Relationship of Effective Circulating Volume with Sublingual Red Blood Cell Velocity and Microvessel Pressure Difference: A Clinical Investigation and Computational Fluid Dynamics Modeling

Abstract

The characteristics of physiologic hemodynamic coherence are not well-investigated. We examined the physiological relationship between circulating blood volume, sublingual microcirculatory perfusion, and tissue oxygenation in anesthetized individuals with steady-state physiology. We assessed the correlation of mean circulatory filling pressure analogue (Pmca) with sublingual microcirculatory perfusion and red blood cell (RBC) velocity using SDF+ imaging and a modified optical flow-based algorithm. We also reconstructed the 2D microvessels and applied computational fluid dynamics (CFD) to evaluate the correlation of Pmca and RBC velocity with the obtained pressure and velocity fields in microvessels from CFD (pressure difference, (Δp)). Twenty adults with a median age of 39.5 years (IQR 35.5−44.5) were included in the study. Sublingual velocity distributions were similar and followed a log-normal distribution. A constant Pmca value of 14 mmHg was observed in all individuals with sublingual RBC velocity 6−24 μm s−1, while a Pmca < 14 mmHg was observed in those with RBC velocity > 24 μm s−1. When Pmca ranged between 11 mmHg and 15 mmHg, Δp fluctuated between 0.02 Pa and 0.1 Pa. In conclusion, the intact regulatory mechanisms maintain a physiological coupling between systemic hemodynamics, sublingual microcirculatory perfusion, and tissue oxygenation when Pmca is 14 mmHg.

Keywords: anesthesia; cardiovascular dynamics; hemodynamic coherence; hemodynamics; microcirculation; oxygen transport; physiology; red blood cell velocity; tissue perfusion.

Figure 1

(A) is the reconstructed microvessel from a recorded sublingual microcirculation video; (B) is the obtained flow field after application of computational fluid dynamics (the arrow shows the flow direction); and (C) is the obtained pressure field after application of computational fluid dynamics for the specific microvessel.

Figure 2

Probability density functions of velocity distributions. Note the velocity values (mean) and the standard deviation (SD). Transverse axis: RBC velocity (pix dt^-1); Vertical axis: Probability density function (dimensionless).

Figure 3

Correlation of Pmca with sublingual microcirculation RBC velocity in individuals with steady-state physiology, effective coupling between the macro- and microcirculation, and normal tissue oxygen extraction ratio. A constant Pmca of 14 mmHg was observed in individuals with RBC velocity 6–24 μm s⁻¹, while a Pmca of <14 mmHg was observed in those with RBC velocity > 24 μm s⁻¹. Pmca, mean circulatory filling pressure analogue; RBC, red blood cell.

Figure 4

Computational fluid dynamics simulation: (A) correlation between Pmca and mean RBC velocity; (B) correlation between Pmca and maximum RBC velocity in the domain; and (C) correlation between Pmca and pressure difference (Δp).