Clinical Hemorheology and Microcirculation

It could be claimed that expansion in clinical aspects of haemorheology has largely been fuelled by the development of a simple test of blood flow properties, i.e., analysis of filterability. With time, the level of sophistication in equipment, theory and sample preparation has increased. Theories for the development of flow in the filter have been described and these enable cellular parameters, such as transit times, to be calculated from experimental data. These theories can be quite general, and applied to filtration of red or white cells. Ideally, experimental design requires an understanding of cell behavior at the filter and of the effects of factors such as the sample concentration and volume, and the presence of different types of cells or subpopulations. Otherwise, results are susceptible to misinterpretation, particularly if impurities or mixed populations of cells are present. It is thus very important to know accurately the constituents of the test suspension. In clinical applications the trend has been to move away from whole blood filtration, toward use of relatively pure suspension of separated red cells and white cells. In the area of red cell filtration this has led to reappraisal of some previously reported abnormalities. The relatively new study of white cell filtration should benefit from previous experience, but there is the added problem of the reactivity of the cells under test, and their mixed nature. In any case, critical evaluation of the meaning and clinical relevance of results is necessary.

A brief outline of the background of viscoelasticity of blood is given in order to derive for blood relevant parameters. In this context, especially one method, the sinusoidal oscillating capillary rheometry, is discussed. The determination of different parameters to characterize blood samples, e.g. the viscoelastic phase angle, an aggregation index, a flexibility index, hematocrit and plasma viscosity is described. Most of these parameters show increased values in disease; however they can be influenced by drug treatment.

Both blood density and sound speed are closely related to total protein concentration in blood and, as a consequence, to rheologically important parameters of blood. Two methods that permit continuous measurement of these properties, the mechanical oscillator technique and the new ultrasonic technique, were used for measuring blood protein concentration over a continuous period of time in a group of hemodialysis patients and in volunteers. It was seen that the concentration of the components of blood varies considerably. This variability is related to transport phenomena within as well as to the flow of masses across the cardiovascular compartment. From the continuous measurement of concentrations during hemodialysis treatment, relative changes in blood volume can be recorded in order to control the fluid balance of the patient. Rapid fluctuations at the macroscopic scale with periods of 5 to 30 seconds are due to heterogeneities at the microscopic scale and to the particular rheological behavior of the red blood cells at the level of the capillaries and the small blood vessels. The amplitude of rapid oscillations increased up to 1.2 % in terms of hematocrit values when there was rhythmic, spontaneous breathing at various frequencies. The measurement of concentrations at an accessible measuring site may be used to investigate the rheology of blood in the human microvasculature.

The interrelationship between systemic oxygen transport and hematocrit, has been studied under various conditions, while the influence of plasma viscosity on oxygen transport and tissue oxygenation has not entirely been explored.

In experiments in dogs the plasma viscosity was increased either by isovolemic hemodilution with 6% hydroxyethyl starch (HES) 200/0.62, or 6% dextran-70, or by infusion of dextran-500 in a volume equivalent to 4% of blood volume from baseline to 3 mPa·s. Cardiac output and regional blood flow were assessed by means of radioactive labelled microspheres and local tissue oxygenation by means of pO₂ multiwire surface electrodes.

In normotensive heal thy animals elevated plasma viscosity did neither jeopardize systemic nor regional blood flow; local tissue oxygenation of skeletal muscle remained unchanged or was even improved. We conclude that among the rheological factors influencing oxygen transport, the hematocrit plays the predominant role, while plasma viscosity is of minor importance.

Continuously measured oxygen uptake during constant work exercise (15′ SOW) reveals increasing oxygen consumption in individuals with elevated blood viscosity parameters, indicating persistent contribution of anaerobic glycolysis during steady state exercise far below expected “anaerobic threshold”. Improvement of viscosity parameters by prostaglandin E₁ infusion (Prostavasin^®) 40 µg i.v., naftidrofurylhydrogenoxalat (Dusodril^® pi) 400 mg i.v. or hemodilution with 500 ml 6% hydroxyethylamylum MW 40000 (Onkohaes^®) in 5 patients results in significant reduction of this oxygen gradient in subsequent exercise test. Integrated V02 during exercise above the mean value at rest or the quotient of V˙O₂ during 15 min by V˙O₂ during 30 min (including recovery time) are not differing significantly due to high variations inter- and intraindividually. Oxygen gradient during submaximal constant exercise permits direct clinical determination of microcirculatory performance in involved muscle tissue as a function of blood Viscosity.

Orbitometry (Hartert) is a rheological ex-vivo method to follow up physical assembly of a coagulum in layers during natural intensity of flow by orbital movement. Fibrin elasticity in the Orbitometers mode of Resonance Thrombography is differentiated from platelet activity as well as e.g. from the effects of disseminated coagulation – minimal in liver disease and maximal during disturbances of delivery. Transition into the mode of dynamic Tendography (Hartert) will e.g. register all fast going tests lasting minutes or seconds. It is comparable to an accelerated form of Thrombelastography (Hartert), the intercourse of which with coagulum yet is an exclusively static operation. Another category is measurement of blood and plasma viscosity. In concentrated blood it seizes plasticity of blood cells as well as their intensity of aggregation in orbital flow.

