Axel R. Pries

Resistance to blood flow in microvessels in vivo.

Resistance to blood flow through peripheral vascular beds strongly influences cardiovascular function and transport to tissue. For a given vascular architecture, flow resistance is determined by the rheological behavior of blood flowing through microvessels. A new approach for calculating the contribution of blood rheology to microvascular flow resistance is presented. Morphology (diameter and length), flow velocity, hematocrit, and topological position were determined for all vessel segments (up to 913) of terminal microcirculatory networks in the rat mesentery by intravital microscopy. Flow velocity and hematocrit were also predicted from mathematical flow simulations, in which the assumed dependence of flow resistance on diameter, hematocrit, and shear rate was optimized to minimize the deviation between measured and predicted values. For microvessels with diameters below approximately 40 microns, the resulting flow resistances are markedly higher and show a stronger dependence on hematocrit than previously estimated from measurements of blood flow in narrow glass tubes. For example, flow resistance in 10-microns microvessels at normal hematocrit is found to exceed that of a corresponding glass tube by a factor of approximately 4. In separate experiments, flow resistance of microvascular networks was estimated from direct measurements of total pressure drop and volume flow, at systemic hematocrits intentionally varied from 0.08 to 0.68. The results agree closely with predictions based on the above-optimized resistance but not with predictions based on glass-tube data. The unexpectedly high flow resistance in small microvessels may be related to interactions between blood components and the inner vessel surface that do not occur in smooth-walled tubes.

Blood flow in microvascular networks. Experiments and simulation.

A theoretical model has been developed to simulate blood flow through large microcirculatory networks. The model takes into account the dependence of apparent viscosity of blood on vessel diameter and hematocrit (the Fahraeus-Lindqvist effect), the reduction of intravascular hematocrit relative to the inflow hematocrit of a vessel (the Fahraeus effect), and the disproportionate distribution of red blood cells and plasma at arteriolar bifurcations (phase separation). The model was used to simulate flow in three microvascular networks in the rat mesentery with 436,583, and 913 vessel segments, respectively, using experimental data (length, diameter, and topological organization) obtained from the same networks. Measurements of hematocrit and flow direction in all vessel segments of these networks tested the validity of model results. These tests demonstrate that the prediction of parameters for individual vessel segments in large networks exhibits a high degree of uncertainty; for example, the squared coefficient of correlation between predicted and measured hematocrit of single vessel segments ranges only between 0.15 and 0.33. In contrast, the simulation of integrated characteristics of the network hemodynamics, such as the mean segment hematocrit or the distribution of blood flow velocities, is very precise. In addition, the following conclusions were derived from the comparison of predicted and measured values: 1) The low capillary hematocrits found in mesenteric microcirculatory networks as well as their heterogeneity can be explained on the basis of the Fahraeus effect and phase-separation phenomena. 2) The apparent viscosity of blood in vessels of the investigated tissue with diameters less than 15 microns is substantially higher than expected compared with measurements in glass tubes with the same diameter.

Biophysical aspects of blood flow in the microvasculature

The main function of the microvasculature is transport of materials. Water and solutes are carried by blood through the microvessels and exchanged, through vessel walls, with the surrounding tissues. This transport function is highly dependent on the architecture of the microvasculature and on the biophysical behavior of blood flowing through it. For example, the hydrodynamic resistance of a microvascular network, which determines the overall blood flow for a given perfusion pressure, depends on the number, size and arrangement of microvessels, the passive and active mechanisms governing their diameters, and on the apparent viscosity of blood flowing in them. Suspended elements in blood, especially red blood cells, strongly influence the apparent viscosity, which varies with several factors, including vessel diameter, hematocrit and blood flow velocity. The distribution of blood flows and red cell fluxes within a network, which influences the spatial pattern of mass transport, is determined by the mechanics of red cell motion in individual diverging bifurcations. Here, our current understanding of the biophysical processes governing blood flow in the microvasculature is reviewed, and some directions for future research are indicated.

Red Cell Distribution at Microvascular Bifurcations

The distribution of red cell and blood volume flow was studied at 65 arteriolar bifurcations in the rat mesentery. Hematocrit and flow velocity were measured simultaneously in all three vessel segments constituting a bifurcation. Blood flow distribution was manipulated by irreversibly occluding downstream side branches of one of the daughter vessels. The dependence of fractional red cell volume flow on fractional blood flow was described using a three-parameter (X0, B, A) logit function. The critical volume flow fraction below which only plasma enters a downstream branch (X0), the nonlinearity of the relation between red cell and blood volume flow (B), and the asymmetry of that relation which is described by the parameter A decrease with increasing diameter of the vessel feeding the bifurcation. At diameters above 30 microns, phase separation is very limited. In addition, the nonlinearity parameter B decreases with decreasing hematocrit in the feeding vessel. The asymmetry parameter A strongly depends on the diameter ratio between the two daughter branches: For a given fractional blood flow, the smaller branch receives more red cells than the larger branch. Using a model for plasma skimming based on the assumption of a planar separating surface, the shape of the radial hematocrit profile in the feeding vessel has been calculated. The model predicts a decrease in local hematocrit from the vessel axis toward the wall with a distinct marginal zone free from cell centers. With increasing vessel diameter the hematocrit profile becomes more blunted while the width of the marginal zone increases.

