| This series of studies examines the respiratory-heart rate interaction and evaluates a measure of this phenomenon, generated through spectral analysis, as a non-invasive estimate of neural influences on the heart. The respiratory influence on heart rate, respiratory sinus arrhythmia (RSA), is a naturally occurring cardiac arrhythmia in which heart rate fluctuates with a periodicity similar to respiration. Studies of the physiological mechanisms of RSA suggest that the primary efferent neural contribution to RSA is via the vagus. In fact, it has been suggested that the relationship between RSA and vagal activity is so strong, that RSA may be used as a non-invasive estimate of parasympathetic control of the heart.;amplitude of heart rate fluctuations around the mean heart rate.;However, in freely behaving, unanesthetized subjects RSA is often.;masked by other physiological influences on the heart. Therefore,;Traditionally, RSA has been quantified by measuring the.;spectral analysis, a time-series procedure, was used to partition the.;various influences on the heart by decomposing the heart period.;variance into its constituent frequencies. The heart period variability.;that occurs at the respiratory frequencies was defined as V, an(' ).;estimate of RSA.;Since RSA has been reported to be related to vagal activity, it.;was hypothesized that V would parallel changes in vagal control of(' ).;the heart. Studies are presented which examine changes in V, in(' ).;addition to other non-invasive estimates, heart period and heart.;period variance, as a function of manipulations of neural tone to.;the heart. Specifically, cardiac neural tone was manipulated by: (a) pharmacological manipulation of the baroreceptor reflex via injection or continuous infusion of phenylephrine or nitroprusside in rats and cats; and (b) electrical stimulation of barosensory fibers (aortic depressor nerve) in rabbits. In addition, the relative contributions of the sympathetic and parasympathetic nervous systems were examined following selective pharmacological blockade with beta-adrenergic blockers or atropine.;The synthesis of these studies suggests that in anesthetized and.;freely behaving animals, across species, using different types of.;neural tone manipulations, V is a robust, predictable estimate of(' ).;cardiac vagal tone. Moreover, V appears to be less influenced by(' ).;extra-vagal factors than other non-invasive cardiac measures, specifically heart period and heart period variance. |
