Local anesthesia is our friend for a multitude of Emergency Department procedures. The main goal is to assure adequate coverage of the area of injury or pathology. Field block anesthesia describes the infiltration of a local anesthetic to an area surrounding a surgical and/or procedure site, resulting in anesthesia of the nerve supply of area of skin supplying the surgical field (2).This method of anesthesia is particularly effective for repair of large contaminated wounds, or incision and drainage of abscesses. The main advantage of the field block technique is that it provides anesthesia without disrupting the anatomy of the affected area (1,3).

The most commonly used anesthetic for a field block is lidocaine. One and two percent solutions are available in the ED. Onset is rapid (approximately 2-5 minutes) and lasts anywhere from 30 – 60 minutes (3). Epinephrine can be added to lidocaine as well. It causes vasoconstriction which can decrease the clearance of the lidocaine from the desired site (3). In areas near terminal arterial branches with compromised perfusion ( e.g ear lobe, penis, tip of the nose), the use of epinephrine should be avoided (3). Buffering of lidocaine with sodium bicarbonate (in a 9:1, lidocaine:8.4% sodium bicarbonate ratio) or warming the anesthetic to body temperature can decrease the amount of pain with injection (4). When the affected area involves infected tissue, this usually results in a more acidic environment which can hydrolyze the local anesthetic and make it less effective (4). Adding sodium bicarbonate can also increase the pH of the anesthetic solution and improve its analgesic effect. (4). Safe doses are generally 4mg/kg of lidocaine or 7mg/kg of lidocaine with epinephrine.

Performing a Field Block

Equipment

Gloves, mask and applicable personal protective equipment
Syringes (5 to 10 mL depending on the volume of anesthetic being used)
Lidocaine (or desired anesthetic)
25 or 27G 1 or 1.5 inch needle
Chlorhexidine (or betadine)

Technique

Imagine a square or diamond shaped margin surrounding the affected area/wound. This is the ideal shape in which to perform a field block. After cleaning the area with antiseptic solution, pass the needle at one point of the diamond or square into the subcutaneous layer, and inject the anesthetic while slowly withdrawing the needle, making sure not to exit the skin. After injecting a small volume, the needle can be turned to the opposite side to inject anesthetic into the other side of the diamond of square. This method can be repeated on the opposite side of the wound at a new injection site (5).

It’s important to aim deep enough to target nerve branches. You don’t want to make an excessively large wheal. Additionally, it is paramount that you give the lidocaine enough time to work. You can set a 10 minute timer – if not in the midst of sedating the patient – in order to assure that the field block is successful.

Potential Complications

Common complications include inadequate anesthesia provided, warmth on palpation (with use of epinephrine), and risk of infection. Depending on the location, there is a risk of damage of nearby nerves or injection in major vessels. Allergic reactions to the anesthetic are rare but can occur, ranging anywhere from contact dermatitis, localized angioedema to severe sysetmic reactions – such as seizure (4). Reports of systemic toxicity following local anesthetic injections generally occur when the maximum dose is exceeded. Systemic reactions include agitation, confusion, anxiety, tinnitus, seizures, loss of consciousness, and/or cardiovascular involvement; arrhythmias, bradycardia, cardiac arrest.