Local anesthesia in wound care
Image 1 Quite often we need to inject local anesthetics around wounds to debride them properly. Other areas of use are numbing a wound to take biopsies or numbing a toe to treat an ingrown toenail. While you may be quite comfortable using local anesthetics we hope this chapter includes a few pearls that are new to you. image: shutterstock
Local anesthetics can be found in creams, ointments, sprays and gels. Be aware that most producers of these products specify that the products should only be used on intact skin. In other words- these products are not made to be used in open wounds. However, worldwide, millions of patients receive topical anesthetics into their wounds every year - for lack of an alternative. Every caregiver we know in wound care uses these products off-label directly in wounds. Most producers acknowledge off-the record that this practice exists and that it seems to be safe.
Be aware of the following aspects to use topical anesthetics in a safe manner:
Topical anesthetics have dose limits beyond which they are toxic. Although the absorption through a wound surface is not so high we have to accomodate for some absorption. We therefore use the same dose limits as we would for subcutaneous injections.
Potent local anesthetics like priolocaine are particularly toxic for infants. In small children it is very important to double check the maximum allowed doses.
The larger the wound surface - the more chance of absorption of the product. When using topical anesthetics on extensive wounds like burns, for example, you have to absolutely respect the maximum allowed dosages.
Some topical anesthetic gels and sprays contain alcohol and can burn intensely upon application. Read the label and do not use products containing alcohol in these wounds.
You can drip regular local anesthetic which is meant for injection, onto the wound and use it topically. If you can inject it under and around the wound it must be safe to drip it into the wound, right? And this is therefore the least controversial method of using topical anesthetics.
Image X Topical analgesics come i many different formulations. Most are based on lidocaine. 1. Lidocaine cream often comes in strengths of 4% or 5%. As it is lipophilic it penetrates slightly better than lidocaine gel. 2. Lidocaine gel often comes in strengths of 2%. The type of lidocaine gel used to numb the urthra prior to catherization is also suitable. 3. Note that some formulations contain high concentrations of lidocaine and you have to use them with caution when treating extensive wound areas such as burns. Nr 3. in the diagram i J-Caine which is marketed as local analgesics prior to tatoos. 4. Lidocain spray often contains additives which burn! We do not recommend using these unless you have no other alternatives. 5. Emla cream stands out from the crowd as it is the most effective of the topical analgesics. Be aware though that it is toxic for infants < 3months!
None of the topical anesthetics numb the tissue deeply. At best they might anesthetize tissue a millimeter beneath the wound bed. The prilocaine/lidocaine combination is probably the most potent topical product as it penetrates slightly deeper than other formulations. If the wound surface is covered by necrotic tissue or a thick layer of fibrine then the anesthetic will not reach nerve endings very well. Debride what you can first without causing the patient pain. When the patient starts to feel som discomfort, stop and apply the anesthetic. You have to be patient with topical anesthetics- they work slowly. Apply them to the wound- cover with a nonabsorptive dressing and let the anesthetic dwell in the wound for at least 15-20 minutes.
Topical anesthetics do not work well if you have to do extensive debridements - in these cases it is more effective to inject the anesthetic in or around the ulcer, or as a local or regional block. The latter techniques will be discussed in the end of the chapter.
Using anesthetics for subcutaneous injections in wound care
1% Lidocaine is quite cheap and usually available even i remote clinics in Africa. In most cases you only need 0,5%. You can dilute the 1% Lidocaine with an equal part of sterile saline to make 0,5% which will save you money and is really equally effective. It may take a couple of minutes longer for the 0,5% solution to work and the effect may dissipate earlier but by that time you are already done with the debridement. We rarely use bupivacaine or other more potent anesthetics in wound care.
We rarely use local anesthetics with epinefrine/adrenaline. The exception may be patients using strong blood thinning medication where we would like to take a biopsy. There is no documentation that the temporary bloodvessel constriction caused by epinefrine/adrenaline is detrimental to wound healing, but we usually see no need to use it. When we went to medical school we were taught that the use of epinephrine/adrenaline was contraindicated in fingers/toes, nose and ears and around the penis, as it could cause irreversible necrosis of tissues in these anatomical areas. It turned out that this was somewhat of a medical myth and today we are no so concerned using epipenphrine/adrenaline in these areas.
However, we are very careful when we do a toe block in patients with critical ischemic disease. Firstly we would never use local anesthetic with epinephrine/adrenaline in these patients. Secondly we would use a needle as thin as possible ( preferably 27G) to avoid damaging the few remaining blood vessels that are in the toe. Thirdly, use as little of the anesthetic solution as possible. Avoid pressing large volumes into these delicate areas. The pressure in the tissues from the injected volume can severely reduce blood circulation for several hours!
When we use local anesthetic to numb the tissues prior to a biopsy of the wound we usually only need very minute amounts of the anesthetic. Half a millileter is usally more than enough for each biopsy site.
How do we numb an entire wound for deeper debridement? Here we can use a number of injection techniques. Firstly, since we only need the numbing effect to last 20-30 minutes, using 0,5% Lidocaine is usually sufficient. The most effective method of numbing the wound would be to inject at several sites around the wound through the intact skin. Often this will also be the most painful way to do the injections. The gentlest method is to carefully inject lidocaine throught the necrotic wound bed ( the dead tissue is without sensation) to just below the necrotic layer. Obviously you do not know for sure where the necrotic tissue /healthy tissue interface is but often you can feel it in the resistance of the needle as you gently push it deeper. In most cases we use this technique. Always be patient when using local anesthetics. Make it a rule to wait at least 10 minutes for the product to work properly.
Table 1 Some of the properties of different types of local anesthetics. Note that maximum doses are dependant on whether epinephrine/adrenaline is added. For most purposes in wound care lidocaine is sufficient and we can often dilute it to 0,5% with saline.
Image X Hypodermic needles come in different thicknesses and lengths. Understandably, the thinner the needle the less painful the injection is. We often use 27G needles for local anesthesia. When using such thin needles the challenge is to press the local anesthetic through the needle. Using a small syringe makes this a lot easier. See image x.
Image X The laws of physics tell us that it will be easier to press the anesthetic through the thin 27G needle if we use a smaller syringe with lower diameter piston. When using a 27 G needle it is best to use a 2,5ml syringe or even a 1ml syringe. This means that you may have to pull up the anesthetic several times but it will make it so much easier to inject.
Local and regional blocks
A local anesthetic block will numb a smaller area of tissue. Local blocks are often used to numb an entire finger or toe. Regional blocks are very useful to numbing larger anatomical regions. A regional hand block for example can numb the entire hand.
As mentioned above, local blocks are often used around the toes or fingers. These are also called ring blocks. Usually only about 2-3 milliliters of lidocaine are necessary on each side of the toe/finger. Wait 10 minutes to make sure that the anesthetic has enough effect.
Important! The correct method is to inject the lidocaine from the dorsal side of the finger or toe! We have een many examples on the internet where caregivers inject from the inside of the foot/hand. The number of nerve endings on the insides of the feet/ hands is much higher here. Subsequently - piercing the skin with a needle from the plantar/palmar side is so much more painful! Do not do this!
Regional blocks are so useful for African clinics! While som regional blocks like a plexus block of the upper extremity require quite some training, blocks in other anatomical regions can be easier to learn.
Numbing an entire hand is farily easy with a regional hand block. To achieve this you need to inject lidocaine at the sites of the radial nerve, median nerve and ulnar nerve.
Since most chronic ulcers are located below the knee, learning to perform a popliteal block is very useful.