26 Effects of vagal blockade on the complexity of heart rate variability in rats M. Baumert 1 , E. Nalivaiko 2 and D. Abbott 1 1 Centre for Biomedical Engineering, School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, Australia; 2 Department of Human Physiology and Centre for Neuroscience, Flinders University, Adelaide, Australia. Abstract— In this paper we investigate the influence of va- gal blockage on heart rate variability complexity measures. Nine conscious rats are injected with methyl-scopolamine brobide (50 µg/kg s.c.). We analyze 10 minute segments of beat-to-beat intervals before and after injection by standard time and frequency domain methods, compression entropy, sample entropy, Poincaré plot, detrended fluctuation analysis and symbolic dynamics. All parameter domains show changes in heart rate variability after vagal blockade, indicating a decrease in heart rate complexity. In conclusion, vagal modu- lation plays an important role in the generation of heart com- plexity in rats or, in other words, heart rate complexity meas- ures are sensitive to vagal heart rate modulation. Keywords— heart rate variability, complexity, vagal block- ade, rat I. INTRODUCTION The heart rate underlies beat-to-beat variations, reflecting modulations mediated by vagal and sympathetic branches of the autonomic nervous system. Heart rate variability (HRV) analysis has shown prognostic significance in patients after acute myocardial infarction [1] and in the diagnosis of auto- nomic neuropathy [2]. Furthermore, it is used in various research settings such as sports [3] or obstetrics [4]. The quantification of HRV is basically a time series analysis task, and numerous approaches have been pro- posed, including traditional time and frequency domain measures [5], but also measures from complex systems science [4,6,7,8]. Although the sensitivity of some of those new measures often appeared superior to standard time and frequency domain measures, their physiological meaning is hardly understood and their interpretation remains difficult. To assess their sensitivity to vagal heart rate modulation we investigate the impact of vagal blockade on HRV com- plexity measures in a rat model. II. METHODS A. Animal preparation and experimental protocol The study is performed on nine male Wistar Hooded rats weighing 250-300 g. Experiments are conducted in accor- dance with the European Community Council Directive of 24 November 1986 (86/609/EEC), and are approved by the Flinders University Animal Welfare Committee. During preliminary surgery, telemetric ECG transmitters (TA11CA-F40, Data Science International, USA) are im- planted into the peritoneal cavity under isoflurane (1.5% in 100% oxygen) anesthesia. On the day of experiment, ECG is recorded before and after administration of methyl- scopolamine bromide (50 µg/kg s.c., Sigma, USA), a vagal blocker that does not cross blood-brain barrier. Analogue signal is acquired using the MacLab interface and Chart software (ADInstruments, Sydney, Australia). B. Heart rate variability analysis Pre-processing: RR intervals series are extracted from the ECG recording, using the Chart® software (ADInstru- ments, Sydney, Australia). Subsequently, RR time series are scanned and manually edited. Artifacts and ectopic beats are filtered, resulting in a normal-to-normal (NN) interval time series. For further analysis we select a segment of a 10 min- ute length, starting 15 minutes prior to the vagal blockade injection in order to obtain HRV baseline values. To ana- lyze HRV during vagal blockade, we select ten minute ep- ochs, beginning five minutes after injection. 100 120 140 160 180 200 220 0 10 20 30 40 50 60 time [min] NN interval [ms] injection Fig. 1 Beat-to-beat interval time series in a conscious rat prior and after injection of methyl-scopolamine. The NN interval as well as heart rate variability deceases.