Lidocaine is one of the most frequently used local anesthetics in medicine and dentistry, providing temporary numbness to a specific area of the body. This common drug, which belongs to the amino amide class, blocks nerve signals to prevent the feeling of pain during minor procedures. While often administered as a “plain” solution, lidocaine is more frequently combined with another agent, epinephrine, to enhance its effects. Understanding the distinction between these two formulations is important, as the choice between them dictates the safety and effectiveness of the anesthetic.
Understanding the Role of Epinephrine
Epinephrine, also known as adrenaline, is a naturally occurring hormone that acts as a potent vasoconstrictor. Its addition to lidocaine is a standard practice, as it causes the local blood vessels to narrow at the injection site. This vasoconstriction serves multiple important functions that improve the anesthetic’s performance. By constricting the blood vessels, epinephrine significantly reduces local blood flow, which minimizes bleeding during a procedure, providing a clearer surgical field.
More importantly, the vasoconstrictive action slows the rate at which the body absorbs the lidocaine into the systemic circulation. This crucial pharmacokinetic effect keeps a higher concentration of the anesthetic localized in the target tissue for a longer period. This extended retention effectively prolongs the duration of the numbing effect, often by two to four times compared to plain lidocaine. The decreased systemic absorption also acts as a safety measure, lowering the peak concentration of lidocaine in the bloodstream, thereby reducing the risk of systemic toxicity.
How Plain Lidocaine Works and Its Duration
Lidocaine works at a molecular level by reversibly blocking voltage-gated sodium channels within the nerve cell membranes. This action prevents the rapid influx of sodium ions that is necessary to generate an electrical impulse, stopping the nerve from transmitting pain signals to the brain. This mechanism of action is the same whether or not epinephrine is included in the formulation.
The duration of the anesthetic effect, however, is significantly shorter with plain lidocaine because the drug is more quickly metabolized and cleared from the injection site. Without the localizing effect of a vasoconstrictor, the body’s natural blood flow washes the lidocaine away more rapidly. Plain lidocaine typically provides anesthesia for a relatively short time, often ranging from 45 to 90 minutes. This contrasts sharply with the combined formulation, which can provide effective pain relief for up to 200 minutes or longer.
When Plain Lidocaine is Necessary
The formulation without epinephrine becomes the preferred choice in two distinct clinical situations, primarily to avoid the potentially harmful effects of vasoconstriction or the systemic effects of adrenaline.
End-Artery Locations
The first scenario involves specific anatomical locations where the blood supply is considered a terminal or end-artery, meaning there is poor collateral blood flow. Injecting a vasoconstrictor into these areas could severely restrict blood flow, potentially leading to tissue ischemia or necrosis. Classical sites that require plain lidocaine include areas like the fingers, toes, penis, nose, and earlobes. Though some recent studies suggest that low concentrations of epinephrine may be safe even in some of these areas, the traditional and cautious practice is to avoid vasoconstrictors in these locations.
Medical Contraindications
The second scenario involves patients with specific health conditions or those taking certain medications where the systemic effects of epinephrine could be dangerous. Patients with uncontrolled hypertension, severe coronary artery disease, or certain cardiac arrhythmias are at risk because even small amounts of absorbed epinephrine can cause unwanted increases in heart rate and blood pressure.
Furthermore, patients taking certain medications, such as non-selective beta-blockers, tricyclic antidepressants, or monoamine oxidase inhibitors (MAOIs), may experience a heightened or dangerous reaction to the small dose of epinephrine in the local anesthetic. For these medically compromised individuals, the choice of plain lidocaine eliminates the risk of an adverse cardiovascular event triggered by the adrenaline. Using the plain formulation is a safety-driven decision, accepting a shorter duration of action to prevent a serious complication.
Recognizing Signs of Systemic Absorption
Since plain lidocaine is absorbed into the bloodstream more quickly, the risk of Local Anesthetic Systemic Toxicity (LAST) is a greater consideration, especially if a large volume or dose is administered. Systemic toxicity occurs when the concentration of lidocaine in the blood reaches a level high enough to affect the central nervous system (CNS) and the cardiovascular system.
Early signs of CNS toxicity are often subtle and include a metallic taste or a tingling sensation around the lips and tongue, known as perioral numbness. Other CNS symptoms may progress to lightheadedness, dizziness, visual disturbances, and confusion. More severe signs can appear, such as muscle twitching, tremors, and potentially seizures. Cardiovascular toxicity involves changes in heart rhythm and blood pressure, potentially leading to a slow heart rate, low blood pressure, and even cardiac arrest. Recognizing these symptoms promptly is important for patient safety, as early intervention is necessary.