A between-subjects comparison of respiratory sinus arrhythmia and baroreceptor cardiac reflex sensitivity as non-invasive measures of tonic parasympathetic cardiac control.

Respiratory sinus arrhythmia (RSA) has been used as an index of parasympathetic cardiac control. However, recent psychophysiological research casts serious doubts upon the usefulness of RSA as an index of vagal influences upon the heart in psychophysiological as well as in clinical studies. It suggests the need to look for another measure. In this exploratory study we investigated whether the baroreflex sensitivity (BRS) could serve as an alternative tool to investigate between-subject tonic parasympathetic influences on the heart. In nine healthy subjects we examined the effects of intravenous atropine (0.03 mg/kg i.v.), intravenous metoprolol (10-15 mg i.v.), and of saline as a placebo condition upon RSA, BRS, and related cardiovascular and respiratory variables, both under resting and under mental task conditions. After parasympathetic blockade, RSA and BRS display values near zero, showing their vagal origin. After beta-adrenergic blockade, when heart period is predominantly under vagal control, RSA fails to predict heart period variability. Using BRS, however, it is possible to predict more than 97% of heart period variance during beta-blockade. Finally, both the vagal and beta-adrenergic blockade show that BRS is a better predictor of parasympathetic cardiac control during blood pressure increases than during blood pressure decreases.

RCT Entities:   Population Interventions Outcomes