Vascular and Interventional Radiology
Local Anesthetics, Dr. Shah
History:
First local anesthetic was cocaine, which was isolated from cocoa leaves by Albert Niemann in Germany in the 1860’s. First reported use was in 1884 by Sigmund Freud, who used it to wean a patient from a morphine addiction. His colleague, Karl Kollar first noticed its anesthetic effect and introduced it as a topical ocular anesthetic in 1884. The first synthetic local anesthetic was procaine (Novocain). Lidocaine (Xylocaine) was invented in the 1940s.
Actions:
Local anesthetics work by reversibly blocking nerve conduction. The anesthetic penetrates the nerve membrane and reduces the influx of sodium ions into the nerve cytoplasm. If the sodium ions cannot flow into the neuron, then potassium ions cannot flow out, thereby inhibiting depolarization of the nerve.
Esters: Amides:
Procaine Lidocaine
Cocaine Bupivacaine [Marcaine, Sensorcaine]
Tetracaine [Pontocaine] Mepivacaine [Carbocaine, Polocaine]
Benzocaine Prilocaine [Citanest]
Chloroprocaine
Allergic Reactions:
True allergic reactions to local anesthetics are rare and usually involve an ester agent. Allergic reactions are very seldom caused by amide agents. There is no cross-reactivity between the amide and ester agents. Therefore, if a patient is allergic to an agent from one class, an agent from the other class can be substituted.
Benadryl as a local anesthetic:
If a patient is allergic to both esters and amides, diphenhydramine (Benadryl) can be used as follows – Dilute a 1-mL dose of diphenhydramine 50 mg/mL with 9 mL sterile saline to make a 0.5% solution for local infiltration. Note, however, that compared to a local anesthetic such as lidocaine, this mixture causes more pain on infiltration, is less efficacious, and carries an increased risk of tissue necrosis.
EMLA cream: Eutectic Mixture of Local Anesthetics
Topical local anesthetic composed of mixture of lidocaine (2.5%) and prilocaine (2.5%). A eutectic mixture is defined as a mixture of two substances (in this case crystalline bases of lidocaine and prilocaine) that has a melting point lower than that of either substance by itself. Onset of action – When applied to the face or damaged skin, onset is quick (15 minutes). For most other sites, an anesthetic depth of 3 mm is achieved after 60 minutes and increases 1 mm per 30 minutes up to 5 mm at 120 minutes. Dosage – 1-2 g EMLA per 10 cm2 of skin is applied and covered with occlusive adhesive dressing (e.g. Tegaderm) and applied for at least 30-60 minutes.
Methods to decrease the pain of injection:
Ø Use small needles (25 gauge or smaller)
Ø Buffer the slightly acidic lidocaine with sodium bicarbonate in a 10:1 ratio (10 mL of 1% lidocaine to 1 mEq/mL of sodium bicarbonate)
Ø Inject slowly (slow injection over 10 seconds is less painful than injection of the same volume over 2 seconds)
Ø Apply skin pressure or pinching near the site of injection while injecting the anesthetic
Ø Warm the anesthetic to body temperature (the cooler the lidocaine, the more painful the injection)
From Dr. Namyslowski:
• Esters
- cocaine, procaine, tetracaine, chlorprocaine
• Amides
- lidocaine, mepivacaine, prilocaine,
bupivacaine, etidocaine, ropivacaine
• Rare “allergic reactions” reported; most commonly represent misinterpretation of symptoms (vasovagal rxn, effect of epinephrine; intravasc. inj.)
- likely due to PABA-like preservatives used in
ester compounds
- anesthetic itself unlikely to be the primary antigen
- consider 1cc hypodermic test injection
From Dr. Shah; Pharmacy does carry a preservative free lidocaine.