The easiest and most reliable way to anesthetize a wound is to inject a
local anesthetic. There are two techniques: (1) direct injection of the
local anesthetic agent into the area around the wound and (2) injection
of the anesthetic agent around a sensory nerve that supplies sensation
to the injured area. Both methods are addressed below, but first the two
most commonly used anesthetic agents are discussed. Neither needs to
be refrigerated, which is important in the rural setting.
Lidocaine (Lignocaine)
Lidocaine is the most commonly used and least expensive agent. The
usual total dose that can safely be given is 3–5 mg/kg body weight. Do
not give more than this amount at one time. The anesthesia becomes effective
after 5–10 minutes and lasts, on average, from 45 minutes to 1 hour.
Bupivacaine (Marcaine)
Bupivacaine is a longer-acting agent than lidocaine, but it is also more
expensive. The usual total dose that can safely be administered at one
time is 2.0–3.0 mg/kg. Bupivacaine takes a few minutes longer to
become effective than lidocaine (10–15 vs. 5–10 minutes), but its effect
can last 2–4 hours.
The longer duration of effect can be valuable. Some wounds take more
than 1 hour to clean and suture. In addition, bupivacaine gives residual
pain control after the procedure is completed. Hand injuries are especially
prone to pain, making bupivacaine a good choice for treating
hand and finger injuries.
If both lidocaine and bupivacaine are available, they can be mixed together
in equal parts and administered with one syringe. This combination
gives the advantages of the quicker onset of anesthesia from the
lidocaine with the longer duration of action of the bupivacaine.
Calculating the Amount to Administer
To calculate the mg dose, multiply the ml of solution that you plan to
give by the concentration of the solution (mg/ml). The following table
converts the commercially available anesthetic solutions to the mg/ml
concentration of the anesthetic agent.
Concentration of Agent in Commercially Available Anesthetic Solutions
Example: You expect to inject 30 ml of 1.0% lidocaine to anesthetize a
relatively large wound:
30 ml × 10 mg/ml = 300 mg of lidocaine
The patient is a 70-kg man. A 70-kg man can receive 3–5 mg/kg or
210–350 mg of lidocaine. Your 30-ml dose is on the high end of the
“safe” range.
Additives
It is sometimes useful to add additional drugs to the local anesthetic
solutions to optimize their effect.
Bicarbonate
Both lidocaine and bupivacaine are acidic and therefore painful when
injected. One way to lessen this pain is to add injectable sodium bicarbonate
to the local anesthetic solution. Your patient will be grateful for
this extra step. It is essential to use commercially prepared bicarbonate
for injections. Do not try home-grown formulations. Be careful: adding
too much bicarbonate to the anesthetic solution can lead to the formation
of crystals. The proper mixtures are as follows:
• Lidocaine: add 1 ml of bicarbonate to each 9 ml of lidocaine before
injection.
• Bupivacaine: add 1 ml of bicarbonate to each 19 ml of bupivacaine
before injection.
Epinephrine
Epinephrine is a vasoconstrictor that shrinks blood vessels and thus
reduces bleeding from the wound and surrounding skin edges. This
makes wound examination and repair easier to perform. Epinephrine
also decreases absorption of the anesthetic agent, which
may allow safe injection of more than the usually recommended
amount of anesthetic agent. Epinephrine requires 5–7 minutes to take
effect. The maximal dosages of lidocaine and bupivacaine with epinephrine
are as follows:
• Lidocaine with epinephrine increases to 7 mg/kg, and its effect lasts
11⁄2–2 hours.
• Bupivacaine with epinephrine: dosing essentially stays the same at
2.0–3.0 mg/kg, and its effect still lasts 2–4 hours.
Lidocaine and bupivacaine are available in solutions premixed with
epinephrine , but you can add it to the anesthetic solution yourself. Be
very careful, however, because the amount of epinephrine to add is
very small:
1. Add 0.25 ml of 1:1000 epinephrine to 50 ml of local agent (50 ml is
the usual-sized vial). This will give a 1:200,000 dilution—safe for
most procedures.
2. Err on the side of adding a little less rather than a little more if you
are drawing the epinephrine with a syringe larger than 1 ml.
Contraindications to Adding Epinephrine
In certain circumstances the vasoconstricting effects of epinephrine can
be detrimental and may lead to tissue loss. Examples include:
• Digital block (numbing the whole finger)
• On the tip of the nose
• On the penis
• Injury that results in a very ragged and irregular laceration. Epinephrine
worsens the already compromised circulation of the skin edges.
Indications for Adding Epinephrine
• Straight cut with healthy-looking skin edges
• On the face, oral mucosa, and scalp, which have excellent blood circulation
Overview of Anesthetic Agents: Dosage and Duration of Action
* You can add the same amount of bicarbonate to solutions with epinephrine as you add to plain solutions
without epinephrine (see above for proper amounts).
