We compared laser-Doppler velocimetry with plethysmographically determined changes in skin blood flow (SkBF) in five studies on four men. Increments in SkBF were induced by raising whole-body skin temperature to 39 degrees C for 50-70 min. We found laser-Doppler blood flow (LDF) to correlate well with total forearm blood flow (FBF) within each study (r = 0.94-0.98), but the relationship varied among studies. Thus the slopes for the LDF vs. FBF relationship varied from 40 to 122 mV X ml-1 X 100 ml X min. The value for LDF at zero FBF, extrapolated from the regression relationships, ranged from 246 to 599 mV above the value for LDF set with the probe on a stationary object. The value for LDF when blood flow to the arm was mechanically occluded ranged from 110 to 230 mV. In a second series, we measured the LDF values from six sites on forearms of each of four normothermic men. There was marked regional variation, with 1.8- to 5.7-fold ranges in LDF within a given subject. Values for LDF during occlusion of the forearm were more consistent within and between subjects. Thus LDF appears to provide a good indicator of the response pattern of SkBF from the region of illuminated skin. However, variability in the relationship to total SkBF (probably arising from variation in the number of perfused capillaries in the small volume of tissue) and uncertainties in the value of LDF at zero SkBF make quantitative use difficult.

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Two important optical properties of red blood cells (RBCs), their microscopic scattering cross sections sigma(s), and the mean cosine of their scattering angles micro, contribute to the optical behavior of whole blood. Therefore, the ability of Mie theory to predict values of sigma(s) and was tested by experiment. In addition, the effect of red blood cell size on sigma(s) and micro was investigated in two ways: (1) by studying erythrocytes from the dog, goat, and human, three species known to have different RBC sizes and (2) by allowing the RBCs from each species to shrink or swell osmotically. Values of sigma(s) obtained by measuring the collimated transmittance of dilute RBC suspensions illuminated with a He-Ne laser agreed with those predicted by Mie theory. Moreover, measured as values were directly proportional to RBC volume. By contrast, values of from Mie theory were consistently greater than those obtained experimentally by making angular scattering measurements in a goniometer. Thus Mie theory appears to yield adequate values for the RBC's microscopic scattering cross section, but by treating the RBC as a sphere with an equal volume, Mie theory fails to take the RBC's anisotropy into account and thus yields spuriously high values for micro.

To measure blood flow in the intestinal mucosa we built a laser-Doppler flowmeter that consists of a helium-neon laser, an electronic circuit, and a pair of fiber-optic light guides that conduct laser light to the tissue and carry the backscattered light to a photodetector. Because light scattered by moving red blood cells experiences a shift in its frequency, we measured blood flow by detecting the mean Doppler frequency. In isolated loops of canine small bowel, we raised perfusion pressure and found the increases in laser mucosal blood flow were significantly correlated with total blood flow measured by an electromagnetic probe. During infusions of isoproterenol (a selective vasodilator of the mucosa), laser mucosal blood flow increased before total flow increased. Similarly, adenosine (a selective dilator of the muscularis) increased total flow, whereas local mucosal blood flow fell or was unchanged. In addition, reactive hyperemia was sometimes observed in the mucosa but not in the muscularis. These observations indicate that the laser-Doppler technique measures blood flow in the surface tissue and does not reflect blood flow throughout the other tissues of the bowel wall. Instrumental problems identified in this study were 1) the difficulty of calibrating the laser mucosal blood flowmeter in absolute units, 2) the uncertainty of the volume of tissue in which local mucosal blood flow is measured, and 3) the problem of maintaining contact between the optical probe and the tissue. Nevertheless, the method holds great promise because it can detect small ischemic areas, because it could be used in combination with endoscopy, and because it yields a continuous measurement of blood flow in either the muscularis or mucosa.

In autoperfused preparations of feline jejunum, blood flow was measured from the mucosal surface with laser-Doppler velocimetry (LDV) and hydrogen gas (H2) clearance techniques while blood flow was altered by intra-arterial infusions of isoproterenol. LDV and H2 clearance estimates of blood flow were compared with total-wall and mucosal-submucosal blood flows measured with the radiolabeled microsphere technique. Over the range (26.3-73.6 ml X min-1 X 100 g-1) of blood flows attained, a series of direct linear relationships were obtained among LDV, H2 clearance, and microsphere estimates of jejunal blood flow. The slopes of these relationships indicated that the H2 clearance technique over-estimates total intestinal blood flow but reflects mucosal-submucosal flow as measured with microspheres. LDV measurements of blood flow from the mucosal surface were equally well correlated with total and mucosal-submucosal blood flow measured by microspheres, thereby not allowing for a definitive conclusion on the measurement depth of the LDV method. However, the ability of the LDV method to detect changes in blood flow in the perfused gut, even through 3 mm of unperfused tissue, casts a doubt on the assumption that the LDV method has a spatial resolution of less than 0.5-1.0 mm. The results of this study indicate that the H2 clearance technique can be used to measure mucosal blood flow in the small intestine. By contrast, the precise measurement depth of the LDV method is still uncertain and requires further evaluation.