Rheology of human blood, near and at zero flow. Effects of temperature and hematocrit level

Static normal human blood possesses a distinctive yield stress. When the yield stress is exceeded, the same blood has a stress-shear rate function under creeping flow conditions closely following Casson's model, which implies reversible aggregation of red cells in rouleaux and flow dominated by movement of rouleaux. The yield stress is essentially independent of temperature and its cube root varies linearly with hematocrit value. The dynamic rheological properties in the creeping flow range are such that the relative viscosity of blood to water is almost independent of temperature. Questions raised by these data are discussed, including red cell aggregation promoted by elements in the plasma.

MYOCARDIAL BLOOD FLOW AND OXYGEN UPTAKE DURING ACUTE RED CELL VOLUME INCREMENTS

The role of red cell mass in the regulation of coronary blood flow and myocardial oxygen consumption has been examined after acute transfusion of sedimented red cells. Patients with substantial blood-loss anemia had a lower than normal left ventricular oxygen uptake, despite a high myocardial blood flow at rest. A 50% rise of hematocrit reduced coronary flow markedly. Although systemic arterial pressure, cardiac output, and left heart work rose, myocardial oxygen consumption was unchanged.

Red cell transfusion of normovolemic dogs resulted in a decrease of both myocardial flow and oxygen uptake associated with an elevation of time-tension index and left ventricular work, accelerated isometric contraction and prolonged systole. There was no evidence of ischemia or change of left ventricular hemodynamic parameters that would account for the reduced myocardial blood flow and oxygen uptake. These findings, in the presence of an increased oxygen carrying capacity and extraction, would appear to be related principally to enhanced blood viscosity.

IN VIVO BLOOD VISCOSITY AND HINDRANCE

This paper presents an in vivo method for measuring peripheral resistance and differentiating between the role of blood viscosity and hindrance. A vascular bed was connected to a pressure generator and recording system. At rest the vascular bed was perfused by the animal, but during the short duration of an experimental run the perfusing pressure was supplied solely by the pressure generator. The apparatus was constructed so that the flow rate was proportional to the rate of change of pressure. The shape of a portion of the curve of pressure versus time is determined solely by total resistance in the vascular bed. Total resistance is a function of hindrance and blood viscosity, and therefore a change in either of these produces a change in the shape of the pressure versus time curves. Since the change in shape produced by a change in viscosity differs from that produced by a change in hindrance, it is possible to differentiate between these two factors.

VISCOSITY AND CLOTTING OF BLOOD IN VENOUS THROMBOSIS AND CORONARY OCCLUSIONS

Viscosity of blood was measured in the absence of anticoagulants by means of an improved cone-in-cone viscometer. Data obtained in a study of a series of patients suffering from venous thrombosis and coronary occlusion were compared with data from a control group of healthy subjects. The probability that the large difference in viscosity observed between normal and abnormal blood is due to chance is approximately 1 in 1,000,000. The viscosity of blood remains constant, at any rate of shear, during the first two to four minutes after removal of blood. Clotting starts after various time periods, depending not only on the intrinsic properties of the particular blood sample, but also on the velocity gradient at which such clotting takes place.

The following general trends were established : at 36°C the onset of clotting is much more rapid at high rates of shear than at low rates of shear. It appears, however, that at lowered temperatures this trend is reversed.

The causes of aggregation of red cells, and the role of this aggregation in clotting, are discussed. It is emphasized that blood is thixotropic, i.e., that its viscosity is both time and rate-of-shear dependent. Thixotropy of blood is due, mainly, to a reversible aggregation of red cells. This suggests the existence of intermittent sedimentation of red cell aggregates during flow in blood vessels. Owing to the existence of a velocity profile, in which the axial region is under zero or near-zero rate of shear, such aggregates should be present especially in large veins and arteries. It appears that an increased thixotropy and viscosity are symptomatic of some thrombotic states.

EFFECT OF POSTHEMORRHAGIC ANEMIA ON THE RENAL CIRCULATION OF THE UNANESTHETIZED RABBIT

In the unanesthetized rabbit made acutely anemic by bleeding with plasma replacement, there was no change in renal blood flow (RBF), a reduction in glomerular filtration rate (GFR), and a reduction in renal vascular resistance with a fall in the filtration fraction. There was evidence of renal tissue hypoxia and reduction in renal PAH extraction ratio. The extraction ratio was 97% at a hematocrit ratio of 34.5%, 91% at a hematocrit of 11.5%, and 84% with hematocrits between 6 and 10%. The effects of carboxyhemoglobinemia at normal blood viscosity were compared with the effects of anemia; a smaller reduction in renal PAH extraction ratio was found and there was no reduction in filtration fraction. The reductions in renal vascular resistance and filtration fraction in anemia were partly reversed by breathing 100% O 2 , viscosity changes again being minimized. It was concluded that renal tissue hypoxia and reduction in red cell concentration contribute in varying degrees to the changes in the renal vascular bed and the PAH extraction ratio.

SCREEN FILTRATION PRESSURE OF HOMOLOGOUS AND HETEROLOGOUS BLOOD AND ELECTROENCEPHALOGRAM

The character and duration of electrical activity arising from a completely isolated cat's head was dependent upon the screen filtration pressure (SFP) of the heparinized oxygenated blood with which it was perfused. If the SFP was above normal the amplitude and frequency of the EEG first decreased, then the brain waves disappeared. The duration of time from the beginning of perfusion until these changes in the EEG occurred was inversely related to the SFP of the perfused blood. Also, the perfusion rate was inversely related to the SFP provided the perfusion pressure remained the same. It is believed that the increase in SFP and changes in EEG and blood flow were due to the presence in the blood of aggregates of blood cells (platelets and leucocytes) which obstructed the brain capillaries. It would appear that heterologous (dog) as well as homologous (cat) blood can be effectively used to perfuse the isolated cat's head provided the blood has a normal SFP. In practice, this was made possible by filtering the blood continuously through Pyrex glass wool.

BLOOD VISCOSITY IN HEALTHY SUBJECTS AND PATIENTS WITH CORONARY HEART DISEASE

Viscosity of whole blood and plasma was measured in 258 apparently healthy subjects of both sexes from 5 to 60 years of age, and in 86 patients with unequivocal evidence of chronic coronary heart disease. Children and young healthy females had the lowest viscosity readings. Healthy young and middle-aged males had significantly higher blood viscosity than females. Patients with coronary heart disease had significantly higher blood viscosity values than healthy groups of the same sex. It is suggested that the higher viscosity of whole blood and of plasma is a contributory factor in development of clinical manifestations of coronary heart disease and possibly of the basic vascular lesion itself.