Methods for the simultaneous measurement of pressure differentials and flow in single unbranched vessels of the microcirculation for rheological studies

Methods and illustrative applications for in vivo study of the rheological behavior of blood in the microcirculation are presented. Techniques were developed for the simultaneous measurement of pressure differential (drop) and red blood cell velocity in single unbranched microvessels ranging in luminal diameter from 7 to 54 μm. The servo-null micropressure technique was modified by hydraulically coupling two separate systems to a high-resolution differential pressure transducer to facilitate measurement of pressure drops to within ±0.02 cm H₂O. Simultaneous flow measurements of red cells plus plasma were made by a variation of the "two-slit" photometric technique. Measurements of these parameters and vessel geometry (length and diameter) in the mesentery of the cat permitted computation of rheological parameters such as resistance, resistivity, "apparent viscosity," and intravascular wall shear stress. The results indicate the persistence of a pulsatile component in pressure drop throughout all levels of the microvasculature. Apparent viscosity was found to increase dramatically in the "true capillaries," from 1.0 to 5.6 cP, as luminal diameter decreased from 9 μm to the size of a red blood cell, nominally 7 μm. The concomitant rise in capillary wall shear stress was from 18 to 40 dynes/cm². In larger microvessels, flow vs ΔP curves were established and found to be reasonably well represented by a Casson rheological model. Asymptotic apparent viscosities decreased from approximately 3.3 cP in arterioles and venules 45 to 54 μm in diameter to on the order of 2 cP in microvessels from 17 to 23 μm in diameter. For this illustrative sampling of data, wall shear stresses as great as 88 dynes/cm² were found in the immediate precapillary vessels, 23 μm in diameter.