Thoughts on anaesthesia in chronobiological studies with electrocardiograms.

Abstract

The design and development of experimental, in vivo chronobiological animal models may help reveal some of the relationships between circadian rhythms and biological function, which can be very difficult to study in humans. To perform ethically acceptable cardiovascular research involving animals, the use of general anaesthesia is often necessary. However, the agents used for general anaesthesia exert a variety of effects on the cardiovascular system that may significantly impact haemodynamic data and the incidence of arrhythmias (Grund et al. 2004). All anaesthetic drugs alter normal physiology in some way and may confound the results of physiological studies. There is strong evidence from animal model and epidemiological studies showing that disruption of circadian rhythms is a significant risk factor for many cardiovascular diseases and, furthermore, that the emergence of cardiovascular diseases may be a time-dependent process (Culic 2014, Chen & Yang 2015). Given that heart rhythm disorders are also associated with changes in electrophysiological parameters of the myocardium, knowledge of the initial state of these parameters can significantly affect interpretation of the final results. Because some electrophysiological properties of the heart depend on the time of day experiments are performed, it is critically important to identify the causes of various types of ventricular arrhythmias. Problem remains fact that fluctuations in several endogenous functions, which depend on external environmental periodicities, are not often considered in experimental animal models. Unfortunately, there are no consistent methodological data regarding the daytime dependence on, or the synchronization of animals to, the light-dark cycle (12 h light : 12 h dark), because most in vivo experiments involving rats have typically been performed during the non-active (i.e. light) part of the day. Therefore, in addition to the vital importance of circadian timing, the impact of anaesthesia on this timing should also be a major consideration in experimental design. Numerous studies have referred to the impact of general anaesthesia on measurable parameters of different systems; however, relatively few have addressed the effect of general anaesthesia on the circadian variation of biological functions, particularly cardiovascular functions. Although chronobiological studies investigating interactions between general anaesthesia and circadian rhythms are sparse, all suggest that general anaesthetics have a significant effect on circadian rhythms (Dispersyn et al. 2008). Pentobarbital, ketamine/xylazine and zoletil are the most commonly used agents for general anaesthesia in experimental studies using animal models. Table 1 summarizes the results of our chronobiological studies investigating the impact of anaesthesia on cardiac electrophysiology in rats. The results clearly show differences among three types of anaesthetics and their effects on the circadian rhythmicity of myocardial electrophysiological parameters in an in vivo rat model, presented in our studies as ‘light-dark differences’. Although studies from other groups (Ohno et al. 2009, Mihara et al. 2012) have reported moderate or, alternatively, no effects of pentobarbital on the circadian rhythm of locomotor activity and the secretion of specific hormones in mice and rats, the disruptive effect of pentobarbital on circadian oscillations in electrophysiological parameters of the rat heart was evident in our study. Results regarding the effect of ketamine/xylazine anaesthesia on myocardial electrophysiology in rats are supported by the work of Pelissier et al. (1998), who also described the disruptive effect of ketamine on circadian rhythms of locomotor activity, heart rate and body temperature. The effect of ketamine was associated only with modifications, but no loss of circadian rhythms. However, to date, there is no literature evidence addressing the effect of zoletil anaesthesia on the circadian rhythms of vital functions. To conclude, we emphasize that investigators need to understand how animals are affected by anaesthetic drugs when designing and developing anaesthetic protocols, so as to minimize impact on data acquisition and analysis. Therefore, appropriate anaesthesia should be selected not only according to its particular effect on the organism, but also to factors that may be circadian dependent. The authors declare no conflict of interest.