5 - Local Anesthetics Flashcards
Local anesthetics
- Local anesthetics block nerve conduction when applied locally to nerve tissue in appropriate concentrations and act on any part of the nervous system and on every type of nerve fiber
Properties of a good anesthetic
- Not be irritating to tissues
- Not cause permanent damage to nerves
- Low systemic toxicity
- Short onset (can start procedures quicker)
- Long duration of activity (meaning they also work for longer procedures, or if the procedure is short, the patient has longer pain relief after surgery)
Chemical properties of amides
**KNOW THIS **
o All drug names have two “I’s” in the chemical name
o Metabolized by the liver (so avoid in your liver disease patients)
o Most commonly used local anesthetic by podiatry
Chemical properties of esters
KNOW THIS
o Metabolized in the blood by pseudocholinesterase
o Safe in your liver disease patients
List of amides
KNOW THIS
- Lidocaine (Xylocaine)
- Bupivacaine (Marcaine, Sensorcaine)
- Prilocaine (Citanest)
- Mepivacaine (Carbocaine)
- Etidocaine (Duranest)
List of esters
KNOW THIS
- Procaine (Novocain)
- Chloroprocaine (Nesacaine)
- Tetracaine (Pontocaine)
- Cocaine
Basic mechanism of action of anesthetics
- Just know the basics behind how local anesthetics work
- Inhibits excitation (generation) and conduction of the nerve impulse by increasing the threshold for depolarization
Differential sensitivity of nerve fibers
Small nerve fibers and unmyelinated nerves are more susceptible to local nerve block
Order of function that is lost following local nerve block
o ***Pain –> temperature –> touch –> proprioception –> deep pressure –> movement
o Voluntary movement is the last function to go, which is what we want – you could give a high enough dose to knock out muscle movement
“Epicritic sensation in tact” can be used in documenting when you checked all nerve sensations and they are all normal
Effects of pH
Local anesthetics slightly soluble
Addition of water-soluble salts (HCL)
o Increases absorption
o More stability
pKa is approximately 8
Increasing pH will decrease effectiveness of anesthetic
o Add bicarb to make the pH higher and the injection more comfortable (less burning)
o Could compromise the effect of the anesthetic
Prolongation of action by vasoconstriction
- You can use vasoconstrictors in your local nerve blocks in order to induce blood vessels constriction locally and not allow the drug to be taken up into the systemic circulation
Epinephrine
o Increases duration of action
o Decreases systemic toxicity
o Provides hemostasis
Usually supplied in concentration of 1:100,000, but is safer to use if you dilute to 1:200,000
o 5 cc of saline with 5 cc of 1:100,000 1:200,000
o Some physicians do NOT dilute and they don’t have problems (outside rotation sites)
o There are other ways to control bleeding (tourniquet, etc.) that you would not need to take the risk of using the higher concentration and still be able to control bleeding
Calculating dose of anesthetics
COMMON CONCENTRATIONS
- 0.25% of anesthetic contains 2.5mg/ml or cc (i.e. Marcaine, mepivacaine)
- 0.50% contains 5mg/ml or cc
- 1% contains 10mg/ml or cc
- 2% contains 20mg/ml or cc
- 10cc of 1% Lidocaine contains 100mg of Lidocaine
o How many mg of Lidocaine are in 15cc of 2% Lidocaine?
o How many mg of Marcaine are in 15cc of 0.50% Marcaine?
o How many mg of Marcaine are in 10cc of 0.25% Marcaine?
o How many mg of each anesthetic are contained in 10cc of a 50/50 mixture of 2% Lidocaine and 0.50% Marcaine?
NEED TO DO THIS ON EXAM
o How many mg of Lidocaine are in 15cc of 2% Lidocaine?
Answer: 300mg of Lidocaine
o How many mg of Marcaine are in 15cc of 0.50% Marcaine?
Answer: 75mg of Marcaine
o How many mg of Marcaine are in 10cc of 0.25% Marcaine?
Answer: 25mg of Marcaine
o How many mg of each anesthetic are contained in 10cc of a 50/50 mixture of 2% Lidocaine and 0.50% Marcaine?
Answer: 100mg of Lidocaine and 25mg of Marcaine
Toxic dose of lidocaine (4.5 mg/kg)
- Toxic dose of plain lidocaine is 300 mg, but if you have lidocaine with epinephrine, the toxic dose is 500 mg
- This is because it decreases the absorption time of the anesthetic into the systemic circulation
Toxic dose of marcaine (2.5 mg/kg)
Plain = 175 mg
With epinephrine = 225 mg
Toxic dose notes
- You may not want to adjust the amount or the concentration based on if you are doing unilateral or bilateral procedures so you don’t come close to the toxic dose
- This is based on text book calculations when the dose is directly injected into the vasculature (venous), so it might actually be much less when we are injecting into soft tissue (due to the need to absorb into systemic circulation from tissues), so the toxic dose might be much higher in soft tissue injections
How many cc of 2% Lidocaine plain can be given before the toxic dose is reached?
o Answer: 15cc of 2% Lidocaine plain
How many cc of 0.50% Marcaine plain can be given before the toxic dose is reached?
o 35cc of 0.50% Marcaine plain
Lidocaine (Xylocaine)
- Onset: 3-5 minutes
- DOA: 3-4 hours
- Complications: Ventricular fibrillation, cardiac arrest (especially if injection is given intravascularly, not local or soft tissue injection)
Bupivicaine (Marcaine, Sensorcaine)
- Onset: 5-10 minutes
- DOA: 6-8 hours
Note: Often mixed with lidocaine to increase DOA and prolonged anesthesia
o You can get the “best of both worlds” when mixing anesthetics
o Example: give a lidocaine/bupivacaine injection at the onset of surgery to get quick onset (lidocaine) so you can start the procedure immediately, and the patient benefits from 6-8 hours of pain block for the post-op period
Mixing anesthetics: STUDY (level I)
- Done to get the best of both worlds – quicker onset and longer duration
- Double-blind, randomized study revealed that a mixture of bupivacaine/lidocaine does have a quicker onset, but decreases duration.
- Dr. Smith = This has not been my experience – I get quick onset and longer duration
o There is “bad science” and there is conflicting evidence
o This is just his experience and just one study – consider this
Hypersensitivity
- Usually related to esters and related compounds
- Amide reactions related to preservatives
o Rash or asthmatic attacks are the most common allergic reactions
Less painful injections
- Warm anesthetic
- Smaller needle?
o 25, 27 or 30 gauge needle (25 is larger, 30 is smaller)
o The larger needle is actually less painful
o This is because the amount of pressure you need to exert in order to push the same amount of fluid through a smaller needle, the velocity of the fluid increases and is more painful – 25 gauge needle is actually still very small, can’t really feel the difference - Syringe
o Right syringe for the job
o If you’re going to inject 3 cc, use a 3 cc syringe - Topical refrigerants
o Ethyl Chloride “freezes” the skin - Sodium Bicarbonate
o Less burning sensation because it buffers the anesthetic
o Use a 5:1 ratio (2.5 cc anesthetic, 0.5 cc bicarbonate)
Injection technique
- Antiseptic
o Betadine or alcohol and betadine - Needle inserted and a wheal raise
o “Mosquito bite” – from a little bit of anesthetic injected right under the skin - Aspiration to prevent intravascular injection
o Pull back on the needle a little bit, look in the hub of the needle to see if there is blood in there – if it is full of blood, you are probably in the vasculature, so you should reinsert the needle and try again
o Some aspirate every injection they give, some only aspirate when injecting around a large artery or vein - Needle advanced forward and anesthetic deposited
- Redirection of the needle (if needed)
