The Primary Additives Mixed with Lidocaine
Medical professionals combine lidocaine with other substances to modify its properties, such as its duration and the pain experienced during injection. The most common additives are epinephrine and sodium bicarbonate, each serving a unique purpose to improve the anesthetic's performance.
Epinephrine: The Vasoconstrictor
Epinephrine, also known as adrenaline, is a potent vasoconstrictor, meaning it constricts blood vessels. When mixed with lidocaine, it delays the absorption of the local anesthetic from the injection site into the bloodstream. This has several key benefits:
- Prolonged Anesthesia: By slowing the rate at which the body clears the lidocaine, the anesthetic effect lasts significantly longer. This is particularly useful for surgical procedures that require more than the 30–60 minutes of pain relief that plain lidocaine typically provides.
- Reduced Bleeding: The vasoconstrictive effect shrinks the local blood vessels, leading to a bloodless or blood-poor surgical field. This improves the surgeon's visibility and reduces blood loss during the procedure.
- Increased Maximum Dose: With slower systemic absorption, the maximum safe dose of lidocaine can be increased, from around 4 mg/kg to 7 mg/kg.
Historically, there was a belief that epinephrine should not be used in areas with 'end arteries,' such as fingers, toes, and the nose, due to concerns of ischemia and tissue necrosis. However, modern medical evidence and extensive clinical experience have shown that in patients with normal circulation, low concentrations of lidocaine with epinephrine are safe for digital blocks and other procedures. The mixture is often prepared in concentrations such as 1:100,000 or 1:200,000.
Sodium Bicarbonate: The Buffer
Commercially available lidocaine solutions are acidic, with epinephrine-containing versions being even more so. This acidity is a primary cause of the stinging or burning sensation patients feel during the injection. To counteract this, doctors can buffer the solution by adding sodium bicarbonate, raising the pH closer to the body's natural level.
- Less Painful Injection: Buffering with bicarbonate significantly reduces the pain of local anesthetic infiltration, particularly for deeper injections and in sensitive areas. This improves patient comfort and can reduce anxiety.
- Faster Onset: By raising the pH, more of the lidocaine molecules are converted into their non-ionized, fat-soluble form. This form can more easily penetrate nerve membranes, leading to a more rapid onset of anesthesia.
- Mixing Precautions: Buffering is typically done just before administration, as the mixture is less stable over time. A common mixing ratio is 1 mL of 8.4% sodium bicarbonate to 10 mL of 1% lidocaine with epinephrine.
Combining Lidocaine with Other Anesthetics
In some cases, lidocaine is mixed with other local anesthetics to blend their different properties. A common combination is with bupivacaine, a longer-acting agent.
- Lidocaine and Bupivacaine: This mixture combines the rapid onset of lidocaine with the prolonged duration of bupivacaine. This aims to provide both quick pain relief at the beginning of a procedure and lasting analgesia afterward. However, some studies have shown conflicting results regarding the onset and duration benefits of mixing, suggesting that in some applications, using a single anesthetic might be equally or more effective.
- Lidocaine and Prilocaine (EMLA): This eutectic mixture is formulated as a topical cream for dermal analgesia. It is applied to the skin, often under an occlusive dressing, to numb the area before procedures like needle punctures.
Preservatives and Other Agents
Besides therapeutic additives, other substances may be present in lidocaine formulations, depending on how they are packaged.
- Preservatives in Multi-Dose Vials: Some multiple-dose vials contain preservatives like methylparaben to prevent bacterial growth. However, these preservatives can cause allergic reactions in some individuals. Preservative-free lidocaine must be used for spinal or epidural injections.
- Sulfite Stabilizers: Epinephrine-containing lidocaine often includes sodium metabisulfite as an antioxidant to prevent the epinephrine from degrading. This can trigger allergic reactions in patients with sulfite sensitivity.
Comparison of Lidocaine Mixtures
| Feature | Plain Lidocaine | Lidocaine with Epinephrine | Buffered Lidocaine | Lidocaine with Bupivacaine |
|---|---|---|---|---|
| Purpose | Simple local anesthesia | Prolong anesthesia, reduce bleeding | Reduce injection pain, speed onset | Combine rapid onset with long duration |
| Onset | Fast | Slightly slower due to vasoconstriction | Faster | Rapid |
| Duration | Short (30-60 min) | Long (significantly prolonged) | Unaffected or slightly increased | Long (prolonged) |
| Injection Pain | Moderate (stinging/burning) | Moderate to High (very acidic) | Low (pH is neutralized) | Moderate (acidic) |
| Common Use | Minor procedures, suture placement | Surgical procedures, dental work | Infiltration of sensitive tissues | Nerve blocks for prolonged analgesia |
| Considerations | Use when prolonged effect is not needed | Avoid in patients with certain conditions; use with caution in extremities | Mix just prior to use, as stability is limited | Clinical benefits debated; monitor for total dose of both agents |
Conclusion
Doctors mix with lidocaine various substances to tailor its effects for specific procedures and patient needs. The addition of epinephrine prolongs anesthesia and controls bleeding by constricting blood vessels, while sodium bicarbonate reduces the pain of injection by buffering the solution's acidity. Combining lidocaine with other anesthetics like bupivacaine can provide a synergistic effect of rapid onset and long duration, though some evidence questions its clinical advantage over single-agent use. Additionally, clinicians must be aware of preservatives in multi-dose vials and potential drug interactions. These specialized mixtures and applications, such as tumescent anesthesia, highlight how doctors leverage pharmacology to enhance patient safety and comfort during a wide range of medical interventions.
