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Local Anesthetics
PHAR 341
| Question | Answer |
|---|---|
| What are the components of the pain pathway? | Nociceptors, peripheral nerves, Brainstem or Spinal cord, thalamus and cortex. |
| What is the order in which local anesthetics causes function to be lost? | 1. Pain and Temp 2. Touch 3. Proprioception 4. Motor Control |
| What function does A-alpha fibers have? | Motor |
| What function does A-beta fibers have? | Fine touch |
| What function does A-gamma fibers have? | Muscle tone |
| What function does A-delta fibers have? | Pain, Temperature, touch |
| Are A fibers myelinated? | yes |
| What function does B fibers have? | Pre-ganglionic Sympathetic |
| What function does C-sympathetic fibers have? | Post-ganglionic Sympathetic |
| Are B fibers myelinated? | Yes |
| What function does C-sensory fibers have? | Pain, Temperature and touch |
| Are C fibers myelinated? | No |
| How are APs propagated? | Due to passive process which allows current to flow down the axon and active process involving the opening of Na+ and K+ channels |
| What is the MOA of LAs? | Block conduction in nerve fibers by blocking the Na+ channels and thus prevents depolarization. |
| What category is Tetradotoxin? | Natural Toxin - LA |
| What category is Saxitoxin? | Natural Toxin - LA |
| How do Natural Toxins work? | They bind to a specific receptor on the outside of the Na+ Channels and prevents Na+ influx. |
| What category is Procaine? | Ester LA |
| What category is Tetracaine? | Ester LA |
| What category is Benzocaine? | Ester LA |
| What category is Lidocaine? | Amide LA |
| What category is Mepivacaine? | Amide LA |
| What category is Ropivacaine? | Amide LA |
| What category is Bupivacaine? | Amide LA |
| What is the typical structure of a Amide/Ester LA? | Lipophilic group (usually an aromatic ring) linked by an amide or ester group to a hydrophilic group. |
| Why are quaternary amines not ideal as a hydrophilic group for LA? | Quaternary amines are not IONIZABLE. This is a problem because LA need to be basic to cross the axon membrane. |
| Why do ester LA usually have a shorter duration of action than amide LA? | Because ester links are more prone to hydrolysis. |
| Which form of the LA is more soluble? | Protonated form |
| What is the active form of a LA? | Protonated form b/c it cannot readily exit from closed channels |
| Where are LA less effective? | In acidic/infected tissues b/c there is a smaller % of LA in the non-ionized form that is then available for diffusion across the membrane. |
| How are ester LA broken down? | By plasma esterases (pseudocholinesterase). These cause rapid breakdown of the ester in the blood. |
| How are amide LA broken down? | In the liver by N-dealkylation and Hydrolysis (by CYP 450) |
| Where are ester and amide LA metabolites excreted? | In urine |
| True or False: Higher concentrations (mM) of LA are used to achieve a block | True: increased driving force to cross the axon membrane |
| True or False: Hydrophobicity will decrease potency (base is less soluble) | False: this increases the ability of the drug to cross an axon membrane (increased lipophilicity) |
| True or False: Rate of metabolism is unimportant to the duration of a LA block | True: redistribution determines the block length (toxicity is unaffected), not metabolism. |
| True or False: Lowering of the pH can increase block efficacy and duration (increased blood flow and increased protonated form) | False: will decrease the amount of uncharged drug so less is available to cross the axon membrane |
| True or False: co-injected vasoconstrictors (e.g. epinephrine) can lengthen block duration | True: delays the redistribution into the systemic circulation; useful for drugs that leave very quickly (e.g. lidocaine) |
| What is the faster LA? Small drug with high lipophilicity or a large drug with low lipophilicity? | Small drug with high lipophilicity |
| If lipophilicity increase, what is the patient at risk of experiencing? | increased drug potency and cardio-toxicity |
| How is Procaine normally administered? | Dental infiltration |
| How is Tetracaine normally administered? | Dental infiltration |
| How is Benzocaine normally administered? | Topically (skin, mucous membranes) |
| Which is the only LA that is in its basic form at pH 7.4? | Benzocaine |
| How is Cocaine normally administered? | Topical - Eyes |
| How is Lidocaine normally administered? | All: Topical, infiltration, nerve block, epidural/caudal, spinal/subarachnoid |
| How is Mepivacaine normally administered? | Infiltration and Nerve block |
| How is Bupivacaine normally administered? | All: Topical, infiltration, nerve block, epidural/caudal, spinal/subarachnoid |
| How is Ropivacaine normally administered? | Nerve block, epidural/caudal |
| Where are topical/surface anesthesia used? | Commonly on mucous membranes (mouth, pharynx, trachea) and on cut/burned skin |
| How is infiltration anesthesia administered? | Given SC or intradermally into the tissues that are to be treated. A ring or filed block is another variation. |
| How is a nerve block created? What is it mainly used for? | LA injected around but not in the nerve innervating the surgical field. Mainly used for dental procedures and limb surgeries |
| Where is the subarachnoid/Intrathecal injection administered? | Into the CSF |
| Where is the Epidural/caudal injection administered? | Into the dorsal roots |
| Which spinal anesthesia is more segmented? | Epidural/caudal |
| What are CNS affects of LA? | Initial nervousness, dizziness, blurred vision & tremors (often missed b/c they are very transient) later drowsiness, convulsion, unconsciousness & respiratory arrest Headaches are particularly common in pts treated with spinal anesthesia (hydrated |
| What are the cardiotoxicity effects of LA? | Hypotension, cardiovascular collapse, bradycardia, cardiac arrest Direct effect of LA on cardiac and arteriolar Na+ channels Indirect effect due to sympathetic blockade (spinal anesthesia) and CNS |
| What is the order in which a spinal anesthesia blocks? | SNS nerves to PS nerves to Pain/temp to touch to deep pressure to motor control |
| What are the local effects of LA? | Hypersensitivity: Dermatitis to breathing difficulties more common in esters Prolonged anesthesia: may last up to several weeks after injection |
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