R-R VARIABILITY DETECTS INCREASES IN VAGAL MODULATION WITH PHENYLEPHRINE INFUSION

High-frequency power, measured from power spectral analysis of R-R var iability, reflects vagal modulation of the sinus node. Unexpectedly, a recent study reported a decrease in high-frequency power during the i nfusion of phenylephrine despite a prolongation of R-R intervals, indi cating an increase in vagal activity. To better define the limitations of high-frequency power to quantify vagal modulation, we measured hig h-frequency power during the infusion of phenylephrine (0.4, 0.8, and 1.2 mu g . kg(-1) . min(-1)) into 10 normal subjects. We found increas ing doses of phenylephrine produced progressive increases in systolic blood pressure from 118 +/- 4 to 129 +/- 5 mmHg (P < 0.005), R-R inter vals from 881 +/- 44 to 1,274 +/- 69 ms (P < 0.0001), and the logarith m of high-frequency power from 5.83 +/- 0.22 to 7.73 +/- 0.24 ln(ms(2) ) (P < 0.0001). The conclusion was high-frequency power increases with increasing doses of phenylephrine. These data strongly support the ab ility of high-frequency power to detect an increase in vagal modulatio n during baroreceptor activation from an increase in systolic blood pr essure with the infusion of phenylephrine.