Mechanism of Action

Lidocaine HCl stabilizes the neuronal membrane by inhibiting the ionic fluxes required for the initiation and conduction of impulses, thereby effecting local anesthetic action.

Hemodynamics

Excessive blood levels may cause changes in cardiac output, total peripheral resistance, and mean arterial pressure.  With central neural blockade these changes may be attributable to block of autonomic fibers, a direct depressant effect of the local anesthetic agent on various components of the cardiovascular system.  The net effect is normally a modest hypotension when the recommended dosages are not exceeded.

Pharmacokinetics and Metabolism

Information derived from diverse formulations, concentrations and usages reveals that lidocaine HCl is completely absorbed following parenteral administration, its rate of absorption depending, for example, upon various factors such as the site of administration and the presence or absence of a vasoconstrictor agent.  Except for intravascular administration, the highest blood levels are obtained following intercostal nerve block and the lowest after subcutaneous administration.

The plasma binding of lidocaine HCl is dependent on drug concentration, and the fraction bound decreases with increasing concentration.  At concentrations of 1 to 4 mcg of free base/mL, 60 to 80% of lidocaine HCl is protein bound.  Binding is also dependent on the plasma concentration of the alpha-1-acid glycoprotein.

Lidocaine HCl crosses the blood-brain and placental barriers, presumably by passive diffusion.

Lidocaine HCl is metabolized rapidly by the liver, and metabolites and unchanged drug are excreted by the kidneys.  Biotransformation includes oxidative N-dealkylation, ring hydroxylation, cleavage of the amide linkage, and conjugation.  N-dealkylation, a major pathway of biotransformation, yields the metabolites monoethylglycinexylidide and glycinexylidide.  The pharmacological/toxicological actions of these metabolites are similar to, but less potent than, those of lidocaine HCl.  Approximately 90% of lidocaine HCl administered is excreted in the form of various metabolites, and less than 10% is excreted unchanged.  The primary metabolite in urine is a conjugate of 4-hydroxy-2,6-dimethylaniline.

The elimination half-life of lidocaine HCl following an intravenous bolus injection is typically 1.5 to 2 hours.  Because of the rapid rate at which lidocaine HCl is metabolized, any condition that affects liver function may alter lidocaine kinetics.  The half-life may be prolonged two-fold or more in patients with liver dysfunction.  Renal dysfunction does not affect lidocaine HCl kinetics but may increase the accumulation of metabolites.

Factors such as acidosis and the use of CNS stimulants and depressants affect the CNS levels of lidocaine HCl required to produce overt systemic effects.  Objective adverse manifestations become increasingly apparent with increasing venous plasma levels above 6 mcg free base/mL.  In the rhesus monkey arterial blood levels of 18 to 21 mcg/mL have been shown to be threshold for convulsive activity.

General

The safety and effectiveness of lidocaine HCl depend on proper dosage, correct technique, adequate precautions, and readiness for emergencies.  Standard textbooks should be consulted for specific techniques and precautions for various regional anesthetic procedures.

Resuscitative equipment, oxygen, and other resuscitative drugs should be available for immediate use (see WARNINGS and ADVERSE REACTIONS).  The lowest dosage that results in effective anesthesia should be used to avoid high plasma levels and serious adverse effects.  Syringe aspirations should also be performed before and during each supplemental injection when using indwelling catheter techniques.  An intravascular injection is still possible even if aspirations for blood are negative.  Repeated doses of lidocaine HCl may cause significant increases in blood levels with each repeated dose, because of slow accumulation of the drug or its metabolites.  Tolerance to elevated blood levels varies with the status of the patient.  Debilitated, elderly patients, acutely ill patients, and children should be given reduced doses commensurate with their age and physical condition.  Lidocaine HCl should also be used with caution in patients with severe shock or heart block.

Careful and constant monitoring of cardiovascular and respiratory (adequacy of ventilation) vital signs and the patient’s state of consciousness should be accomplished after each local anesthetic injection.  It should be kept in mind at such times that restlessness, anxiety, tinnitus, dizziness, blurred vision, tremors, depression or drowsiness may be early warning signs of central nervous system toxicity.

