AUTONOMIC CONTROL OF SKIN MICROVESSELS - ASSESSMENT BY POWER SPECTRUMOF PHOTOPLETHYSMOGRAPHIC WAVES

1. Although it is well known that the microvessels of the skin constan tly undergo spontaneous variations in volume, the significance of thes e rhythmic changes remains uncertain, 2. In 10 healthy males and in 15 patients in intensive care, we assessed the origin of the autonomic i nfluences on spontaneous fluctuations in the microcirculation of the s kin, obtained by an infra-red photoplethysmographic device; we used sp ectral analysis techniques to compare these fluctuations (which were r ecorded simultaneously in two sites) with those of blood pressure, in order to test the presence of autonomic control of any synchronous flu ctuations in these different measurements from the cardiovascular syst em, In order to minimize mechanical fluctuations caused by occasional slow breaths, rather than nervously mediated fluctuations in skin bloo d flow, respiration was controlled at 15 breaths/min (0.25 Hz), 3. Spo ntaneous infra-red photoplethysmographic fluctuations were observed in different body areas (left index finger and left ear lobe, right and left index finger), and all were evident at 0.1 Hz, as well as respira tion-related components at 0.25 Hz, Active standing increased the powe r of the 0.1 Hz fluctuations (sympathetic activity) in both blood pres sure (from 62.7 +/- 7.1 to 79.2 +/- 3.7 normalized units, P < 0.05) an d IRP (finger: from 68.5 +/- 6.4 to 86.9 +/- 3.4 normalized units, P < 0.05; ear: from 59.0 +/- 5.9 to 88.1 +/- 2.0, P < 0.01), There was a high (> 0.5) coherence between the fluctuations obtained in blood pres sure, in IRP signals obtained simultaneously at the finger and at the ear, and in R-R interval, This synchronization between the oscillation s in all these signals, which were unrelated to the respiratory freque ncy or to the pulse rate, suggests a common neural, non-local origin, The phase between IRP and blood pressure was positive in the 0.1 Hz re gion (+ 1.65 +/- 0.41 radians, i.e. IRP was leading blood pressure, sh owing that 0.1 Hz fluctuations were not passively transmitted to the s kin microvessels from large arteries) and negative in the 0.25 Hz regi on (-0.74 +/- 0.19 radians, P < 0.01 compared with phase in the 0.1 Hz region, i.e. IRP was lagging behind blood pressure, suggesting possib le passive transmission to the skin microvessels of blood pressure flu ctuations caused by respiration), Fluctuations at lower frequency were observed in all IRP recordings, suggesting a local origin for these, intra-arterial and IRP fluctuations were compared in the 15 intensive care patients and gave similar results, 4. The skin microcirculation i s thus not only under local control, but also reflects changes in symp athetic activity; the effect of these changes on the skin microcircula tion can be easily evaluated by the spectral analysis of the IRP signa l obtained simultaneously in multiple areas, in conjunction with the s pectra of R-R interval and blood pressure.