There are multiple topical anesthetics that we use in the ED. The common theme is that all of these drugs with -caine cause sodium channel inhibition in nerves, which blocks axonal transmission leading to the typical numbness and and localized weakness. I admit that it can be a bit confusing as to which one you should use and when. The aim of this post is therefore to help provide some clarity.
This gel contains 4% Lidocaine, 0.1% epinephrine and 0.5% tetracaine. It is used for topical laceration repair and according to some studies can provide adequate local anesthesia for scalp and facial lacerations in up to 90% of patients. Apply 1-3mL to the open wound. It should then remain there for 20-30 minutes to allow for absorption. Cover the wound with an occlusive bandage and tell the patient not to remove it. LET is great for lacs of the face of the scalp. It is generally contraindicated in areas with end arterial supply digits, penis, nose, and ears because pi is a vasoconstrictor. In large wounds the amount needed to cover can be toxic. You can also see excess absorption if it is applied to mucous membranes – so don’t do that. However, in most uses it is very safe. A trial of 203 children by Harman et al, in 2013 involving kids 3 mos to 17 years saw no toxicity when 3mL of LET was applied to a wound prior to tissue adhesive. The amount of lido in 3 mL of LET is approximately 135 mg – but the amount absorbed is generally way less than this and is limited by the size of the lac. Epi will lead to less absorption locally due to the vasoconstriction.
You may be familiar with 5mg/kg at the ceiling for lidocaine dosing. The lack of toxicity with LET means that this number does not need to be slavishly followed. Finally, since lidocaine can lead to methemoglobinemia be cautious in using it in infants under age 1 month.
EMLA stands for eutectic mixture of local anesthetics and is comprised of 2.5% lidocaine and 2.5% prilocaine in a cream. This cream consists of microscopic droplets that penetrate intact skin up to a depth of 2-3mm via diffusion down the concentration gradient. It can reduce the pain of venous/arterial puncture, lumbar puncture, access of ports and in wound repair/abscess I&D (prior to subcutaneous injection of lidocaine).
The usual dose is 1-2 grams applied per 10 square cm of skin. Cover with an occlusive dressing for 45-60 minutes.
Per Gajraj et al, the maximum area for usage is: The maximum application areas recommended for children are
- Less than 10 kg – 100 sq cm
- 10 to 20 kg – 600 sq cm
- Greater than 20 kg – 2000 sq cm
It does require approximately one hour to achieve peak effect and can last for 60-120 minutes after removal. It does not work as well in diseased/denuded skin (like eczema) with a shorter duration of action. You should not use it in patients with risk factors for methemoglobinemia (G6PD deficiency). In infants younger than 3 months be careful with how much you apply.
LMX is liposomal lidocaine and it used to be known as ELA-Max. LMX 4 is 4% lidocaine, LMX 5 is 5% lidocaine. It is designed to be applied to intact skin. It has a more rapid onset because the liposomes penetrate better and prevent metabolism. It compares foavorably with EMLA in IV stick pain. It does not technically require an occlusive dressing (but it is probably a good idea to prevent accidental removal). It takes 30 minutes to work as opposed to 60 minutes for EMLA.
The risk of significant systemic absorption is very low. Nevertheless it should only be applied to intact skin.
The dose is 1 to 2 g of LMX per 10 sq cm of skin with similar max doses to EMLA.