Lidocaine is a widely used local anesthetic that creates numbness or a loss of feeling in a specific area of the body. It is injected by trained healthcare professionals for a range of medical and dental procedures to prevent or manage pain. The precise injection site is critical for achieving effective pain control and depends heavily on the type of procedure being performed.
How Lidocaine Works as a Local Anesthetic
At its core, lidocaine functions by temporarily blocking nerve signals in a localized area. Nerve cells transmit pain signals to the brain via ionic fluxes, particularly through sodium channels. Lidocaine stabilizes the neuronal membrane and inhibits these ionic fluxes, preventing the nerve from transmitting signals. Unlike general anesthetics, lidocaine does not cause unconsciousness. The specific location, volume, and concentration of the injection determine the area and duration of numbness.
Common Lidocaine Injection Techniques and Sites
There are several techniques for injecting lidocaine, each targeting a different level of tissue or a specific nerve pathway.
Local Infiltration
This is the most common and straightforward technique, where the anesthetic is injected directly into or around the tissue that needs to be numbed.
- Subcutaneous Injection: This involves injecting lidocaine into the subcutaneous fat layer just beneath the skin. It is the preferred method for local wound infiltration and preparing skin for minor procedures like IV insertions or biopsies, as it is less painful than intradermal injection. The technique involves inserting the needle into the subcutaneous layer and slowly injecting the solution while advancing or withdrawing the needle. The goal is to distribute the anesthetic across the surgical site.
- Intradermal Injection: This involves injecting into the dermis, the layer of skin directly below the surface. While effective, it is generally more painful than subcutaneous injection and less frequently used when other methods are an option.
Regional and Nerve Blocks
For larger areas or deeper tissue, a nerve block is used. This technique involves injecting lidocaine near a specific nerve or group of nerves, which anesthetizes the entire region supplied by that nerve.
- Peripheral Nerve Blocks: These target nerves in the limbs. For example, a digital nerve block for finger surgery involves injecting near the nerves in the web space of the finger. Other examples include blocking the brachial plexus for shoulder or arm surgery, or the femoral nerve for knee procedures.
- Dental Nerve Blocks: In dentistry, lidocaine is injected near branches of the trigeminal nerve to numb the gums and oral cavity for procedures like extractions or fillings.
Central Neural Blocks
This advanced form of regional anesthesia is typically performed by an anesthesiologist.
- Epidural Blocks: Lidocaine is injected into the epidural space surrounding the spinal nerves in the lower back. This is commonly used for pain management during childbirth or for certain surgeries.
- Spinal Injections: Lidocaine may also be injected as part of diagnostic or therapeutic spinal injections to help pinpoint and treat chronic back pain.
Intravenous Regional Anesthesia (Bier Block)
For surgical procedures on a limb, lidocaine may be injected intravenously into the arm or leg while a tourniquet is applied to keep the anesthetic isolated. Only certain formulations of lidocaine are used for this technique.
Important Considerations for Lidocaine Injection
To ensure efficacy and minimize complications, several factors must be considered during the injection process:
Patient Safety
- Trained Professionals: Lidocaine injections must be administered only by healthcare professionals who are trained in the diagnosis and management of dose-related toxicity.
- Aspiration: Before injection, the plunger should be pulled back slightly to aspirate and ensure the needle is not in a blood vessel. Accidental intravascular injection can lead to systemic toxicity.
- Monitoring: Patients receiving large blocks, especially in sensitive areas like the head and neck, should be carefully monitored for signs of systemic toxicity.
Minimizing Injection Pain
The injection itself can cause a brief stinging sensation. Techniques to minimize this include:
- Warming and Buffering: Warming the lidocaine solution to body temperature and buffering it with sodium bicarbonate significantly reduces the pain of injection.
- Small Gauge Needles: Using a smaller gauge needle (e.g., 27 to 30 gauge) for the injection can minimize discomfort.
- Slow Injection: Administering the anesthetic slowly reduces the pain caused by rapid tissue expansion.
Additives to Lidocaine
- Epinephrine: Epinephrine is often added to lidocaine to constrict blood vessels at the injection site. This slows the absorption of lidocaine, prolonging its effect and allowing for a higher maximum dose. However, epinephrine should be avoided in areas with limited blood supply, such as the fingers, toes, nose, and ears, to prevent tissue damage.
Comparison of Lidocaine Injection Methods
| Feature | Local Infiltration | Nerve Block | Central Neural Block (Epidural) |
|---|---|---|---|
| Target Area | Directly into or around the tissue at the procedure site (e.g., a wound, skin biopsy site) | Near a specific nerve or group of nerves that supply a larger area (e.g., a limb, mandible) | Into the epidural space around the spinal cord to numb multiple nerve roots |
| Depth of Injection | Subcutaneous tissue (just beneath the skin), intradermal | Extra-neural or paraneural spaces adjacent to the target nerve | Into the epidural space of the spine, guided by anatomical landmarks and patient feedback |
| Onset Time | Very rapid, often within seconds to a few minutes | Generally 10-20 minutes, as the anesthetic needs to diffuse to the nerve | Typically 15-30 minutes, depends on dosage and location |
| Area of Numbness | Small, localized area | Larger region distal to the injection site, e.g., an entire finger or forearm | Wide area of the body below the level of injection, e.g., lower abdomen and legs |
| Procedure Type | Minor skin procedures, wound repair, IV starts | Dental procedures, orthopedic surgery on limbs | Labor and delivery, certain surgeries involving the abdomen or lower extremities |
Conclusion
Deciding where to inject lidocaine is not a single answer but depends entirely on the clinical context. For minor procedures, a simple local infiltration into the subcutaneous tissue is sufficient. For more extensive or deep tissue work, a nerve block or a central neural block may be required to achieve broader anesthesia. Techniques to minimize injection pain and the use of additives like epinephrine are also important considerations. Regardless of the method, the administration of injectable lidocaine should always be performed by a qualified healthcare professional who understands the proper anatomical sites and safety protocols to ensure patient well-being and a successful outcome.
