Influence of nociceptive stimuli on heart rate variability in equine general anesthesia

Abstract

Heart rate variability (HRV) is a parameter to assess autonomic nervous system activity. Previous studies showed, that HRV in horses varied significantly between basal conditions and psychological stress and pain situations. In human medicine HRV is already used for acquisition of intraoperative nociceptive surgical stimuli. Aim of the present study was to identify the influence of nociceptive measures on autonomic nervous system in general anesthesia with HRV analysis. For this purpose enucleation and castration were taken as an example as often performed surgeries in the horse. According to the oculocardiac reflex horses undergoing ocular surgery con develop sudden bradyarrhythmias to the point of cardiac arrest because of vagal stimulation. A total of 20 horses were examined in general anesthesia (10 enucleations, 10 castrations). Pre-medication und method of general anesthesia were standardized. As pre-medication all horses obtained xylazinhydrochloride combined with levomethadone intravenously. General anesthesia was induced with ketamine und guaifenesine intravenously and after intubation maintained with isoflurane in oxygen (inhalative anesthesia). The ECG data were taken by a telemetric ECG before, during and after surgery and via Bluetooth recorded on a notebook. Afterwards HRV was assessed in 5 different stages (preoperative (T1), throughout stable anesthesia with isoflurane (T2), during surgical manipulation (T3), at nociceptive stimulation (T4) and after surgery (T5)) from ECG sequences of 5-minute-length via time and frequency domain analysis. As time related parameters mean heart rate, mean RR-interval and their standard deviations were calculated. As frequency related parameters LF-power (low-frequency, mainly representing sympathetic activity) and HF-power (high-frequency, representing parasympathetic activity) were evaluated. All horses in the enucleation group showed a significant heart rate decrease (p< 0,05) and a lower blood pressure decrease at severe traction on the external eye muscles. Simultaneously HF-power increased significantly (p= 0,007). Two horses developed a massive pulse and blood pressure decrease and atropine had to be applied intravenously. In the castration group heart rate increased only slightly at traction and penetration of the spermatic cord, but LF-power increased significantly (p=0,0 1). Nociceptive stimulation at enucleation leads to a vagale excitation due to a subliminal activation of the oculocardiac reflex, whereas at castration sympathetic excitation comes to the fore. Also during general anesthesia heart rate variability seems to be a sensitive and non-invasive parameter to assess sympathovagale stimulations on the autonomic nervous system. To draw conclusions from sufficient analgesia and depth of anesthesia further studies are requested.