EFFECTS OF ISOFLURANE ON CARDIOVASCULAR-SYSTEM AND SYMPATHOVAGAL BALANCE IN NEW-ZEALAND WHITE-RABBITS

We investigated the effects of isoflurane on the rabbit cardiovascular system at several end-tidal concentrations. Furthermore, because isof lurane has been reported to produce tachycardia while reducing sympath etic nervous activity and baroreflex function, we evaluated whether th e chronotropic effects of isoflurane could be due to a vagal withdrawa l. EGG, mean arterial pressure (MAP), and heart rate (HR) were obtaine d in rabbits the conscious, unsedated state and during isoflurane anes thesia by telemetric device. Measurements of pH, oxygen, carbon dioxid e, plasma catecholamines, baroreflex sensitivity, and spectral analysi s of HR variability were made in nonanesthetized and anesthetized anim als. Isoflurane caused an increase in HR at 0.5, 1, and 1.5 minimum al veolar concentration (MAC) and a decrease in systolic and diastolic bl ood pressure (SEP, DBP) and MAP at 1 and 1.5 MAC. Biochemical analysis showed that isoflurane-mediated cardiovascular effects were not accom panied by any significant changes in plasma norepinephrine (NE) and ep inephrine (Epi) levels. Neither were any significant differences in pl asma catecholamine levels noted between anesthetized and awake animals . The analysis of spectral components of HR variability and baroreflex function indicated that isoflurane induced a marked reduction in the low- and high-frequency spectral power of HR variability and in barore flex sensitivity. Tachycardia under isoflurane was suppressed dose dep endently by the administration of clonidine or atenolol and was not in fluenced by bilateral vagotomy. Collectively, our results indicate tha t cardiovascular effects induced by isoflurane in smaller animals such as rabbits are similar to those observed in humans and other animal s pecies. We showed that isoflurane-induced tachycardia is mainly the re sult of a vagal withdrawal rather than a baroreflex response, even tho ugh a marginal role of baroreflex in heart response to higher concentr ations of isoflurane cannot be excluded.