Lidocaine patches (e.g., Lidoderm or Ztlido) are a common topical treatment prescribed for localized pain conditions, most notably for post-herpetic neuralgia following a shingles infection. This transdermal delivery system provides targeted pain relief directly at the site of discomfort. Patients often wonder if the medication, applied through the skin, can affect systemic functions, specifically whether it might increase blood pressure. Understanding the mechanism of action and the amount entering the bloodstream clarifies the relationship between topical application and systemic regulation.
How the Lidocaine Patch Works
Lidocaine is an amide-type local anesthetic whose primary function is to block pain signals in the peripheral nervous system. It achieves this effect by acting as a voltage-gated sodium channel blocker. These sodium channels are necessary for the conduction of electrical impulses along nerve fibers.
By binding to the sodium channels in peripheral nerve cells, lidocaine prevents sodium ions from flowing into the cell. This action stabilizes the nerve membrane, making it difficult for the nerve to fire and send a pain signal. The patch ensures a controlled, slow release of medication directly into the skin layers where pain-sensing nerve endings are located.
The goal of this topical administration is to achieve a high local concentration while maintaining minimal levels in the systemic circulation. This localized action distinguishes the patch from injectable or intravenous forms of lidocaine. The controlled release aims to provide prolonged pain relief without the systemic side effects associated with circulating drug levels.
Systemic Effects on Blood Pressure
Because the patch is designed for localized action, the amount of lidocaine absorbed into the bloodstream is very low. Studies indicate that typically less than 5% of the total drug is absorbed systemically when used as directed. This minimal systemic exposure results in plasma concentrations significantly below the level considered toxic.
For example, the average peak blood concentration following proper patch use is generally around 0.13 micrograms per milliliter (\(\mu\)g/mL). This concentration is far below the threshold of approximately 5 \(\mu\)g/mL, at which serious systemic toxicity is expected. Because systemic levels are so low, the patch is not expected to cause a rise in blood pressure in healthy individuals.
The known cardiovascular effects of lidocaine, associated with high-dose intravenous administration, are more often related to depression of the cardiac system. High plasma concentrations can lead to a decrease in heart rate (bradycardia) and a drop in blood pressure (hypotension). Clinical trials focusing on the patch formulation rarely list hypertension as a common side effect. The risk of any significant cardiovascular effect, including a blood pressure increase, remains negligible when the product is used strictly according to the prescribed directions. Any substantial change in blood pressure while using the patch usually signals excessive systemic absorption.
Warning Signs of Systemic Overdose
A significant elevation in blood pressure is not the typical sign of lidocaine toxicity, but any major change in blood pressure or heart function often accompanies a systemic overdose. This condition, known as Local Anesthetic Systemic Toxicity (LAST), occurs when the medication’s absorption exceeds safe limits and affects the central nervous system (CNS) and the cardiovascular system. Initial symptoms of toxicity are often related to the CNS.
Early warning signs include lightheadedness, confusion, and drowsiness, signaling that too much medication has entered the circulation. Patients may also experience auditory disturbances like ringing in the ears (tinnitus) or a metallic taste in the mouth. These symptoms typically occur before more severe reactions.
As systemic levels rise further, toxicity can progress to neurological signs like muscle twitching or tremors. Severe overdose can ultimately lead to seizures, unconsciousness, and significant changes in heart rhythm or blood pressure. If any of these neurological or cardiorespiratory symptoms develop, the patch must be removed immediately, and emergency medical attention should be sought.
Patient Considerations and Safe Application
To maintain the low systemic absorption profile and avoid toxicity, patients must adhere to specific application guidelines.
Application Rules
The most important rule is to apply the patch only to intact, non-irritated skin. Applying it to broken, scraped, or cut skin drastically increases the rate and amount of lidocaine absorbed into the bloodstream. Patients should never apply more than the prescribed number of patches at once, nor should they wear them for longer than the recommended duration, typically 12 hours on and 12 hours off within a 24-hour period. Exposing the patch to external heat, such as a heating pad, electric blanket, or prolonged sun exposure, can accelerate the release and absorption of the drug. Heat exposure should be strictly avoided while the patch is being worn.
High-Risk Patients
Certain patient populations require extra caution, especially those with pre-existing conditions that affect the body’s ability to process and eliminate the drug. Individuals with severe liver impairment, heart failure, or existing arrhythmias may be at a higher risk of developing systemic toxicity because their bodies cannot clear the lidocaine efficiently.
Disposal
If the patch needs to be cut to fit a smaller area, it should only be done prior to removing the release liner. The used patch must be folded sticky-side to sticky-side before safe disposal to prevent accidental exposure to children or pets.