An association between vasomotion and oxygen extraction

摘要

Vasomotion is defined as a spontaneous local oscillation in vascular tone whose function is unclear but may have a beneficial effect on tissue oxygenation. Optical reflectance spectroscopy and laser Doppler fluximetry provide unique insights into the possible mechanisms of vasomotion in the cutaneous microcirculation through the simultaneous measurement of changes in concentration of oxyhemoglobin ([HbO_2]), deoxyhemoglobin ([Hb]), and mean blood saturation (S_(mb)O_2) along with blood volume and flux. The effect of vasomotion at frequencies <0.02 Hz attributed to endothelial activity was studied in the dorsal forearm skin of 24 healthy males. Fourier analysis identified periodic fluctuations in S_(mb)O_2 in 19 out of 24 subjects, predominantly where skin temperatures were >29.3°C (X2 = 6.19, P < 0.02). A consistent minimum threshold in S_(mb)O_2 (mean: 39.4%, range: 24.0-50.6%) was seen to precede a sudden transient surge in flux, inducing a fast rise in S_(mb)O_2. The integral increase in flux correlated with the integral increase in [HbO_2] (Pearson's correlation r~2 = 0.50, P < 0.001) and with little change in blood volume suggests vasodilation upstream, responding to a low S_(mb)O_2 downstream. This transient surge in flux was followed by a sustained period where blood volume and flux remained relatively constant and a steady decrease in [HbO_2] and equal and opposite increase in [Hb] was considered to provide a measure of oxygen extraction. A measure of this oxygen extraction has been approximated by the mean half-life of the decay in S_(mb)O_2 during this period. A comparison of the mean half-life in the 8 normal subjects [body mass index (BMI) <26.0 kg/m~2] of 12.2 s and the 11 obese subjects (BMI >29.5 kg/m~2) of 18.8 s was statistically significant (Mann Whitney, P < 0.004). The S_(mb)O_2 fluctuated spontaneously in this saw tooth manner by an average of 9.0% (range 4.0-16.2%) from mean S_(mb)O_2 values ranging from 30 to 52%. These observations support the hypothesis that red blood cells may act as sensors of local tissue hypoxia, through the oxygenation status of the hemoglobin, and initiate improved local perfusion to the tissue through hypoxic vasodilation.