Ischaemic heart disease in women: are there sex differences in pathophysiology and risk factors?Position Paper from the Working Group on Coronary Pathophysiology and Microcirculation of the European Society of Cardiology

Cardiovascular disease (CVD) is the leading cause of death in women, and knowledge of the clinical consequences of atherosclerosis and CVD in women has grown tremendously over the past 20 years. Research efforts have increased and many reports on various aspects of ischaemic heart disease (IHD) in women have been published highlighting sex differences in pathophysiology, presentation, and treatment of IHD. Data, however, remain limited. A description of the state of the science, with recognition of the shortcomings of current data, is necessary to guide future research and move the field forward. In this report, we identify gaps in existing literature and make recommendations for future research. Women largely share similar cardiovascular risk factors for IHD with men; however, women with suspected or confirmed IHD have less coronary atherosclerosis than men, even though they are older and have more cardiovascular risk factors than men. Coronary endothelial dysfunction and microvascular disease have been proposed as important determinants in the aetiology and prognosis of IHD in women, but research is limited on whether sex differences in these mechanisms truly exist. Differences in the epidemiology of IHD between women and men remain largely unexplained, as we are still unable to explain why women are protected towards IHD until older age compared with men. Eventually, a better understanding of these processes and mechanisms may improve the prevention and the clinical management of IHD in women.

Structural adaptation and stability of microvascular networks: theory and simulations

A theoretical model was developed to simulate long-term changes of vessel diameters during structural adaptation of microvascular networks in response to tissue needs. The diameter of each vascular segment was assumed to change with time in response to four local stimuli: endothelial wall shear stress (τw), intravascular pressure (P), a flow-dependent metabolic stimulus (M), and a stimulus conducted from distal to proximal segments along vascular walls (C). Increases in τw, M, or C or decreases in P were assumed to stimulate diameter increases. Hemodynamic quantities were estimated using a mathematical model of network flow. Simulations were continued until equilibrium states were reached in which the stimuli were in balance. Predictions were compared with data from intravital microscopy of the rat mesentery, including topological position, diameter, length, and flow velocity for each segment of complete networks. Stable equilibrium states, with realistic distributions of velocities and diameters, were achieved only when all four stimuli were included. According to the model, responses to τw and P ensure that diameters are smaller in peripheral than in proximal segments and are larger in venules than in corresponding arterioles, whereas M prevents collapse of networks to single pathways and C suppresses generation of large proximal shunts.A theoretical model was developed to simulate long-term changes of vessel diameters during structural adaptation of microvascular networks in response to tissue needs. The diameter of each vascular segment was assumed to change with time in response to four local stimuli: endothelial wall shear stress (tauw), intravascular pressure (P), a flow-dependent metabolic stimulus (M), and a stimulus conducted from distal to proximal segments along vascular walls (C). Increases in tauw, M, or C or decreases in P were assumed to stimulate diameter increases. Hemodynamic quantities were estimated using a mathematical model of network flow. Simulations were continued until equilibrium states were reached in which the stimuli were in balance. Predictions were compared with data from intravital microscopy of the rat mesentery, including topological position, diameter, length, and flow velocity for each segment of complete networks. Stable equilibrium states, with realistic distributions of velocities and diameters, were achieved only when all four stimuli were included. According to the model, responses to tauw and P ensure that diameters are smaller in peripheral than in proximal segments and are larger in venules than in corresponding arterioles, whereas M prevents collapse of networks to single pathways and C suppresses generation of large proximal shunts.

Selective blockade of endothelial Ca2+‐activated small‐ and intermediate‐conductance K+‐channels suppresses EDHF‐mediated vasodilation

Activation of Ca2+‐activated K+‐channels (KCa) has been suggested to play a key role in endothelium‐derived hyperpolarizing factor (EDHF)‐mediated vasodilation. However, due to the low selectivity of commonly used KCa‐channel blockers it is still elusive which endothelial KCa‐subtypes mediate hyperpolarization and thus initiate EDHF‐mediated vasodilation. Using the non‐cytochrome P450 blocking clotrimazole‐derivatives, 1‐[(2‐chlorophenyl) diphenylmethyl]‐1H‐pyrazole (TRAM‐34) and 2‐(2‐chlorophenyl)‐2,2‐diphenylacetonitrile (TRAM‐39) as highly selective IK1‐inhibitors, we investigated the role of the intermediate‐conductance KCa (rIK1) in endothelial hyperpolarization and EDHF‐mediated vasodilation. Expression and function of rIK1 and small‐conductance KCa (rSK3) were demonstrated in situ in single endothelial cells of rat carotid arteries (CA). rIK1‐currents were blocked by TRAM‐34 or TRAM‐39, while rSK3 was blocked by apamin. In current‐clamp experiments, endothelial hyperpolarization in response to acetylcholine was abolished by the combination of apamin and TRAM‐34. In phenylephrine‐preconstricted CA, acetylcholine‐induced NO and prostacyclin‐independent vasodilation was almost completely blocked by ChTX, CLT, TRAM‐34, or TRAM‐39 in combination with the SK3‐blocker apamin. Apamin, TRAM‐34, and CLT alone or sulphaphenzole, a blocker of the cytochrome P450 isoform 2C9, were ineffective in blocking the EDHF‐response. In experiments without blocking NO and prostacyclin synthesis, the combined blockade of SK3 and IK1 reduced endothelium‐dependent vasodilation. In conclusion, the use of selective IK1‐inhibitors together with the SK3‐blocker apamin revealed that activation of both KCa, rIK1 and rSK3 is crucial in mediating endothelial hyperpolarization and generation of the EDHF‐signal while the cytochrome P450 pathway seems to play a minor or no role in rat CA.

