1. We have developed a simple noninvasive method to assess the spontaneous baroreflex (BRS; reflex heart rate response to change in systemic blood pressure) in man. The BRS is impaired in many cardiovascular disorders and is known to be of major prognostic significance; however, the invasive nature of traditional BRS assessment has thus far limited its clinical application. 2. Sixteen healthy subjects (7M, 9F; age 28 +/- 3 years) were studied at baseline and during stepwise i.v. infusion of phenylephrine and bolus i.v. dosing of nitroprusside. Invasive BRS was derived from linear regression of the change in systolic blood pressure (SBP) and RR interval from baseline following these perturbations. Noninvasive BRS was derived from approximately 1500 gated beat-to-beat SBP and RR interval data points acquired by Finapres BP and continuous ECG monitoring. Slopes were derived from linear regression of three consecutive baroreflex-mediated data points (following a phase shift of one RR interval from its accompanying SBP value) and were averaged to determine BRS. Correlations between invasive and noninvasive BRS measures were found to be highly significant (r = 0.78; P = 0.0009; slope = 0.87). 3. Significant correlations between noninvasively determined BRS and heart rate variability derived measures of autonomic activity (24h standard deviation of normal RR intervals, root mean square of successive RR intervals, low frequency and high frequency power) were also observed (r = 0.79-0.83; P = 0.003-0.008). 4. These results support the validity of this noninvasive method of measurement of BRS in man and of parasympathetic contribution to spontaneous baroreflex.

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