Effects of halothane and isoflurane on baroreflex sensitivity in horses

Baroreflex sensitivity (BS) was used to quantitatively assess the effects of halothane and isoflurane on the heart rate/arterial pressure relationship during steady-state (10 minutes) and dynamic pressure changes in adult horses. Arterial pressure was decreased in response to nitroglycerin or sodium nitroprusside and increased in response to phenylephrine HCl. Mean (+/- SEM) BS in awake horses was 28.9 +/- 2.6 and 13.2 +/- 2.0 ms/mm of Hg during steady-state decreases and increases in systolic arterial pressure (SAP), respectively. Halothane and isoflurane either significantly (P < 0.05) decreased or eliminated BS during steady-state decreases in SAP, with no significant differences detected between anesthetic agents. During steady-state decreases in SAP, significant (P < 0.05) correlation between R-R interval and arterial pressure was not observed for 6 of 10 and 4 of 11 halothane and isoflurane anesthesia periods, respectively. Halothane significantly (P < 0.05) decreased BS during steady-state increases in SAP to 7.9 +/- 0.6 and 6.5 +/- 1.1 ms/mm of Hg during low and high minimal alveolar concentration (MAC) multiples, respectively. Isoflurane decreased BS during steady-state increases in SAP to 9.6 +/- 1.5 and 6.6 +/- 1.1 ms/mm of Hg during low and high MAC anesthesia, respectively, with high MAC of isoflurane decreasing BS significantly (P < 0.05), compared with awake and low MAC values. Plasma catecholamine (epinephrine and norepinephrine) concentrations increased significantly (P < 0.05), compared with baseline values during steady-state vasodilator infusions in halothane- and isoflurane-anesthetized horses. Steady-state infusions of phenylephrine in anesthetized horses resulted in arrhythmia development, with premature atrial and ventricular complexes seen in halothane-anesthetized horses and increased heart rate and atrial premature complexes seen less frequently in isoflurane-anesthetized horses. Dynamic BS was 25.0 +/- 4.5 and 20.1 +/- 2.8 ms/mm of Hg for decreasing and increasing SAP, respectively, in awake horses. The R-R interval and SAP were linearly correlated during dynamic decreases in SAP in 7 of 9 halothane and 8 of 10 isoflurane anesthesia periods. Baroreflex sensitivity decreased to 15.0 +/- 6.8 and 13.3 +/- 3.5 ms/mm of Hg during anesthesia with low MAC of halothane and isoflurane, respectively. High MAC of halothane and isoflurane significantly (P < 0.05) decreased BS during dynamic decreases in SAP in 7.8 +/- 1.8 and 7.2 +/- 1.3 ms/mm of Hg, respectively. There were no significant differences in BS depression between halothane and isoflurane.