Changes in Respiratory Rate, Blood Pressure and Heart Rate Variability in Rabbits during Orthostasis

Mokr˘ J, T. RemeAova, K. Javorka: Changes in Respiratory Rate, Blood Pressure and Heart Rate Variability in Rabbits during Orthostasis. Acta Vet. Brno 2006, 75: 3-12. The purpose of the study was to evaluate the changes of respiratory rate, systemic blood pressure and heart rate variability parameters (HRV) during orthostasis in anaesthetized rabbits. Furthermore, these changes were influenced by affecting the renin-angiotensin-aldosterone (RAA) system and autonomic nervous system (ANS) to study the mechanisms participating in activity of spectral frequency bands of HRV in rabbits. Ten adult rabbits (Chinchilla) were anaesthetized by ketamine and flunitrazepam. The systemic blood pressure, tidal volume and respiratory rate were measured. HRV was evaluated by microcomputer system VariaPulse TF3E. The R-R intervals were derived from the electrocardiogram signal from subcutaneous needle electrodes. The evaluation of HRV in very low (VLF; 0.01-0.05 Hz), low (LF; 0.05-0.15 Hz) and high frequency bands (HF; 0.15-2.0 Hz) was made and parameters of frequency and time analysis were calculated. The measurements were made in horizontal (supine) position, in orthostasis (the angle of 60 °) and again in supine position before and after enalapril (0.5 mg/kg b.w.), metipranolol (0.2 mg/kg b.w.), and after subsequent bilateral cervical vagotomy. The orthostasis in anaesthetized rabbits is accompanied by depression of respiratory rate reversed only by vagotomy. Furthermore, decrease of systemic blood pressure, unchanged heart rate and increased characteristics of heart rate variability were found, with predominant increase of spectral power in LF and VLF bands. This elevation can be eliminated only by complete blockade of ANS. Although the participation of ANS or RAA system in modification of individual HRV frequency bands is not as specific as in humans, we confirmed the participation of RAA system in determination of the VLF band. Heart rate variability, renin-angiotensin-aldosteron, blood pressure, respiratory rate, autonomic nervous system, enalapril, rabbits Orthostatic test is generally used for evaluation of the reactivity of autonomous nervous system (ANS), endocrine system and cardiovascular system to stress. Changing the position from lying to standing results in the blood accumulation in capacity vessels (lower extremities, pelvic and gluteal area) below the level of the heart. This leads to diminishing of the venous return, cardiac filling and cardiac output as well as decrease in systemic arterial blood pressure. The compensatory mechanisms are mediated via reflexes and humoral substances to keep adequate organ perfusion during orthostasis. The reflex mechanisms are activated by lower afferent impulses from baroreceptors with almost immediate commence. The humoral mechanisms are, however, much slower and they start to participate on compensation after prolonged orthostasis. The changes of respiration during orthostasis are still not completely known. The body transition from supine to orthostatic position may lead to slowing down or temporary apnea via Hering-Breuer inflation reflex by a decrease of abdominal organs and traction through