  The variation of blood flow characteristics caused by the probe pressure
during noninvasive studies is of particular interest within the context of
fundamental and applied research. It has been shown previously that the weak
local pressure induces vasodilation, whereas the increased pressure is able to
stop the blood flow in the compressed area, as well as to significantly change
optical signals.
  The blood flow oscillations measured by laser Doppler flowmetry (LDF)
characterize the functional state of the microvascular system and can be used
for noninvasive diagnostics of its abnormality. This study was intended to
identify the patterns of the relationship between the oscillating components of
blood flow registered by the LDF method under different levels of pressure
applied to an optical fiber probe.
  For this purpose we have developed an original optical probe capable of
regulating the applied pressure. The developed protocol included six sequential
records of the blood perfusion at pressure within the 0 to 200 mmHg range with
unloading at the last stage.
  Using wavelet analyses, we traced the variation of energy of oscillations for
these records in five frequency bands associated with different vascular tone
regulation mechanisms. Six young volunteers of the same age (three males and
three females) were included in this preliminary study and the protocol was
repeated five times in each volunteer. Totally 30 LDF records were available
for the analyses. As expected, the LDF signal increases at weak pressure (30
mmHg) and decreases at increased pressure. The statistically stable
amplification of endothelial associated blood flow oscillations under the 90
mmHg pressure allowed us to put forward a hypothesis that the endothelial
activity increases. The possible causes of this phenomenon are discussed.
