Dm. Bloomfield et al., R-R VARIABILITY DETECTS INCREASES IN VAGAL MODULATION WITH PHENYLEPHRINE INFUSION, American journal of physiology. Heart and circulatory physiology, 43(5), 1998, pp. 1761-1766
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.