vascularity of the injection site, and absence of a vasoactive substance
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