Local anesthetic drugs are extremely effective, inexpensive and easy to use. When local anesthetic drugs are administered, pain impulses originating in the periphery are blocked and prevented from reaching the central nervous system. This blockade has several positive consequences:
• The sensation of pain is alleviated or even eliminated for the duration of the block. Local anesthetic drugs work by blocking sodium channels in nerve membranes. Decreased permeability to sodium slows the rate of depolarization so that the threshold potential is not achieved and an action potential is not propagated, thus the pain impulse is not propagated. Local anesthetics bind more readily to 'open' channels, thus rapidly firing nerves are more susceptible to blockade.
• The likelihood that 'wind-up' or hypersensitization will occur is greatly decreased because the portion of the pain pathway called 'transmission' is blocked. Transmission involves the conductance of pain impulses from the peripheral nociceptors to the dorsal horn neurons in the spinal cord. The neurons in the dorsal horn are responsible for central sensitization. By blocking input to these neurons, central sensitization (or 'wind up') is less likely to occur.
• The analgesia allows the patient to be maintained under a lighter plane of anesthesia and this makes the anesthetic episode safer for the patient. In fact, local anesthetic drugs decrease the minimum alveolar concentration (MAC) of all anesthetic gases.
Furthermore, local anesthetic blocks are extremely cost effective and can increase profits to the clinic.
Commonly used local anesthetic drugs in veterinary medicine include
o Onset of action: rapid (less than 5 minutes)
o Duration of action: 60-120 minutes
o Dose 2-6 mg/kg (use the lower end of the dose in cats)
o Convulsive dose in dogs: 11-20 mg/kg
o Lethal dose in dogs: 16-28 mg/kg
o 'Toxic dose' in cats reported as 6-10 mg/kg
o The general recommendation for clinical use is ≤ 6 mg/kg in the dog and ≤ 3-4 mg/kg in the cat.
o Onset of action: approximately 5-10 minutes after injection (up to 20 minutes)
o Duration of action: 4 to 6 hours
o Dose 1-2 mg/kg (use the lower end of the dose in cats)
o Convulsive dose in dogs: 3.5-4.5 mg/kg
o Lethal dose in dogs: 5-11 mg/kg
o Data is mostly anecdotal in the cat but the general feeling is that 3 mg/kg is the toxic dose.
o The general recommendation for clinical use is ≤ 2 mg/kg in the dog and ≤ 1 mg/kg in the cat.
Adverse events caused by local anesthetic drugs
• Adverse events are extremely rare but can include any of the following:
• Local tissue effects – swelling, bleeding, inflammation, 'tingling'? (unknown if this occurs in animals)
• Anaphylaxis – rare, more common with esters (but still rare)
• Central nervous system – muscle tremors, seizure, coma
o At lower concentrations, depression of inhibitory neurons occurs and can cause cerebral excitation, which may lead to seizures. At higher concentrations, profound CNS depression with subsequent coma, respiratory arrest and death can occur. The latter is more likely following IV boluses of large doses.
o Cardiovascular system – the myocardial conduction system is sensitive to local anesthetics and IV boluses can result in cardiovascular collapse. ONLY LIDOCAINE CAN BE ADMINISTERED IV.
o Methemoglobinemia – rare, but can occur in cats.