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Hello, today we will be talking about local anesthesia techniques Part I of a two part series.
Male Speaker: Production of this presented lecture was made possible by an educational grant from Gill Podiatry Supply and Equipment Company, helping you put your best foot forward.
Dr. Khurram Khan: The objective of today’s lecture is to talk about the different types of local, we will also go over the Pharmacokinetics of local anesthesia, we will talk about the toe block and do an anatomical review as well as hallux block with an anatomical review.
First a little bit of history, local anesthesia follow general anesthesia by about 40 years. Cocaine being one of the first ones in 1860, it had multiple uses and the first synthetic was procaine in 1905 followed by lidocaine the most commonly used local anesthetic in 1943.
Local anesthesia is considered any technique that renders part of the body insensitive to pain without affecting consciousness. It’s a technique that can be used for relief of non-surgical pain and to enable diagnosis of the cause of some chronic pain conditions.
Peripheral nerve block is injecting local anesthetic near the course of a named nerve. It can be used for surgical procedures and the distribution of the blocked nerve. The advantages are relatively small dose of local anesthetic can be used to cover a large area. It can also have a rapid onset. The disadvantages, the placement of a tourniquet maybe limited given the area of the block. And also the block may not work properly.
Local anesthetics are medications which produce a reversible loss of sensation in the localized part of the body when applied directly on to our nerve tissue or mucus membrane. It limits the propagation of the action potential. Some of the desirable characteristics of local anesthetics include rapid onset of action, a reversible block of nerve conduction, low degree of systemic toxicity, it should be soluble in water and stable in solution. And it should be effective in all parts of the nervous system including all types of nerve fibers and muscle fibers.
The mechanism of action of local anesthetics includes it binds the sodium channels, it slows and/or prevents axonal conduction and their weak basis and that access to receptor site is dependent on the pKa, lipid solubility and the molecular size and level of neuronal activity.
Local anesthetics have a Lipophilic and hydrophilic end making them ionizable. A low pH means that it is more in ionized state and unable to cross the membrane. Adding sodium bicarbonate puts more local anesthetic in a non-ionized state. So the question is does buffering anesthetics reduce pain? Based on an article in JAPMA entitled, “Buffered Lidocaine decreases the pain of digital anesthesia in the foot” a randomized double blind study demonstrated that 24 out of 30 participants indicated on a visual analog scale that buffered lidocaine is less painful than plain lidocaine. The pain decrease by about 50% for more than half of the participants.
The pharmacokinetics of local anesthetics with regards to absorption are based on multiple factors. The first one being the site of injection and its vascularity, the second one being the total dosage of medication given, the third one being the specific drug characteristics specifically tendency to produce vasodilatation. In the presence of a vasoconstrictor can also affect the peak plasma concentration. The effects of vasoconstrictors decrease the rate of systemic absorption and decrease system intoxicity. They can increase the local drug concentration and increase the neuron uptake of the local anesthetic. And they can also increase the local duration of action and in lidocaine’s example, may increase it up to two fold with epinephrine.
When looking at the vasoconstrictor effects and local anesthetics we have to remember that it’s the ratio. Epinephrine is added to local anesthetics in extremely dilute concentrations, best expressed as a ratio of grams of drug in terms of total CCs of solution. So one to one thousand preparation of epinephrine would be one gram of epinephrine and 1000 CCs of solution which is equal to 1000 milligrams of epinephrine and 1000 CCs of solution which is equal to 1 milligram of epi and one CC of solution, therefore a one to 200,000 solution of epinephrine would be 1 gram of epi and 200000 CCs of solution which is equal to 1000 milligrams of epi and 200000 CCs of solution or 5 micrograms of epi in 1 CC solution of local anesthetic.
The formulation of local anesthetic is administered as very dilute solution which can be expressed as part of an active drug, part 100 parts of solution and that can be in grams percent. So for our example a 2% solution is equal to 2 grams of medication over a 100 CCs of solutions which is equal to 2000 milligrams over a 100 CCs which is equal to 20 milligrams over 1 CC of solution. So with regards to percent and CCs a 1% solution is equal to 10 milligrams of medication per CC. A 2% is equal to 20 milligrams of medication per CC. 0.5 is equal to 5 milligrams per CC and 0.25 is equal to 2.5 milligrams per CC.
There are two classes of local anesthetics amides and esters. One trick to remembering which medication belongs in which anesthetic is the rule of two IIs. Eters each medication only has one I cocaine, chloroprocaine, procaine etc. The amides have two IIs, Bupivacaine, Lidocaine, Ropivacaine etc.
One of the most famous esters is cocaine a schedule II substance. It’s medical use is limited to surface or topical anesthetics, this is typically for the eye or for the nose. We must remember to avoid epinephrine because cocaine already has vasoconstrictor properties and it’s the only one.
