Clinical Hemorheology and Microcirculation

Normal Hemorheology: Plasma proteins, plasma viscosity, and red blood cell (RBC) aggregation are very low in the fetus and the preterm infant, but increase with increasing gestational and postnatal age. Moreover, both the viscosity (Fåhræus-Lindqvist effect) and the hematocrit reductions (Fåhræus effect) in narrow tubes are more pronounced in the fetus and neonate than in adults. As a result of these peculiar hemorheological properties, viscosity of blood (measured in 50 μm diameter tubes) with given hematocrit is markedly lower in the fetus (−50% at 25 wk gestation) and neonate (−25%) than in adults. Blood viscosity in 50-μm tubes is similar in neonates with a hematocrit of 0.70 l/l as in adults with a hematocrit of 0.50 l/l. This may explain why polycythemia does not impair systemic, cerebral and intestinal RBC transport in neonates as long as the hematocrit does not exceed 0.70 l/l and why polycythemic neonates are less susceptible to serious complications than polycythemic children and adults.

Pathological Hemorheology: Since high flow conditions in the fetus and neonate are strongly dependent on the favorable rheologic properties of their blood, hemorheolocal abnormalities may contribute to the high risk of the fetus and neonate to ischemic damage of vital organs (e.g. brain) in a variety of disorders: 1) Perinatal asphyxia may be associated with impaired RBC deformability, increased RBC aggregation and leukocyte count, thereby aggravating tissue hypoxia. 2) In growth-retarded fetuses, increased hematocrit, RBC aggregation and leukocyte count may contribute to the high risk of ischemic damage of vital organs (e.g. brain). 3) Fetal and neonatal cells are less protected from radicals than adults, thereby predisposing them to oxidative damage. 4) Neonatal septic shock may be associated with markedly decreased RBC defo rm ability , increase in the count of rigid, immature leukocytes, RBC aggregation and plasma viscosity. 5) Infants of poorly-controlled insulin-dependent diabetic mothers show markedly increased hematocrit and decreased RBC deformability. This may increase their risk of thromboses.

Deformability of normocytic hypochromic human erythrocytes was studied as a function of their mean corpuscular hemoglobin content (MCHC). Erythrocyte populations with subnormal MCHC but with normal cell shape and volume and preserved cell surface area, were obtained by incomplete lysis, followed by cell resealing and density gradient separation. Erythrocyte rheological properties were measured in the erythrodeformeter, apparatus in which the erythrocyte laser diffraction pattern is recorded in shear flow. It was found that the deform ability index, proportional to the ellipticity of the diffraction pattern at the maximum shear stress, has a negative correlation with MCHC. Membrane elastic shear modulus and membrane surface viscosity have a positive correlation with MCHC. These facts are indicative of a concentration dependent membrane-hemoglobin interaction, and thus hemoglobin concentration must be included as one of the factors influencing erythrocyte deformability, even at low concentrations. We propose a simple model to demonstrate that the maximum oxygen delivery to tissues occurs at the physiological MCHC values.

Lipid A is responsible for the endotoxic activities of gram-negative bacteria. Red blood cell (RBC) deformation was studied in adults, in full-term and preterrn infants during 60 min of in vitro incubation with lipid A (100 μg/ml) using a rheoscope. Compared to control values without lipid A incubation, RBC deformation (shear stress of 4 Pa) in adults, preterm, and from full-term neonates were decreased after 15 min (11%, 10%, 8%, respectively), after 30 min (11%, 6%, 3%, respectively) and after 60 min (8%, 5%, 2%, respectively) of incubation with lipid A. The decrease in RBC deformation after 15 min was significant (p<0.05) in the four groups. After 30 and 60 min, RBC deformation in the neonates recovered to the control values. In adults, RBC deformation improved after 30 and 60 min but did not reach preincubation values. These results indicate that neonatal RBC deformability recovers more rapidly than adult RBC during lipid A incubation.

The erythrocyte deformability, the membrane lipid fluidity and the anion permeability of erythrocytes for lung cancer patients were measured and the results were compared with those of healthy persons. The experimental results showed that the blood viscosity of lung cancer patients increased, the erythrocyte deformability of lung cancer patients decreased, the erythrocyte membrane lipid fluidity and membrane anion permeability of lung cancer patients decreased as well.

We examined erythrocyte aggregability (RBC-A) in 108 patients with acute-atage (less than 72 hours after onset) cerebral infarction (85 males, 23 females; age, 40–78 (61±10 (mean±SD)) YO) and 52 age-matched healthy volunteers (37 males and 15 females, 59±9 YO). The subtypes of these patients were atherothrombotic infarction (N=31, 62+10 YO), lacunar infarction (N=58, 61±9 YO) and cardioembolic infarction (N=19, 60±10 YO). RBC-A was examined using the whole-blood erythrocyte aggregometer developed by us (Am. J. Physio!. 251, H1205-H1210, 1986) with concomitant measurement of hematocrit, albumin:globulin ratio and fibrinogen concentration. RBC-A values in atherothrombotic infarction (0.153±0.026/s), lacunar infarction (0.154±0.021/s) and cardioem bolic infarction (0.163±0.022/s) were significantly (P<0.01) higher than that in age-mlitched healthy volunteers (0.122±0.027/s). Fibrinogen concentrations in atherothrombotic infarction (391±93 mg/dl), lacunar infarction (333±79 mg/dl) and cardioembolic infarction (423±66 mg/dl) were also significantly (P<0.01) higher than that in agematched healthy volunteers (294±73 mg/dl). Fibrinogen concentration in atherothrombotic infarction and cardioembolic infarction were significantly (P<0.01) higher than that in lacunar infarction. Albumin;globulin ratio in cardioembolic infarction (1.42±0.26) was significantly (P<0.05) lower than those in atherothrombotic infarction (1.66±0.29), lacunar infarction (1.76±0.31) and healthy volunteers (1.79±0.31) We conclude that RBC-A was enhanced in all subtypes of acute-stage cerebral infarction and there were no differences in RBC-A among subtypes.

