R-R variability detects increases in vagal modulation with phenylephrine infusion.

High-frequency power, measured from power spectral analysis of R-R variability, reflects vagal modulation of the sinus node. Unexpectedly, a recent study reported a decrease in high-frequency power during the infusion of phenylephrine despite a prolongation of R-R intervals, indicating an increase in vagal activity. To better define the limitations of high-frequency power to quantify vagal modulation, we measured high-frequency power during the infusion of phenylephrine (0.4, 0.8, and 1.2 μg ⋅ kg-1 ⋅ min-1) into 10 normal subjects. We found increasing doses of phenylephrine produced progressive increases in systolic blood pressure from 118 ± 4 to 129 ± 5 mmHg ( P < 0.005), R-R intervals from 881 ± 44 to 1,274 ± 69 ms ( P < 0.0001), and the logarithm of high-frequency power from 5.83 ± 0.22 to 7.73 ± 0.24 ln(ms2) ( P < 0.0001). The conclusion was high-frequency power increases with increasing doses of phenylephrine. These data strongly support the ability of high-frequency power to detect an increase in vagal modulation during baroreceptor activation from an increase in systolic blood pressure with the infusion of phenylephrine.