Cranial versus caudal thoracic epidural anesthesia using three volumes of lidocaine in conscious Beagle dogs.

OBJECTIVE: To compare the effects of epidural injection of three volumes of lidocaine injected at the third (T3) or eleventh thoracic vertebra (T11) in conscious dogs to induce thoracic epidural anesthesia (TEA) and to measure the epidural dispersion of iohexol under similar conditions.

STUDY DESIGN: Prospective crossover experiment.

ANIMALS: A group of five Beagle dogs weighing 10.4 ± 0.5 kg (mean ± standard deviation).

METHODS: Each dog was anesthetized twice, separated by 1 week, for inserting an epidural catheter at the lumbosacral space and advancing the tip to T3 (treatment TEAT3 ) or T11 (treatment TEAT11 ). For each treatment, three volumes of 2% lidocaine (0.05, 0.10 and 0.20 mL kg-1 ) were administered at 24 hour intervals, and sensory blockade (SB) of dermatomes was estimated by pinching the skin with mosquito forceps. Under identical conditions of injection volume and site, iohexol was administered 3 hours after lidocaine injection to identify epidural distribution (ED) using computed tomography. The effects of injection site and volume on SB of thoracic dermatomes and ED were analyzed using a linear mixed model (p < 0.05).

RESULTS: Thoracic SB and ED significantly increased as the volume increased (p < 0.001 and p < 0.001, respectively), and significantly decreased in TEAT3 than in TEAT11 (p = 0.011 and p = 0.002, respectively). Cervical SB was obtained in three of five dogs in TEAT3 and two of five dogs in TEAT11 injected with 0.20 mL kg-1 . One dog showed temporary inspiratory stridor probably caused by bilateral laryngeal paralysis, but no hypoxia.

CONCLUSIONS AND CLINICAL RELEVANCE: TEA induced at T3 produced less thoracic SB than did TEA at T11 with the same volumes of lidocaine. The cervical SB obtained with the highest volume of lidocaine may increase the risk of laryngeal paralysis and pulmonary aspiration.