Choosing a local anesthetic
The effectiveness of a particular drug is determined by its concentration at the site of administration. It depends on the dose and concentration of the administered solution, its diffusion into tissues and elimination, which in turn is determined by the vasculature in this zone. The latter condition can be changed by the joint use of vasoconstrictors, for example, epinephrine. Toxicity depends on the systemic concentration of the drug formed as a result of its absorption into the blood, or accidental intra-vascular injection. The appointment of vasoconstrictors also reduces these effects. Thus, the correct choice of a local anesthetic plays an important role. Increased lipid solubility Choosing a local anesthetic enhances absorption. Increased ionization, which depends on the drug’s pKa and local pH, decreases absorption. The values of the maximum safe doses are approximate, since they depend on many factors. Use the minimum dose and concentration to achieve an effective block. After careful aspiration, to prevent accidental intravenous or intrathecal injection, the required amount of the drug is administered fractionally. Local anesthetics quickly penetrate the placenta and can adversely affect the fetus, depending on the dose and technique of administration. Solutions containing preservatives should not be used for epidural or intrathecal administration.
Variants of application and characteristics of various local anesthetics are presented in the tables or prolonged subcutaneous administration of lidocaine is used for neuropathic pain. Alkalinization with sodium bicarbonate accelerates the onset of the effect and strengthens the last Bupivacaine, a prototype of local long-acting anesthetics used for infiltrative anesthesia, peripheral nerve block and nerve plexus blockade, epidural and spinal anesthesia and analgesia. The action occurs within 5 minutes after infiltration, 20 minutes after epidural administration and blockade of the nerve plexuses, and faster with intrathecal administration, although the peak of the effect is achieved a little later – after 15 minutes at intrathecal and 30-45 minutes with the epydural route of administration.