Lidocaine

Lidocaine (INN) (IPA: ) or lignocaine (former British Approved Name)(International Phonetic Alphabet]]: ) is a common local anesthetic and antiarrhythmic drug. Lidocaine is used topically to relieve itching, burning and pain from skin inflammations, injected as a dental anesthetic, and in minor surgery. The most commonly encountered lidocaine preparations are marketed by Abraxis Pharmaceutical Products under the brand names Xylocaine and Xylocard, though lidocaine is also found in many other proprietary preparations.

History
Lidocaine, the first amino amide-type local anesthetic, was developed first by Nils Löfgren and Bengt Lundqvist in 1943 and first marketed in 1948.

Pharmacokinetics
Lidocaine has a more rapid onset of action and longer duration of action than amino ester-type local anesthetics such as procaine. It is approximately 90% metabolized in the liver by CYP1A2 (and to a minor extent CYP3A4) to the pharmacologically-active metabolites monoethylglycinexylidide and glycinexylidide.

The elimination half-life of lidocaine is approximately 1.5–2 hours in most patients. This may be prolonged in patients with hepatic impairment (average 343 minutes) or congestive heart failure (average 136 minutes). (Thomson et al., 1973)

Anesthesia
Lidocaine alters depolarization in neurons, by blocking the fast voltage gated sodium (Na+) channels in the cell membrane. With sufficient blockade, the membrane of the presynaptic neuron will not depolarize and so fail to transmit an action potential, leading to its anesthetic effects. Careful titration allows for a high degree of selectivity in the blockage of sensory neurons, whereas higher concentrations will also affect other modalities of neuron signalling.

Indications
Indications for the use of lidocaine include:
 * Topical, infiltration, nerve block, ophthalmic, epidural and intrathecal anaesthesia, IV regional anaesthesia (IVRA)
 * Treatment of serious ventricular arrhythmias (IV preparations), including VF (Ventricular Fibrillation), and VT (Ventricular Tachycardia) associated with cardiac arrest
 * Neuropathic pain, including postherpetic neuralgia

Contraindications
Contraindications for the use of lidocaine include:
 * Heart block, second or third degree (without pacemaker)
 * Severe sinoatrial block (without pacemaker)
 * Serious adverse drug reaction to lignocaine or amide local anaesthetics
 * Concurrent treatment with quinidine, flecainide, disopyramide, procainamide (Class I antiarrhythmic agents)
 * Prior use of Amiodarone Hydrochloride
 * Hypotension not due to Arrhythmia
 * Bradycardia
 * Accelerated Idioventricular rhythm

Adverse drug reactions
Adverse drug reactions (ADRs) are rare when lidocaine is used as a local anesthetic and is administered correctly. Most ADRs associated with lidocaine for anesthesia relate to administration technique (resulting in systemic exposure) or pharmacological effects of anesthesia, however allergic reactions can rarely occur.

Systemic exposure to excessive quantities of lidocaine mainly result in central nervous system (CNS) and cardiovascular effects – CNS effects usually occur at lower blood plasma concentrations and additional cardiovascular effects present at higher concentrations, though cardiovascular collapse may also occur with low concentrations. CNS effects may include CNS excitation (nervousness, tingling around the mouth, tinnitus, tremor, dizziness, blurred vision, seizures) followed by depression (drowsiness, loss of consciousness, respiratory depression and apnea). Cardiovascular effects include hypotension, bradycardia, arrhythmias, and/or cardiac arrest – some of which may be due to hypoxemia secondary to respiratory depression. (Rossi, 2006)

ADRs associated with the use of intravenous lidocaine are similar to toxic effects from systemic exposure above. These are dose-related and more frequent at high infusion rates (≥3 mg/minute). Common ADRs include: headache, dizziness, drowsiness, confusion, visual disturbances, tinnitus, tremor, and/or paraesthesia. Infrequent ADRs assocaited with the use of lidocaine include: hypotension, bradycardia, arrhythmias, cardiac arrest, muscle twitching, seizures, coma, and/or respiratory depression. (Rossi, 2006)

Dosage forms
Lidocaine, usually in the form of lidocaine hydrochloride, is available in various forms including:
 * Injected local anesthetic (sometimes combined with epinephrine)
 * Dermal patch (sometimes combined with prilocaine)
 * Intravenous injection (sometimes combined with epinephrine)
 * Intravenous infusion
 * Nasal instillation/spray (combined with phenylephrine)
 * Oral gel (often referred to as "viscous lidocaine" or abbreviated "lidocaine visc" or "lidocaine hcl visc" in pharmacology; used as teething gel)
 * Oral liquid
 * Topical gel (as with Aloe Vera gels that include Lidocaine)
 * Topical liquid
 * Topical patch (Lidocaine 5% patch is marketed as "Lidoderm" in the US (since 1999) and "Versatis" in the UK (since 2007 by Grünenthal))