Benzocaine is available in many over-the-counter preps for relief in pain and irritation. It’s a serviced anesthetic and it’s only meant to be used topically in ointments and in sprays. Procaine or as it is also known novocaine is topically ineffective. It’s used for infiltration because of low potency and short duration. It produces a significant vasodilatation therefore epinephrine must be used to prolong it’s effect. Tetracaine is a topical anesthetic and it’s also used for spinal anesthesia, but it has a slow onset and more prolonged effect than procaine and also one of the longest duration of the esters.
Moving on to the amides, Lidocaine is the most widely used local anesthetic. It’s effective by all routes and has a faster onset and more intense long lasting affect than procaine. Bupivacaine has a similar onset and duration as Lidocaine but it’s more toxic to neonates, so it is not to be used in surgical anesthetics. Bupivacaine or as it is also known as marcaine has no topical effect, has a slower onset and it’s one of the longer lasting agents. It’s sensory and motor disassociation provides sensory analgesia with minimum motor block. Ropivacaine is antimer of Bupivacaine and is clinically equivalent. Similar to sensory versus motor selectivity as Bupivacaine though it’s significantly less cardiovascular toxicity. Prilocaine has a similar clinical profile to that of Lidocaine. It does cause significantly less vasodilatation than Lidocaine, so therefore less vasoconstrictor needs to be added, but it’s most popular clinical application is for a topical anesthetic as in combo with lidocaine in a eutetcic mixture.
When you combine local analgesics you create what’s called an EMLA cream. EMLA stands for Eutectic Mixture of Local Analgesics. So the mixture of lidocaine and prilocaine one of the most common one becomes an oily mixture. This combination is indicated for derma anesthesia, specifically it is applied to prevent pain associated with intravenous catheter insertion, blood sampling, superficial surgical procedures and topical anesthesia for leg ulcers in terms of cleaning and or debridement.
The next two slides talk about dosages in terms of duration of action and maximum dosage one can infiltrate for a procedure. The most common question asked of students and residents is with regards to lidocaine and marcaine with regards to their duration of action and maximum dosage. So Lidocaine duration of action is considerate to medium which is equal to about 30 to 60 minutes. Without epinephrine the maximum dosage is 4.5 milligrams per kilogram and not to exceed 300 milligrams. With epinephrine you can almost double the duration of action to about 120 to 360 minutes. And with epinephrine the maximum dosage is now 7 milligrams per kilogram.
Moving down to marcaine, it’s duration of action is long, a 120 to 240 minutes without epinephrine. The maximum dosage is 2.5 milligrams per kilogram not to exceed a total dose of a 175 milligrams. With epinephrine it lasts as long as a 180 to 420 minutes. And the total dose is not to exceed 225 milligrams. The reasons there is a maximum dosage of local anesthetics is because of the chance for a toxicity. The most important one being the cardiovascular, myocardial depression and vasodilatation can lead to hypertension and circulatory collapse. Allergic reactions are rare, less than 1% and are usually associated with the preservatives of metabolites of the esters. Those allergic reactions can include going from a simple rash to bronchospasms. The primary way for these medications to become toxic is via intravascular injection or excessive dose. Therefore to prevent toxicity one should ask very often with a slow injection. Always ask about CN toxicity, have monitoring available when giving the injection. Have and prepare resuscitative equipment including CN depressing drugs as well as cardiovascular drugs. And always follow your ABCs.
Before you begin always explain the procedure, the benefits, the risks and complications to the patient and/or patient’s representative and inform the patient of the possibility of paraesthesia join the procedure. Obtain informed consent in accordance with your local hospital protocols. Perform and document neurovascular and muscular skeletal examinations prior to the procedure. Always palpate from N marks. Prepare the site with anesthetic solution and while maintaining sterile technique place a wheel of local anesthetic using a 25 gauge needle or smaller. One tip for injections is to realize that all superficial nerves of the foot should be thought of as a neuronal network rather than a single string of nerves with a well defined and consistent anatomic position.
Some tips for making the injection less painful include, using a vapocoolant spray on the site of injection. You can also distract the patient by talking to them or pinching the skin. A smaller gauge needle 27or 30 gauge may help make the injection less painful. And a smaller CC syringe as well. EMLA cream can be applied that needs to be applied under occlusion for more than a hour. And it may not penetrate as deep as the injection needs to go. Be careful with vapocoolant sprays because the excessive use may cause permanent discoloration in sensitive patients. So you must use with caution and do not over freeze for fear of giving the patient a frostbite injury.