Since amide-type local anesthetics are metabolized by the liver, lidocaine HCl should be used with caution in patients with hepatic disease.  Patients with severe hepatic disease, because of their inability to metabolize local anesthetic normally, are at greater risk of developing toxic plasma concentrations.  Lidocaine HCl should also be used with caution in patients with impaired cardiovascular function since they may be less able to compensate for functional changes associated with the prolongation of A-V conduction produced by these drugs.

Many drugs used during the conduct of anesthesia are considered potential triggering agents for familial malignant hyperthermia.  Since it is not known whether amide-type local anesthetics may trigger this reaction and since the need for supplemental general anesthesia cannot be predicted in advance, it is suggested that a standard protocol for the management of malignant hyperthermia should be available.  Early unexplained signs of tachycardia, tachypnea, labile blood pressure and metabolic acidosis may precede temperature elevation.  Successful outcome is dependent on early diagnosis, prompt discontinuance of the suspect triggering agent(s) and institution of treatment, including oxygen therapy, indicated supportive measures and dantrolene (consult dantrolene sodium intravenous package insert before using).

Lidocaine HCl should be used with caution in persons with known drug sensitivities.  Patients allergic to para-aminobenzoic acid derivatives (procaine, tetracaine, benzocaine, etc.) have not shown cross-sensitivity to lidocaine HCl.

Use in the Head and Neck Area

Small doses of local anesthetics injected into the head and neck area, including retrobulbar, dental and stellate ganglion blocks, may produce adverse reactions similar to systemic toxicity seen with unintentional intravascular injections of larger doses.  Confusion, convulsions, respiratory depression and/or respiratory arrest, and cardiovascular stimulation or depression have been reported.  These reactions may be due to intra-arterial injection of the local anesthetic with retrograde flow to the cerebral circulation.  Patients receiving these blocks should have their circulation and respiration monitored and be constantly observed.  Resuscitative equipment and personnel for treating adverse reactions should be immediately available.  Dosage recommendations should not be exceeded (see DOSAGE AND ADMINISTRATION).

Clinically Significant Drug Interactions

Concurrent administration of vasopressor drugs (for the treatment of hypotension related to obstetric blocks) and ergot-type oxytocic drugs may cause severe, persistent hypertension or cerebrovascular accidents.

Drug/Laboratory Test Interactions

The intramuscular injection of lidocaine HCl may result in an increase in creatine phosphokinase levels.  Thus, the use of this enzyme determination, without isoenzyme separation, as a diagnostic test for the presence of acute myocardial infarction may be compromised by the intramuscular injection of lidocaine HCl.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Studies of lidocaine HCl in animals to evaluate the carcinogenic and mutagenic potential or the effect on fertility have not been conducted.

Pregnancy

Teratogenic Effects: Pregnancy Category B

Reproduction studies have been performed in rats at doses up to 6.6 times the human dose and have revealed no evidence of harm to the fetus caused by lidocaine HCl.  There are, however, no adequate and well-controlled studies in pregnant women.  Animal reproduction studies are not always predictive of human response.  General consideration should be given to this fact before administering lidocaine HCl to women of childbearing potential, especially during early pregnancy when maximum organogenesis takes place.

Labor and Delivery

Local anesthetics rapidly cross the placenta and when used for epidural, paracervical, pudenal, or caudal block anesthesia, can cause varying degrees of maternal, fetal and neonatal toxicity (see CLINICAL PHARMACOLOGY-Pharmacokinetics and Metabolism).  The potential for toxicity depends upon the procedure performed, the type and amount of drug used, and the technique of drug administration.  Adverse reactions in the parturient, fetus and neonate involve alterations of the central nervous system, peripheral vascular tone and cardiac function.

Local anesthetics produce vasodilation by blocking sympathetic nerves. Elevating the patient’s legs and positioning her on her left side will help prevent decreases in blood pressure.  The fetal heart rate also should be monitored continuously, and electronic fetal monitoring is highly advisable.

Paracervical or pudendal anesthesia may alter the forces of parturition through changes in uterine contractility or maternal expulsive efforts.  In one study paracervical block anesthesia was associated with a decrease in the mean duration of first stage labor and facilitation of cervical dilation.  The use of obstetrical anesthesia may increase the need for forceps assistance.

