Washout of a diluent bolus from canine hindlimb as an index of red cell transit time.

Abstract

Our goal was to identify heterogeneity of blood flow distribution in the canine hindlimb, particularly with onset of hypoxia. The red blood cells served as a non-diffusible tracer with the washout of a diluent bolus from the region. We used 2.5 ml of saline and recorded the wash-in and washout curves in flowing blood as changes in hematocrit. To do this, the blood flowed freely through an optical cuvette placed next to the vessel. The light source was a fiberoptic cable carrying light filtered for maximum transmission at 548 nm, an isobestic point for hemoglobin. The detector was a phototransistor opposed to the light source across a 2 mm light path. The output was linearized by a log amplifier and calibrated against hematocrit. This relatively simple system was adequately sensitive, stable, and was not affected by hemoglobin saturation or by flow rate. The fact that only a 2.5 ml bolus was used avoided potential problems of recirculation and changing baseline values because of the insignificant change in the whole body hematocrit.