Remodeling of Blood Vessels: Responses of Diameter and Wall Thickness to Hemodynamic and Metabolic Stimuli

Vascular functions, including tissue perfusion and peripheral resistance, reflect continuous structural adaptation (remodeling) of blood vessels in response to several stimuli. Here, a theoretical model is presented that relates the structural and functional properties of microvascular networks to the adaptive responses of individual segments to hemodynamic and metabolic stimuli. All vessels are assumed to respond, according to a common set of adaptation rules, to changes in wall shear stress, circumferential wall stress, and tissue metabolic status (indicated by partial pressure of oxygen). An increase in vessel diameter with increasing wall shear stress and an increase in wall mass with increased circumferential stress are needed to ensure stable vascular adaptation. The model allows quantitative predictions of the effects of changes in systemic hemodynamic conditions or local adaptation characteristics on vessel structure and on peripheral resistance. Predicted effects of driving pressure on the ratio of wall thickness to vessel diameter are consistent with experimental observations. In addition, peripheral resistance increases by ≈65% for an increase in driving pressure from 50 to 150 mm Hg. Peripheral resistance is predicted to be markedly increased in response to a decrease in vascular sensitivity to wall shear stress, and to be decreased in response to increased tissue metabolic demand. This theoretical approach provides a framework for integrating available information on structural remodeling in the vascular system and predicting responses to changing conditions or altered vascular reactivity, as may occur in hypertension.

The shunt problem: control of functional shunting in normal and tumour vasculature

Networks of blood vessels in normal and tumour tissues have heterogeneous structures, with widely varying blood flow pathway lengths. To achieve efficient blood flow distribution, mechanisms for the structural adaptation of vessel diameters must be able to inhibit the formation of functional shunts (whereby short pathways become enlarged and flow bypasses long pathways). Such adaptation requires information about tissue metabolic status to be communicated upstream to feeding vessels, through conducted responses. We propose that impaired vascular communication in tumour microvascular networks, leading to functional shunting, is a primary cause of dysfunctional microcirculation and local hypoxia in cancer. We suggest that anti-angiogenic treatment of tumours may restore vascular communication and thereby improve or normalize flow distribution in tumour vasculature.

Microvascular blood flow resistance: role of endothelial surface layer

Observations of blood flow in microvascular networks have shown that the resistance to blood flow is about twice that expected from studies using narrow glass tubes. The goal of the present study was to test the hypothesis that a macromolecular layer (glycocalyx) lining the endothelial surface contributes to blood flow resistance. Changes in flow resistance in microvascular networks of the rat mesentery were observed with microinfusion of enzymes targeted at oligosaccharide side chains in the glycocalyx. Infusion of heparinase resulted in a sustained decrease in estimated flow resistance of 14-21%, hydrodynamically equivalent to a uniform increase of vessel diameter by approximately 1 micron. Infusion of neuraminidase led to accumulation of platelets on the endothelium and doubled flow resistance. Additional experiments in untreated vascular networks in which microvascular blood flow was reduced by partial microocclusion of the feeding arteriole showed a substantial increase of flow resistance at low flow rates (average capillary flow velocities < 100 diameters/s). These observations indicate that the glycocalyx has significant hemodynamic relevance that may increase at low flow rates, possibly because of a shear-dependent variation in glycocalyx thickness.Observations of blood flow in microvascular networks have shown that the resistance to blood flow is about twice that expected from studies using narrow glass tubes. The goal of the present study was to test the hypothesis that a macromolecular layer (glycocalyx) lining the endothelial surface contributes to blood flow resistance. Changes in flow resistance in microvascular networks of the rat mesentery were observed with microinfusion of enzymes targeted at oligosaccharide side chains in the glycocalyx. Infusion of heparinase resulted in a sustained decrease in estimated flow resistance of 14-21%, hydrodynamically equivalent to a uniform increase of vessel diameter by ∼1 μm. Infusion of neuraminidase led to accumulation of platelets on the endothelium and doubled flow resistance. Additional experiments in untreated vascular networks in which microvascular blood flow was reduced by partial microocclusion of the feeding arteriole showed a substantial increase of flow resistance at low flow rates (average capillary flow velocities < 100 diameters/s). These observations indicate that the glycocalyx has significant hemodynamic relevance that may increase at low flow rates, possibly because of a shear-dependent variation in glycocalyx thickness.