M. Gruenewald and C. Ilies, “Monitoring the nociception-anti-nociception balance,” Best Pract. Res. Clin. Anaesthesiol., vol. 27, no. 2, pp. 235–247, Jun. 2013.
 D. Stucke, M. Große Ruse, and D. Lebelt, “Measuring heart rate variability in horses to investigate the autonomic nervous system activity – Pros and cons of different methods,” Appl. Anim. Behav. Sci., vol. 166, pp. 1–10, May 2015.
 D. Barnaby et al., “Heart rate variability in emergency department patients with sepsis,” Acad. Emerg. Med. Off. J. Soc. Acad. Emerg. Med., vol. 9, no. 7, pp. 661–670, Jul. 2002.
 T. Tzelos, K. J. Blissitt, and R. E. Clutton, “Electrocardiographic indicators of excitability in horses for predicting recovery quality after general anaesthesia,” Vet. Anaesth. Analg., vol. 42, no. 3, pp. 269–279, May 2015.
 C.-J. Huang, C.-H. Kuok, T. B. J. Kuo, Y.-W. Hsu, and P.-S. Tsai, “Pre-operative measurement of heart rate variability predicts hypotension during general anesthesia,” Acta Anaesthesiol. Scand., vol. 50, no. 5, pp. 542–548, May 2006.
 G. Valenza, L. Citi, J. P. Saul, and R. Barbieri, “Measures of sympathetic and parasympathetic autonomic outflow from heartbeat dynamics,” J. Appl. Physiol. Bethesda Md 1985, vol. 125, no. 1, pp. 19–39, 01 2018.
 R. Logier et al., “PhysioDoloris: a monitoring device for analgesia / nociception balance evaluation using heart rate variability analysis,” Conf. Proc. Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. IEEE Eng. Med. Biol. Soc. Annu. Conf., vol. 2010, pp. 1194–1197, 2010.
 M. Jeanne, R. Logier, J. De Jonckheere, and B. Tavernier, “Validation of a graphic measurement of heart rate variability to assess analgesia/nociception balance during general anesthesia,” Conf. Proc. Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. IEEE Eng. Med. Biol. Soc. Annu. Conf., vol. 2009, pp. 1840–1843, 2009.
 C. Mansour et al., “Performance of the Parasympathetic Tone Activity (PTA) index to assess the intraoperative nociception using different premedication drugs in anaesthetised dogs,” Int. J. Vet. Sci. Med., vol. 8, no. 1, pp. 49–55, Jan. 2020.
 W. Pearce, “The Cardiovascular Autonomic Nervous System and Anaesthesia,” South. Afr. J. Anaesth. Analg., vol. 8, no. 3, pp. 8–24, Jul. 2002.
 T. W. Latson and D. O’Flaherty, “Effects of surgical stimulation on autonomic reflex function: assessment by changes in heart rate variability,” Br. J. Anaesth., vol. 70, no. 3, pp. 301–305, Mar. 1993.
 T. W. Latson, T. H. Ashmore, D. J. Reinhart, K. W. Klein, and A. H. Giesecke, “Autonomic reflex dysfunction in patients presenting for elective surgery is associated with hypotension after anesthesia induction,” Anesthesiology, vol. 80, no. 2, pp. 326–337, Feb. 1994.
 A. T. Mazzeo, E. La Monaca, R. Di Leo, G. Vita, and L. B. Santamaria, “Heart rate variability: a diagnostic and prognostic tool in anesthesia and intensive care,” Acta Anaesthesiol. Scand., vol. 55, no. 7, pp. 797–811, Aug. 2011.
 C. Oel, H. Gerhards, and H. Gehlen, “Effect of retrobulbar nerve block on heart rate variability during enucleation in horses under general anesthesia,” Vet. Ophthalmol., vol. 17, no. 3, pp. 170–174, May 2014.
 Y. Ogawa et al., “Different effects on circulatory control during volatile induction and maintenance of anesthesia and total intravenous anesthesia: autonomic nervous activity and arterial cardiac baroreflex function evaluated by blood pressure and heart rate variability analysis,” J. Clin. Anesth., vol. 18, no. 2, pp. 87–95, Mar. 2006.
 R. Hanss et al., “Does heart rate variability predict hypotension and bradycardia after induction of general anaesthesia in high risk cardiovascular patients?,” Anaesthesia, vol. 63, no. 2, pp. 129–135, Feb. 2008.
 E. Boselli et al., “Prospective observational study of the non-invasive assessment of immediate postoperative pain using the analgesia/nociception index (ANI),” Br. J. Anaesth., vol. 111, no. 3, pp. 453–459, Sep. 2013.
 M. Kuwahara et al., “Assessment of autonomic nervous function by power spectral analysis of heart rate variability in the horse,” J. Auton. Nerv. Syst., vol. 60, no. 1, pp. 43–48, Aug. 1996.