The use of some local anesthetic drug products during labor and delivery may be followed by diminished muscle strength and tone for the first day or two of life.  The long-term significance of these observations is unknown.  Fetal bradycardia may occur in 20 to 30% of patients receiving paracervical nerve block anesthesia with the amide-type local anesthetics and may be associated with fetal acidosis.  Fetal heart rate should always be monitored during paracervical anesthesia.  The physician should weigh the possible advantages against risks when considering a paracervical block in prematurity, toxemia of pregnancy, and fetal distress.  Careful adherence to recommended dosage is of the utmost importance in obstetrical paracervical block.  Failure to achieve adequate analgesia with recommended doses should arouse suspicion of intravascular or fetal intracranial injection.  Cases compatible with unintended fetal intracranial injection of local anesthetic solution have been reported following intended paracervical or pudendal block or both.  Babies so affected present with unexplained neonatal depression at birth, which correlates with high local anesthetic serum levels, and often manifest seizures within six hours.  Prompt use of supportive measures combined with forced urinary excretion of the local anesthetic has been used successfully to manage this complication.

Case reports of maternal convulsions and cardiovascular collapse following use of some local anesthetics for paracervical block in early pregnancy (as anesthesia for elective abortion) suggest that systemic absorption under these circumstances may be rapid.  The recommended maximum dose of each drug should not be exceeded.  Injection should be made slowly and with frequent aspiration.  Allow a 5-minute interval between sides.

Nursing Mothers

It is not known whether this drug is excreted in human milk.  Because many drugs are excreted in human milk, caution should be exercised when lidocaine HCl is administered to a nursing woman.

Pediatric Use

Dosages in children should be reduced, commensurate with age, body weight and physical condition (see DOSAGE AND ADMINISTRATION).

Systemic

Adverse experiences following the administration of lidocaine HCl are similar in nature to those observed with other amide local anesthetic agents.  These adverse experiences are, in general, dose-related and may result from high plasma levels caused by excessive dosage, rapid absorption or inadvertent intravascular injection, or may result from a hypersensitivity, idiosyncrasy or diminished tolerance on the part of the patient.  Serious adverse experiences are generally systemic in nature.

The following types are those most commonly reported:

Central Nervous System

CNS manifestations are excitatory and/or depressant and may be characterized by lightheadedness, nervousness, apprehension, euphoria, confusion, dizziness, drowsiness, tinnitus, blurred or double vision, vomiting, sensations of heat, cold or numbness, twitching, tremors, convulsions, unconsciousness, respiratory depression and arrest.  The excitatory manifestations may be very brief or may not occur at all, in which case the first manifestation of toxicity may be drowsiness merging into unconsciousness and respiratory arrest.

Drowsiness following the administration of lidocaine HCl is usually an early sign of a high blood level of the drug and may occur as a consequence of rapid absorption.

Cardiovascular System

Cardiovascular manifestations are usually depressant and are characterized by bradycardia, hypotension, and cardiovascular collapse, which may lead to cardiac arrest.

Allergic

Allergic reactions are characterized by cutaneous lesions, urticaria, edema or anaphylactoid reactions.  Allergic reactions may occur as a result of sensitivity either to local anesthetic agents or to the methylparaben used as a preservative in the multiple dose vials.  Allergic reactions as a result of sensitivity to lidocaine HCl are extremely rare and, if they occur, should be managed by conventional means. The detection of sensitivity by skin testing is of doubtful value.

Neurologic

The incidences of adverse reactions associated with the use of local anesthetics may be related to the total dose of local anesthetic administered and are also dependent upon the particular drug used, the route of administration and the physical status of the patient.  In a prospective review of 10,440 patients who received lidocaine HCl for spinal anesthesia, the incidences of adverse reactions were reported to be about 3% each for positional headaches, hypotension and backache; 2% for shivering; and less than 1% each for peripheral nerve symptoms, nausea, respiratory inadequacy and double vision.  Many of these observations may be related to local anesthetic techniques, with or without a contribution from the local anesthetic.

There have been reported cases of permanent injury to extraocular muscles requiring surgical repair following retrobulbar administration.

Management of Local Anesthetic Emergencies

The first consideration is prevention, best accomplished by careful and constant monitoring of cardiovascular and respiratory vital signs and the patient’s state of consciousness after each local anesthetic injection.  At the first sign of change, oxygen should be administered.

