Changes in Heart Period, Heart‐Period Variability, and a Spectral Analysis Estimate of Respiratory Sinus Arrhythmia in Response to Pharmacological Manipulations of the Baroreceptor Reflex in Cats

ABSTRACT The purpose of the present study was to assess changes in heart period, heart‐period variance (HPV), and respiratory sinus arrhythmia (RSA) during manipulations of the baroreceptor reflex in anesthetized cats. Hypertension, induced via phenylephrine infusion, reflexly increases parasym‐pathetic activity and decreases sympathetic activity. Hypotension, produced by infusion of nitroprusside, leads to increased sympathetic activity and an inhibition of vagal influences on the heart. Specific autonomic contributions were assessed following administration of practolol, a β‐adrenergic blocker, or atropine methyl nitrate. Spectral analysis was used to quantify the component of HPV associated with respiration, i.e., RSA, which is proposed to be sensitive to vagal influences on the heart. The respiratory component of HPV is described by a statistic, V̂, which is the sum of the spectral densities of the heart period spectrum across the band of frequencies associated with normal respiration. Hypertension produced an increase in V̂ which was blocked by atropine infusion, but unaltered by beta‐adrenergic blockade. Hypotension decreased V̂ to near‐zero values which persisted after atropine and practolol infusion. These data suggest that V̂ is determined primarily by vagal factors. Correlations between V̂ and a previously used criterion measure of vagal tone further support this hypothesis. In contrast, heart period was sensitive to vagal and sympathetic factors. Heart period increases during hypertension were abolished with atropine and heart period decreases due to hypotension were eliminated by beta‐adrenergic blockade. These responses are consistent with the notion that heart period is under the control of both sympathetic and vagal factors. Since in an anesthetized preparation RSA is the major source of heart‐period variability, HPV responded similarly to V̂. The results suggest that the spectral estimate of RSA, V̂, is particularly sensitive to blood pressure induced reflexive changes in vagal efferent influences on the heart. It also appears that V̂ is less influenced by sympathetic factors than heart period, a commonly used estimate of vagal activity.