 E. L. McConachie, S. Giguère, G. Rapoport, and M. H. Barton, “Heart rate variability in horses with acute gastrointestinal disease requiring exploratory laparotomy,” J. Vet. Emerg. Crit. Care San Antonio Tex 2001, vol. 26, no. 2, pp. 269–280, Apr. 2016.
 E. Boselli, R. Logier, L. Bouvet, and B. Allaouchiche, “Prediction of hemodynamic reactivity using dynamic variations of Analgesia/Nociception Index (∆ANI),” J. Clin. Monit. Comput., vol. 30, no. 6, pp. 977–984, Dec. 2016.
 K. Sakata, N. Yoshimura, K. Kito, K. Tanabe, and H. Iida, “Abstract PR216: Prediction of Hypotension During Spinal Anesthesia for Cesarean Section By Heart Rate Variability.,” Anesth. Analg., vol. 123, p. 279, Sep. 2016.
 C. Mansour et al., “Evaluation of the Parasympathetic Tone Activity (PTA) index to assess the analgesia/nociception balance in anaesthetised dogs,” Res. Vet. Sci., vol. 115, pp. 271–277, Dec. 2017.
 D. Aguado, R. Bustamante, V. García-Sanz, P. González-Blanco, and I. A. Gómez de Segura, “Efficacy of the Parasympathetic Tone Activity monitor to assess nociception in healthy dogs anaesthetized with propofol and sevoflurane,” Vet. Anaesth. Analg., vol. 47, no. 1, pp. 103–110, Jan. 2020.
 M. Bootsma, C. A. Swenne, F. P. van Rugge, and E. E. van der Wall, “The sympathovagal balance during control and dobutamine stress MRI,” in Proceedings of Computers in Cardiology Conference, Sep. 1993, pp. 317–320.
 H. Gehlen, J. Loschelder, R. Merle, and M. Walther, “Evaluation of Stress Response under a Standard Euthanasia Protocol in Horses Using Analysis of Heart Rate Variability,” Anim. Open Access J. MDPI, vol. 10, no. 3, Mar. 2020.
 C. L. Kerr, W. N. McDonell, and S. S. Young, “Cardiopulmonary effects of romifidine/ketamine or xylazine/ketamine when used for short duration anesthesia in the horse,” Can. J. Vet. Res. Rev. Can. Rech. Veterinaire, vol. 68, no. 4, pp. 274–282, Oct. 2004.
 J. D. Combie, T. E. Nugent, and T. Tobin, “Pharmacokinetics and protein binding of morphine in horses,” Am. J. Vet. Res., vol. 44, no. 5, pp. 870–874, May 1983.
 M. Jeanne, C. Clément, J. De Jonckheere, R. Logier, and B. Tavernier, “Variations of the analgesia nociception index during general anaesthesia for laparoscopic abdominal surgery,” J. Clin. Monit. Comput., vol. 26, no. 4, pp. 289–294, Aug. 2012.
 Y. Fujiwara et al., “A greater decrease in blood pressure after spinal anaesthesia in patients with low entropy of the RR interval,” Acta Anaesthesiol. Scand., Aug. 2007.
 J. A. Armour, “Intrinsic cardiac neurons involved in cardiac regulation possess alpha 1-, alpha 2-, beta 1- and beta 2-adrenoceptors,” Can. J. Cardiol., vol. 13, no. 3, pp. 277–284, Mar. 1997.
 M. Tanaka and T. Nishikawa, “Arterial baroreflex function in humans anaesthetized with sevoflurane,” Br. J. Anaesth., vol. 82, no. 3, pp. 350–354, Mar. 1999.
 H. Schmidt et al., “Autonomic dysfunction predicts mortality in patients with multiple organ dysfunction syndrome of different age groups,” Crit. Care Med., vol. 33, no. 9, pp. 1994–2002, Sep. 2005.
 D. C. Silverstein, A. Pruett-Saratan II, and K. J. Drobatz, “Measurements of microvascular perfusion in healthy anesthetized dogs using orthogonal polarization spectral imaging: Microvascular perfusion measurements in normal dogs,” J. Vet. Emerg. Crit. Care, vol. 19, no. 6, pp. 579–587, Dec. 2009.
 T. Ledowski, L. Averhoff, W. S. Tiong, and C. Lee, “Analgesia Nociception Index (ANI) to predict intraoperative haemodynamic changes: results of a pilot investigation,” Acta Anaesthesiol. Scand., vol. 58, no. 1, pp. 74–79, Jan. 2014.
 D. Michaloudis et al., “The influence of premedication on heart rate variability,” Anaesthesia, vol. 53, no. 5, pp. 446–453, May 1998.
 D. Oberfeld and T. Franke, “Evaluating the robustness of repeated measures analyses: The case of small sample sizes and nonnormal data,” Behav. Res. Methods, vol. 45, no. 3, pp. 792–812, Sep. 2013.