Methods of assessing vagus nerve activity and reflexes

Abstract

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

Cold-impaired cardiac performance in rats is only partially overcome by cold acclimation

“…A profound decrease in sympathetic tone was noted for control rats at T b 25°C, which was attenuated in CA rats, implying better preservation of autonomic control on acute cooling, as also indicated by the relatively preserved LF/HF ratio for CA vs control rats. Although LF/HF is readily accepted as a measure of sympathovagal balance, we do acknowledge that under different physiological conditions vagal modulation could significantly alter LF (Fazan et al, 2005;Jokkel et al, 1995;Chapleau and Sabharwal, 2011). However, in this study the changes in LF/HF are primarily the result of a reduction in T b , as on rewarming the ratio of LF/HF was returned to pre-cooling levels; this is further supported by blood pressure variability estimates showing similar restoration.…”

Non-invasive Vagus Nerve Stimulation in Healthy Humans Reduces Sympathetic Nerve Activity

“…Ectopic beats could be corrected using a linear spline to average the R- (21). The ratio of low frequency (LF) and high frequency (HF) oscillations of heart rate variability can be used as an index of cardiac autonomic balance such that a decrease in LF/HF ratio indicates a shift in cardiac autonomic balance towards parasympathetic predominance and thus an improvement in HRV (20,22). It is important to note that this may be due to either a decrease in sympathetic activity or an increase in parasympathetic activity.…”

Increase in parasympathetic tone by pyridostigmine prevents ventricular dysfunction during the onset of heart failure

“…All recordings were carried out in conscious freely moving rats during approximately 1 h. Basal AP was recorded during 30 min followed by measurement of the autonomic tone (vagal and sympathetic) by means of methylatropine and propranolol (12). The HR measured when both autonomic blockers were administered was considered the intrinsic HR (iHR).…”