Pulsatile characteristics of microvascular blood flow in single vessels

Numerous studies by direct intravital microscopy have measured the presence of substantial pulsatile components in intravascular pressures and red cell velocities in arterioles, capillaries and venules of the microcirculation. However, few studies have attempted to understand the significance of hemodynamic pulsatility in light of the non-Newtonian blood rheology and the viscoelastic characteristics of the blood vessel wall. To fill this void, studies of the microcirculation are conducted to measure intravascular pressures (P) and pressure gradients (Δ P/1, dual-servo-null method) and RBC velocities (V_rbc, two-slit technique) in the mesentery of the cat. On line digitization of these parameters and simultaneous recordings of the EKG permitted signal averaging in concert with the r-r interval to obtain representative recordings of their pulsatile components. The results clearly demonstrate the presence of a nearly 90 degree phase shift between ΔP/1 and V_rbc as red cells traverse the network from arterioles to venules, presumably due to an increase in microvessel compliance in postcapillary venules. To understand the effects of pulse pressure on the rheology of the microvessel wall, microocclusion experiments were performed to measure P at the proximal end of a vessel while occluding the distal end with a blunt probe. Analysis of the oscillating red cell column trapped within the a vessel from digitized video recordings revealed significant displacements of the vessel wall on the order of 0.01 to 0.05 μm. Preliminary data appear to support the conclusion of greater venule compliance compared to arterioles. The red cell column, however, appears to oscillate in a rigid body motion suggesting that compliance of the vessel wall is primarily responsible for the phasic relationships between pressure gradient and flow of the microvasculature.