One question is EMAL cream effective in hallux blocks? While the study looked at the efficacy of EMAL cream application prior to digital ring block for surgery for ingrown big toe nails. A prospective, double blinded, placebo controlled randomized clinical trial with 81 one patients was performed. And they found no clinical benefit in using EMAL during digital nerve block. The reason given was the depth of the block was not sufficient to prevent pain. Well what about in children? Well there is study done in pediatrics was a randomized control clinical trial of EMAL cream comparing that to vapocoolant spray. And the conclusion was when combined with distraction vapocoolant spray significantly reduces immediate injection pain compared with distraction alone and it’s equally effective and less expensive and faster acting than EMAL cream. The big question is, whether to mix or not to mix your local anesthetics, the thought process being by combining different local anesthetics you can get a faster onset with longer duration. But are there any studies on this? Well Ribotsky in JAPMA 1996 did a study that suggest no clinical advantage with respect to onset and duration of local blockage in using a 50-50 mixture of plain lidocaine and plain bupivacaine in place of the independent use. In 1997 another study was performed and no difference was found in the time until of anesthetic, however the duration of anesthetic affect was longer with both lidocaine and bupivacaine than lidocaine alone.
The different types of block available include the digit block, the hallux block, the inter-metatarsal block, a mayo block a reverse mayo block and the peripheral ankle block. Today’s lecture we will only be talking about the digit and the hallux block, Part II we will be a discussion of the inter-metatarsal block, the mayo, the reverse mayo and the peripheral ankle block.
So let’s begin with a digital block. A digital block is used to anesthetize the individual digits. It’s uses include removal of an ingrown toenail especially of the lesser toes. You can also use it for a biopsy of a toe to perform a close reduction of a fracture and a debridement of digital ulcer. It’s disadvantages include it’s usually contraindicated in patients with severe PVD and epinephrine is contraindicated in digital blocks because of the vasoconstrictor properties.
So which nerves are we blocking in a digital block? Well dorsally the medial, dorsal cutaneous nerve, the internal dorsal cutaneous branch divide into 3 dorsal digital branches one of which supplies medial side of the great toe, the other, the adjacent side of the second and third toes. The intermediate dorsal cutaneous nerve divides into 4 dorsal digital branches, which supply to medial and lateral sides of the 3rd and 4th and of the 4rd and 5th toes. The lateral dorsal cutaneous nerve from the serial nerve serves into the dorsal digital nerve and supplies a lateral side of the 5th toe. Centrally and in medial side the proper digital branches from the common digital branch of the medial plantar nerve supplies to the 2nd, 3rd and 4th toes. On the plantar lateral side a proper plantar digital branches from the common and digital branch of the lateral planter nerves supplies to the lateral aspect of both the 4th and 5th toes. Each proper digital nerve off cutaneous and articular filaments, and opposite the last phalanx since upward a dorsal branch which supplies the structures around the nail. The continuation of this nerve being distributed to the ball of the toe.
There are two techniques that have been described for the digital block. The first one being the V block which is a one poke technique. You start dorsally at the central aspect of the digit and proceed plantarly going dorsal to plantar and oblique angle. This way you cover both, the dorsal and the plantar aspects of the toe. The H block is a two poke technique, you start on adjacent size of the toe and proceed directly from dorsal to plantar. Again covering both the dorsal and the plantar nerves.
Next we will be talking about the hallux block. The hallux block is used to anesthetize the hallux on a distill to the first MPJ. It’s uses include unequal cryptosis of the big toe paranychia of the big toe, skin biopsy and close reduction of a fracture as well as debridement of any ulcers. The disadvantages are loss of propriocetion if allowed to ambulate after the procedure and maybe a risk when driving. The nerves that are being blocked in a hallux block include dorsally, the medial dorsal cutaneous nerve divides into 2 dorsal digital branches one of which supplies the medial side of the great toe dorsally. Plantarly the proper digital nerve of the great toe from the medial plantar nerve supplies the skin on the medial plantar side of the great toe. The first inner space is a medial, terminal branch of the deep perennial nerve which divides into 2 dorsal digital nerve which supplies the adjacent sides of the great and second toe.
The goal of the hallux block is to deposit an aesthetic in all four corners, therefore you should think of the hallux as a square. To begin start out the dorsal medial aspect of the hallux, just distill to the metatarsophalangeal joint and create a wheel. Proceed plantarly to the plantar medial aspect of the hallux, injecting as you go. Slowly withdraw the needle. Next start out to the same dorsal medial aspect of the hallux that you just injected, distilled to the metatarsophalangeal joint. Procedural on the dorsal aspect laterally for the dorsal lateral aspect of the hallux, slowly withdraw the needle. You may be able to achieve this without having to remove the needle from the first injection. Next begin the dorsal lateral aspect of the hallux just distill so that metatarsophalangeal joint. Proceed plantarly and slightly obliquely to the plantar lateral aspect of the hallux. Slowly withdraw the needle. For some this may be an extra step, begin at the plantar medial aspect of the hallux just distill to the metatarsophalangeal joint. Proceed along the plantar aspect laterally to the plantar lateral aspect of the hallux and slowly withdraw the needle.
So in summary, we have talked about the different types of locals, we talked about the pharmacokinetics of locals, we talked about the toe block and did an anatomical review and we talked about the hallux block with an anatomical review. For the rest of the blocks please listen to Part II.
Male Speaker: Production of this presented lecture was made possible by an educational grant from Gill Podiatry Supply and Equipment Company, helping you put your best foot forward.