Systemic Effects
Local anesthetics are absorbed into the blood stream, and if large amounts have been used, the level in the blood can cause toxic effects on the cardiovascular and central nervous systems. Initially, animals may become sedate or drowsy. As the blood and CNS levels increase, the animal may become restless and nauseous and may vomit. At even higher doses the animal will convulse, followed by coma. Death occurs due to respiratory and cardiac arrest.
Procaine is more toxic and more likely to produce a sensitivity reaction than is xylocaine.
Drugs Used in Veterinary Medicine.
Procaine (Novocaine) was one of the earliest (l905) local anesthetics to be synthesized and used as an injectable local, however lidocaine has surpassed it in popularity. Procaine also has the major disadvantage of being a benzoic acid derivative which produces a significant number of allergic type reactions. Lidocaine was synthesized in 1943, it was found to be about 4 times as potent as procaine with approximately the same duration of action. Mepivacaine is generally regarded as the local anesthetic least likely to cause tissue reactions in horses. In veterinary medicine lidocaine is by far the most common injectable local anesthetic. When a longer acting drug is required, bupivacaine is generally used. Listed below are several local anesthetics and their relative potencies, duration of actions, and preparations.