Relationship of heart rate variability to parasympathetic effect

Background-Baroreflex-mediated parasympathetic stimulation has variable eff ects on heart rate variability (HRV). We postulated that a quadratic functi on would describe the relationship between HRV and parasympathetic effect b etter than a linear function. Methods and Results-Twenty-nine normal volunteers (15 women; mean age 39 +/ - 12 years) were studied after P-adrenergic blockade with intravenous propr anolol. Five-minute ECG recordings were made during graded infusions of phe nylephrine and nitroprusside to achieve baroreflex-mediated increases and d ecreases in parasympathetic effect, respectively. Time- and frequency-domai n measures of HRV were calculated from the R-R interval tachograms. The R-R interval and the vagal-sympathetic effect (VSE=R-R interval/intrinsic R-R interval) were used as indices of parasympathetic effect. The data were fit to both quadratic and linear models. In each case, the quadratic model (wi th a negative coefficient for the squared term) was superior to the linear model. There was some evidence that age influenced the responsiveness of th e HRV parameters with changing parasympathetic effect, although the regress ion analysis was significant only in the models for MSSD (P<0.03) and pNNSO (P<0.001). Conclusions-The relationship between HRV and parasympathetic effect is best described by a function in which there is an ascending limb where HRV incr eases as parasympathetic effect increases until it reaches a plateau level: HRV then decreases as parasympathetic effect increases. Because there is m arked interindividual variation in this relationship, differences in HRV be tween individuals may reflect differences in this relationship and/or diffe rences in autonomic effects.