Effect of erythrocyte aggregation and flow rate on cell-free layer formation in arterioles

Abstract

Formation of a cell-free layer is an important dynamic feature of microcirculatory blood flow, which can be influenced by rheological parameters, such as red blood cell aggregation and flow rate. In this study, we investigate the effect of these two rheological parameters on cell-free layer characteristics in the arterioles (20-60 mum inner diameter). For the first time, we provide here the detailed temporal information of the arteriolar cell-free layer in various rheological conditions to better describe the characteristics of the layer variation. The rat cremaster muscle was used to visualize arteriolar flows, and the extent of aggregation was raised by dextran 500 infusion to levels seen in normal human blood. Our results show that cell-free layer formation in the arterioles is enhanced by a combination of flow reduction and red blood cell aggregation. A positive relation (P < 0.005) was found between mean cell-free layer widths and their corresponding SDs for all conditions. An analysis of the frequency and magnitudes of cell-free layer variation from their mean value revealed that the layer deviated with significantly larger magnitudes into the red blood cell core after flow reduction and dextran infusion (P < 0.05). In accordance, the disparity of cell-free layer width distribution found in opposite radial directions from its mean became greater with aggregation in reduced flow conditions. This study shows that the cell-free layer width in arterioles is dependent on both flow rate and red blood cell aggregability, and that the temporal variations in width are asymmetric with a greater excursion into the red blood cell core than toward the vessel wall.

Fig. 1.

Example of temporal variation of cell-free layer width in a 47.1-μm inner diameter (ID) arteriole. Dynamic variations of the cell-free layer width can be observed on both sides of its mean (dashed line), extending either toward vessel wall, or into the red blood cell (RBC) core.

Fig. 2.

Relationship between standard deviation (SD) and mean value of the cell-free layer width before (○) and after (●) aggregation induction. A: normal flow condition. B: reduced flow condition. Linear regression fits (y = 0.09x + 0.51, R² = 0.16 for normal flow, and y = 0.31x + 0.23, R² = 0.24 for reduced flow) are used for combined groups, since no significant effect of dextran infusion is found in either flow condition.

Fig. 3.

A and B: normalized mean value and SD, respectively, of cell-free layer width variations at normal and reduced flow conditions, before and after aggregation induction. C and D: normalized mean value and SD, respectively, of cell-free layer width variations at different pseudoshear rates, before and after aggregation induction. M, aggregation index. *P < 0.05, **P < 0.02, and ***P < 0.001.

Fig. 4.

Frequency of cell-free layer width deviations toward vessel wall. A: relation between the frequency of cell-free layer width deviations toward vessel wall and mean cell-free layer width. The different rheological conditions are represented by ○, normal flow (M = 0); ▵, reduced flow (M = 0); □, normal flow (M = 11.9 ± 3.0), and ×, reduced flow (M = 11.9 ± 3.0). No significant relation is found for all conditions. B: effect of flow rate and RBC aggregation on the frequency of cell-free layer width deviations toward vessel wall. There is no flow rate or aggregation effect on frequency of cell-free layer width deviations.

Fig. 5.

Frequency distribution of cell-free layer variations from the vessel wall. A box plot is utilized in each condition to describe the distribution. A dashed line indicates the mean width of cell-free layer, while the solid line in the box corresponds to the median cell-free layer width value. A and C: normal and dextran-treated rats in normal flow condition, respectively. B and D: normal and dextran-treated rats in reduced flow condition, respectively. Cell-free layer widths deviate with larger magnitudes into RBC core than toward the vessel wall about the mean layer width. This disparity is significantly enhanced (P < 0.05), especially with dextran treatment in reduced flow condition. The total frequency of the cell-free layer widths on each side of the mean are considered as 100%. The reduction of flow increases the frequency of cell-free layer deviations with magnitudes >1.5 μm toward both the vessel wall and the RBC core. All units are in μm.