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Assessing the sensitivity of spontaneous baroreflex control of the heart: deeper insight into complex physiology.

To the Editor:

While several papers support the physiological and clinical relevance of indices quantifying the sensitivity of spontaneous baroreflex control of heart rate (BRS),1 Lipman et al2 claim that they are unable to properly explore baroreflex function because spontaneous BRS was found to be quantitatively different from BRS values provided by the vasoactive drug injection technique and is unrelated to common carotid artery distensibility. We believe that this conclusion is not supported by Lipman’s data, for the following reasons. 

1. The time honored (and still valuable) method for assessing BRS through vasoactive drugs injection3,4 cannot be a “gold standard”5 due to pharmacological alteration of baroreceptor activity through both changes in blood pressure (BP) and unquantifiable mechanical distortion of the vessel;6 direct drug effect on the sinus node;7 simultaneous stimulation of cardiopulmonary stretch receptors; and limited reproducibility of the few spot BRS estimates obtainable.1,8

2. Pharmacological and spontaneous BRS values are significantly correlated in most instances,9 as demonstrated by Parlow et al in a study10 not cited by Lipman et al. Moreover, spontaneous and drug-related BRS values display directionally similar changes under different conditions, thus reflecting virtually superimposable baroreflex physiology. …