Skin blood flow evaluated by the laser speckle method and the transcutaneous oxygen tension: interpretation of the dynamics using a simple model.

Spontaneous fluctuations of skin blood flow were measured by the laser speckle method in a time interval of 1024s. The frequency spectra of the resulting signal (blood flow parameter B) were calculated using the fast Fourier transformation. The mean spectrum was obtained from at least six single measurements on different days in the range 0.02 Hz-0.2 Hz. For five of six subjects no peak exists in the mean spectrum because it can be approximated by an exponential function with a mean frequency ranging between 0.05 Hz and 0.13 Hz (cycle length 7.7s - 20s). The dynamics of skin blood flow during ischaemia and reactive hyperaemia were simultaneously characterized by the blood flow parameter B and the transcutaneous oxygen tension p (24 measurements in 12 subjects). A simple theoretical model including a feedback control system for the stabilization of the tissue oxygen concentration is proposed. It provides an interpretation for the features of the dynamics of blood flow parameter B and transcutaneous oxygen tension p and their mutual relations, namely: (1) The increase in blood flow parameter B following the occlusion is determined by the arterial flow resistance. (2) The half-time for the decline of the blood flow parameter B from the peak of the reactive hyperaemia response to the final steady-state value and the existence of the spontaneous fluctuations of B indicate the active control of blood flow by the oxygen concentration in the tissue. (3) The large time constants of the oxygen tension equilibration during ischaemia and reactive hyperaemia originate from the capacity of the tissue to store oxygen. (4) The cycle length of the fluctuations (5s-50s) corresponds surprisingly well with the time constants for the decline of the blood flow parameter B from the peak to the steady-state (12.5s-42.5s).