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ANESTHETIC PHARMACOLOGY

Epidural Clonidine Suppresses the Baroreceptor-Sympathetic Response Depending on Isoflurane Concentrations in Cats

Yoshikazu Ikeda, MD^*, Kiyonobu Nishikawa, MD^*, Kenji Ohashi, MD, Takashi Mori, MD^*, and Akira Asada, MD^*

*Department of Anesthesiology and Intensive Care Medicine, Osaka City University Medical School, Osaka, Japan; and Department of Anesthesia, Hoshigaoka Kosei-nenkin Hospital, Osaka, Japan

Address correspondence and reprint requests to Dr. Kiyonobu Nishikawa, Department of Anesthesiology and Intensive Care Medicine, Osaka City University Medical School, 1-5-7 Asahi-machi, Abeno-ku, Osaka 545-8586, Japan. Address e-mail to kiyonishikawa{at}msic.med.osaka-cu.ac.jp

Epidural administration of clonidine induces hypotension and bradycardia secondary to decreased sympathetic nerve activity. In this study, we sought to elucidate the change in baroreflex response caused by epidural clonidine. Thirty-six cats were allocated to six groups (n = 6 each) and were given either thoracic epidural clonidine 4 µg/kg or lidocaine 2 mg/kg during 0.5, 1.0, or 1.5 minimum alveolar anesthetic concentration (MAC) isoflurane anesthesia. Heart rate (HR), mean arterial blood pressure (MAP), and cardiac sympathetic nerve activity (CSNA) were measured. Depressor and pressor responses were induced by IV nitroprusside 10 µg/kg and phenylephrine 10 µg/kg, respectively. Baroreflex was evaluated by the change in both CSNA and HR relative to the peak change in MAP (CSNA/MAP and HR/MAP, respectively). These measurements were performed before and 30 min after epidural drug administration. Epidural clonidine and lidocaine decreased HR, MAP, and CSNA by similar extents. CSNA/MAP and HR/MAP for depressor response were suppressed with epidural lidocaine and clonidine in all groups but the clonidine 0.5 MAC isoflurane group (0.197 ± 0.053 to 0.063 ± 0.014 and 0.717 ± 0.156 to 0.177 ± 0.038, respectively, by epidural lidocaine [P < 0.05] but 0.221 ± 0.028 to 0.164 ± 0.041 and 0.721 ± 0.177 to 0.945 ± 0.239, respectively, by epidural clonidine during 0.5 MAC isoflurane). Those for pressor response were suppressed in all groups. We conclude that thoracic epidural clonidine suppresses baroreflex gain during isoflurane anesthesia >1.0 MAC but may offer certain advantages compared with epidural lidocaine during 0.5 MAC isoflurane by virtue of preserving baroreflex sensitivity when inadvertent hypotension occurs.

IMPLICATIONS: Epidural clonidine reduces blood pressure, heart rate, and sympathetic nerve activity but suppresses baroreflex gain, depending on background isoflurane anesthesia.

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