This study of 14 cats examined these commonly practiced local anesthetic techniques in a 2-part experiment. In the first part, half of the cats (group L2) were randomly assigned to receive 0.1 mL of 2% lidocaine topically administered on the larynx (≈0.6 mg/kg); the remaining cats (group L10) received 0.1 mL of 10% lidocaine topically administered on the larynx (2.8 mg/kg). Plasma lidocaine concentrations were then measured and found to be significantly higher in the L10 group as compared with the L2 group (median maximum plasma lidocaine concentrations, 93.6 ng/mL vs 34.1 ng/mL, respectively).
In the second part of the experiment, cats were randomized to receive topical lidocaine as described for the first part. All cats then received intratesticular administration of 2% lidocaine (0.1 mL) prior to neuter. The concurrent use of topical lidocaine (2% or 10%) with intratesticular administration of 0.1 mL/kg of 2% lidocaine resulted in a maximum total lidocaine dose of approximately 2.54 mg/kg and 4.76 mg/kg, respectively, neither of which exceeds the recommended dose for cats.3,4 In addition, the maximum plasma concentrations increased in a dose-dependent manner and were significantly higher than for topical administration alone, although plasma concentrations remained significantly lower than those required to produce seizures in cats.4 The time to reach maximum plasma concentrations did not differ among treatments.
It is important to note that factors other than dose and route (eg, patient age, concurrent anesthetics, hypotension, hypothermia) can alter pharmacokinetics and plasma lidocaine concentrations. However, this study suggests that, in adult cats (≈8.8 lb [≈4 kg]) undergoing surgical neutering, 2% topical lidocaine administered concurrently with intratesticular 2% lidocaine (0.1 mL/kg) results in plasma concentrations unlikely to approach those associated with systemic toxicity. Thus, these techniques can provide useful adjunctive analgesia when neutering cats.