The latest methodical development of Orbitometry is control of platelet activity in its function of adhesion. This is realized by measurement of specific physical effects released in platelet containing coagulum. They generate a structural degradation of fibrin elasticity modul as well as a tendency for coagulum adhesion. The practical use of Adhesiongraphy is control of anticoagulants and platelet protecting substances in their quantitative influence on coagulum structure and on the mentioned platelet activities. A special disturbance of these platelet depending mechanisms obviously is getting evidence in case of v.Willebrand’s syndrome.

In myocardial perfusion, blood acts not only as a source of oxygenation and nutrients and a pathway for removal of metabolites, but also as a coolant and a heat sink. At increasing heart rate the pumping efficiency decreases and the heat generation increases. Increased temperature leads to increase of aggregation of red cells and to apparent increase in the rigidity of red cells. It is also known that clotting of blood becomes more rapid at increased temperatures. Thus, even a marginal increase of temperature at any specific site of the myocardium might form a vicious circle affecting heart nourishment and heart performance. In his studies of cancer, von Ardenne showed localized tissue necrosis at 41°C, while Rigby and Dintenfass considered ‘melting’ of molecular collagen as a factor in the elastic properties and function of myocardium (which might have an example in rheumatic heart disease). Phase transition of collagen depends on its chemical composition which might differ in different parts of the heart. There is also little information permitting definite statement on existence of hot spots in the heart as duration of heat generation required would be of 300 sec duration while current evidence suggests 60 sec duration under experimental conditions. Nevertheless, the total evidence of thermal changes in the blood rheology and in the rheology of myocardium suggests that thermal mechanisms could be of importance in heart pathology.

Acute physical work causes hemorheological changes equivalent to those of hemoconcentration whereas physical training favors an overall improvement of blood viscosity factors with time. Patients with coronary heart disease (CHD) exhibit hemorheological abnormalities which are related to the severity of their disease. Question arose whether these abnormalities further worsen by acute physical stress and whether this can be improved by an endurance training. In 26 patients with CHD a bicycle ergometer test was done at admission and after a supervised performance oriented training period of 2 and 4 weeks. Before and after the ergometer test blood was taken to obtain the follow ing parameter: hematocrit (hct), plasma fibrinogen and red cell aggregation (RCA) measured photometrically. Two groups were selected according their performance capacity (G1 < 100 watts < G2). In G1 fibrinogen level and RCA were higher than in G2 and the exercise induced hemoconcentration was more pronounced. An endurance training of 4 weeks reduced the fibrinogen related RCA significantly just as the exercise induced hemoconcentration. Despite a lower performance capacity the hemorheological adaptation in G1 was relatively more effective than in G2. Hct and plasma viscosity did not change significantly.

Macro and micro-hemorheological analyses on thirteen kinds of cardiovascular and related diseases and healthy young and old controls, 1.102 cases, were carried out together with analyses of hemorheology-related factors. Whole blood viscosity at shear rate of 94.5 and 0.376 s⁻¹ and plasma viscosity at shear rate of 94.5s⁻¹ was measured respectively by Low Shear-30. Whole blood at both shear rates and plasma viscosity in valvular diseases showed significant decrease and those of all the other diseases did not show significant change compared to those of young and old control. Whole blood Passage time measured through Nuclepore membrane with pore size of 5 µm were prolonged significantly in all these diseases with except of valvular diseases. Fibrinogen revealed significant increase in all these diseases. Macrohemorheology presented by whole blood and plasma viscosity in cardiovascular diseases and old control except valvular diseases showed no significant changes, while microhemorheology presented by whole blood passage time of them demonstrated significant abnormalities. The direction of change in macro and microhemorheology in valvular disease was quite different from other that observed in cardiovascular diseases.

We evaluated the microrheological determinants and the red cell membrane individual membrane phospholipids in subjects with VAD. Our study included four groups of patients. In the first group, we evaluated the membrane lipid fluidity using the ghosts marked with DPH. In the second group, the erythrocyte membrane fluidity was employed marking intact red cells with pyrene. In the third group, the membrane fluidity transverse gradient was evaluated marking intact red cells with fatty acid fluorescent probes: 2-AP, 6-AS, 9-AS, 12-AS. In the fourth group we evaluated the red cell membrane phospholipids and the Iex/Im ratio. Examining the first group, it is evident that the fluorescence polarization degree does not discriminate normals from this group. Examining the second group, it is evident that the Iex/Im ratio distinguishes normals from this group. Examining the third group, it is evident that the degree of polarization obtained with each fatty acid fluorescent probe does not distinguish normals from this group. Examining the fourth group, it is evident that no significant difference is present between normals and this group regarding erythrocyte individual phospholipids; a slight significant correlation is evident only between the Iex/Im ratio and phosphatidylethanolamine.