A Comparison of Ultrasound and Laser Doppler for Measuring Capillary Recruitment Using Models in vitro

Abstract

PURPOSE: Contrast enhanced ultrasound (CEU) and laser Doppler flowmetry (LDF) have been used to measure capillary recruitment in vivo in muscle and skin. However, it is imperative that only change in capillary surface area is measured and that measurements are not influenced by change in flow. Accordingly, we compare two methods: CEU and LDF, using artificial capillary tubing systems. METHODS: Phospholipid microbubbles, as contrast medium (approx. 2×108/ml, CEU), or red blood cells (3-9% hematocrit, LDF) were used in isotonic saline. Delivery was by syringe pump and CEU measurements were made using a HDI 5000 with L7-4MHz transducer at a mechanical index of 1.0 that was capable of destroying all bubbles in the ultrasound beam to construct a pulsing interval (time) versus video-intensity curve and calculation of capillary tubing volume (CTV) in the field of interest as well as tubing flow velocity (CTFV). LDF was conducted using a Perimed PeriFlux PF2 operating at 632 nm with a Pf 108 probe. For CEU we used microdialysis tubing (280 μm) and for LDF, polymer tubes of 250, 100 and 50 μm. Measurements were made when tubing diameter was varied with volume delivery rate (VDR) constant. VDR was varied with tubing diameter constant. Tubing capillary surface area was increased by connecting lengths of tubing in zig-zag series, or by flow-sharing with lengths of tubing connected to a manifold (VDR was constant for each, and each pattern was in the field of interest). RESULTS: CEU estimates of CTV were not affected by flow rate and increased whether tubing surface area was increased by connecting tubes in series or parallel. CEU measured changes in CTFV followed prediction. The LDF signal decreased as the diameter of the capillary tubes increased at constant VDR. However, the LDF signal increased as the VDR increased and as the surface area was increased by zig-zag lengths of capillary in series, but it decreased overall when surface area was increased with capillary tubes in parallel. CONCLUSIONS: These modeling studies indicate that CEU measures changes in capillary recruitment and is not affected by VDR. LDF signal derives mostly from non-vectorial speed and should be used with caution to measure capillary recruitment and may fail to detect capillary recruitment if manifold sharing occurs and mean cell speed falls. CEU can measure increases in capillary surface area that occur in exercise or following insulin action and may provide information as to whether capillary recruitment results from either flow redistribution from short to tortuous capillaries and/or by flow sharing into capillaries of similar properties.