The first step in the management of convulsions, as well as underventilation or apnea due to unintended subarachnoid injection of drug solution, consists of immediate attention to the maintenance of a patent airway and assisted or controlled ventilation with oxygen and a delivery system capable of permitting immediate positive airway pressure by mask.  Immediately after the institution of these ventilatory measures, the adequacy of the circulation should be evaluated, keeping in mind that drugs used to treat convulsions sometimes depress the circulation when administered intravenously.  Should convulsions persist despite adequate respiratory support, and if the status of the circulation permits, small increments of an ultra-short acting barbiturate (such as thiopental or thiamylal) or a benzodiazepine (such as diazepam) may be administered intravenously.  The clinician should be familiar, prior to the use of local anesthetics, with these anticonvulsant drugs.  Supportive treatment of circulatory depression may require administration of intravenous fluids and, when appropriate, a vasopressor as directed by the clinical situation (e.g., ephedrine).

If not treated immediately, both convulsions and cardiovascular depression can result in hypoxia, acidosis, bradycardia, arrhythmias and cardiac arrest.  Underventilation or apnea due to unintentional subarachnoid injection of local anesthetic solution may produce these same signs and also lead to cardiac arrest if ventilatory support is not instituted.  If cardiac arrest should occur, standard cardiopulmonary resuscitative measures should be instituted.

Endotracheal intubation, employing drugs and techniques familiar to the clinician, may be indicated, after initial administration of oxygen by mask, if difficulty is encountered in the maintenance of a patent airway or if prolonged ventilatory support (assisted or controlled) is indicated.  Dialysis is of negligible value in the treatment of acute overdosage with lidocaine HCl.

The oral LD 50 of lidocaine HCl in non-fasted female rats is 459 (346 to 773) mg/kg (as the salt) and 214 (159 to 324) mg/kg (as the salt) in fasted female rats.

Adults

For normal healthy adults, the individual maximum recommended dose of lidocaine hydrochloride without epinephrine should not exceed 4.5 mg/kg (2 mg/lb) of body weight and in general it is recommended that the maximum total dose does not exceed 300 mg.

The maximum recommended dose per 90 minute period of lidocaine hydrochloride for paracervical block in obstetrical and non-obstetrical patients is 200 mg total.  One half of the total dose is usually administered to each side.  Inject slowly, five minutes between sides (see also discussion of paracervical block in PRECAUTIONS).

TABLE 1  RECOMMENDED DOSAGES OF LIDOCAINE HYDROCHLORIDE INJECTION

(WITHOUT EPINEPHRINE) FOR VARIOUS ANESTHETIC PROCEDURES IN NORMAL HEALTHY ADULTS

THE ABOVE SUGGESTED CONCENTRATIONS AND VOLUMES SERVE ONLY AS A GUIDE. OTHER VOLUMES AND CONCENTRATIONS MAY BE USED PROVIDED THE TOTAL MAXIMUM RECOMMENDED DOSE IS NOT EXCEEDED.

Children

It is difficult to recommend a maximum dose of any drug for children, since this varies as a function of age and weight.  For children over three years of age who have a normal lean body mass and normal body development, the maximum dose is determined by the child’s age and weight.  For example, in a child of five years weighing 50 lbs., the dose of lidocaine hydrochloride should not exceed 75 to 100 mg (1.5 to 2 mg/lb).

In order to guard against systemic toxicity, the lowest effective concentration and lowest effective dose should be used at all times.  In some cases it will be necessary to dilute available concentrations with 0.9% Sodium Chloride Injection in order to obtain the required final concentration.

Sterilization, Storage and Technical Procedures

Disinfecting agents containing heavy metals, which cause release of respective ions (mercury, zinc, copper, etc.) should not be used for skin or mucous membrane disinfection as they have been related to incidents of swelling and edema.  When chemical disinfection of multi-dose vials is desired, either isopropyl alcohol (91%) or ethyl alcohol (70%) is recommended.  Many commercially available brands of rubbing alcohol, as well as solutions of ethyl alcohol not of USP grade, contain denaturants which are injurious to rubber and therefore are not to be used.

Parenteral drug products should be visually inspected for particulate matter and discoloration prior to administration, whenever the solution and container permit.  The injection is not to be used if its color is pinkish or darker than slightly yellow or if it contains a precipitate.