A technique for the estimation of plasma flow in single capillaries using photobleached dyes

In order to estimate plasma flow in single capillaries, an “indicator bolus” was optically inserted into individual microvessels of the hamster cremaster muscle. This was accomplished using short-duration (200 msec), argon laser pulses to photobleach a 5-μm segment of fluorochrome circulating with the plasma. The subsequent motion of the bleached plasma bolus was then tracked using photomultipliers positioned at three sites along the capillary. The transient passage of the dye appeared as a steep fall in light intensity as the downstream edge of the bleached area entered the sensor field, followed by a steep rise in light intensity as trailing unbleached plasma flowed under the sensor. The behavior of light intensity as the photobleached bolus flowed past a sensor was analyzed using a theoretical model developed to predict the behavior of this type of plasma flow indicator in single capillaries. The characteristic time, τ which equals the capillary segment volume divided by the plasma flow, was taken as an estimate of plasma flow. The model predicts that, for this system, τ of the capillary corresponds closely to the time at which 50% of the full sensor response to the bolus is attained, that is, the t₅₀ . The ratio between the t₅₀ and the characteristic time is found to be a function of the relative sensor width and the flow velocity profile. A procedure is also described to assess the flow velocity profile from in vivo measurements. Using this technique, the ratio of the velocity of the red cell compared to that of plasma is found to be about 1.3.