[Cardiovascular neuroregulation and rhythms of the autonomic nervous system: frequency domain analysis]

Studies using spectral analysis of cardiovascular variability series, as a non invasive tool for assessing the autonomic nervous system activity, have attracted growing interest in the last 20 years. Short-term recordings of heart rate variability distinguish two main spectral components: a high-frequency (HF) component (ranging between 0.15-0.40 Hz), and a low-frequency (LF) component (ranging between 0.04-0.15 Hz), respectively considered markers of parasympathetic and sympathetic control. Spectral analysis of microcirculatory blood flow, by using laser Doppler flowmetry, recently disclosed the presence of similar rhythms. Although a general agreement about the amplitude and frequency of these spectral components has not been gained yet, evidences of an autonomic nervous system role have been collected. Also the pupil diameter spontaneously fluctuates in the HF range. The aim of this study was to design an experimental set up for the simultaneous and continuous recording of heart rate, blood pressure, respiratory activity, peripheral blood flow, and pupil diameter fluctuations in order to investigate whether common spontaneous rhythms could be detected into the variability series. We enrolled 10 normal volunteers (range 24-28 years). The variability series of heart rate, blood pressure, respiratory activity, peripheral blood flow and pupil diameter were obtained at rest and during sympathetic activation by head-up tilt test 70 degrees. Subjects were instructed to breath at 15 breath/min following an audio signal and adjusting their tidal volume to a comfortable level. Peripheral blood flow was non invasively monitored with laser Doppler technique, with the two probes located in the palmar site of the hand and in the sternum. We continuously assess the pupil diameter fluctuations by a custom, portable, infrared TV pupillometer. Spectral density was estimated by autoregressive modeling on 250-point segments. The respiratory rhythm was clearly detected in all the variability series. Oscillations in the LF band were detected in heart rate, blood pressure and in the laser Doppler signals of the hand, as well as in the pupil diameter, but not always in the laser Doppler flow of the sternum. In this last signal an harmonic component at 0.16-0.17 Hz was detected. A reduction in the total variability, with a relative increase in the LF component was observed during head-up tilt test in the heart rate, blood pressure, peripheral blood flow and pupil diameter signals. These findings are consistent with the increased sympathetic modulation induced by the head-up tilt test. These data confirm previous findings and observations, collected with different techniques and for different purposes. Whether these rhythms can be expression of central common oscillators or reflex mechanical factors is of primary importance for possible clinical applications of this approach.