Measurements of red cell deformability are usually performed in isotonic buffer solutions as suspending medium. However, in vivo, red cells are surrounded by plasma in which plasma proteins are present. In this study we investigated whether using plasma (anticoagulated with 1/10 vol. 110 mM trisodium citrate) instead of buffer (phosphate buffered saline solution) as the suspending medium gives rise to differences in the measured red cell deformability parameters. We used ektacytometry, the micropipette, the flow channel (both static and dynamic), and a Cell Transit Analyzer to study this effect. Where necessary, we added 1 g/l bovine serum albumin to prevent echinocyte formation, and poly-vinyl-pyrrolidone to increase the buffer viscosity to match that of plasma.

Plasma was found to be a good alternative for buffer as suspending medium except for ektacytometric measurements. In ektacytometry, interactions between plasma components and the polymer added to increase the medium viscosity caused measurement artefacts. Comparison between the plasma and the buffer measurements using the other techniques showed a decreased red cell deformability in plasma. We found that in plasma membrane elasticity is decreased and membrane viscosity is increased compared with buffer (from micropipette and flow channel measurements) and a decreased filterability in plasma with the Cell Transit Analyzer.

It is unclear whether the plasma composition or the buffer composition is responsible for the observed increased membrane viscosity and decreased membrane elasticity in plasma or, in other words, the decreased viscosity and increased elasticity in buffer. However, our results raise the question whether buffer is a good medium and whether deformability studies in general should be performed in plasma instead of in buffer.

The objective of this study was to compare peripheral microvasculature status non-invasively in heal thy controls and recent stroke cases having history of hypertension. The high risk group of patients viz. hypertensives and diabetics were also included in the study. We used a combination of Laser Doppler flowmetry and reactive hyperemia test along with parallel estimation of blood viscosity factors such as, hematocrit, red cell rigidity, plasma viscosity and biochemical parameters. Reacti ve hyperemia was induced by arterial occlusion for 3.5 min. duration in the forearm of all four groups. Basal skin perfusion before inducing reactive hyperemia (Per_bas), did not vary significantly in any group. However, the increased maximum skin perfusion after inducing reactive hyperemia (Per_max) in the case of stroke and hypertension were found significantly different (p<0.05) as compared to healthy controls. The recovery time (Trh) i.e., time taken for induced maximum perfusion to return to basal value, were significantly less in stroke (p<0.05) and hypertensive groups (p<0.001) whereas it was significantly high in diabetic group (<0.02) as compared to healthy controls. A reduced Trh may be considered as an indicative of myogenic dysfunction while a delayed Trh may be regarded as an indicative of metabolic dysfunction. Further the calculation of two dimensionless microcirculatory parameters reactive hyperemia perfusion index (RHPI) and reactive hyperemia time index (RHTI) were proposed for use in preliminary screening of the degree of myogenic or metabolic dysfunction in various disease conditions. The correlation of above mentioned parameters with blood viscosity factors were discussed.

Platelet membrane fluidity, platelet membrane cholesterol/phospholipid ratio, platelet membrane individual phospholipids and plasma lipid composition were evaluated in diabetics, in subjects with vascular atherosclerotic disease (VAD), in VAD subjects with non-insulin-dependent diabetes mellitus (NIDDM) and in hypertensives. The platelet membrane fluidity did not distinguish normals from diabetics and normals from hypertensives; this microrheological parameter discriminated instead normals from VAD subjects and normals from VAD subjects with NIDDM. Platelet membrane cholesterol/phospholipid ratio discriminated normals from hypertensives only. Of the platelet membrane individual phospholipids, while the phosphatidylcholine (PC) was increased, the phosphatidylserine (PS) was reduced in diabetics; no significant difference was evident between normals and VAD subjects with and without NIDDM, while in hypertensives the PC was significantly increased. In each group of patients the study of correlations between platelet membrane fluidity and platelet membrane lipid pattern, as well as that between platelet membrane fluidity and plasma lipid composition, shows contrasting results.

Hemorheological factors play an important role in the pathogenesis of different cardiovascular diseases. In the present study the fundus appearance and hemorheological parameters (plasma and whole blood viscosity *WBV* by capillary viscosimetry; fibrinogen level by the Clauss method) of 33 hypertensive patients (23 females, 10 males; mean age: 55 years) were examined. The fundus appearance showed retinopathy in all the cases between stages I-III. All the measured hemorheological parameters of the examined patients were in the pathological range (WBV at 90 lis: 5.18 mPas) and were significantly (p < 0.01) higher than in healthy controls (WBV at 90 1/s: 4.18 mPas). The hemorheological factors showed a parallel deterioration with the fundus appearance, namely their values were significantly (p < 0.01) higher in patients with a fundus appearance stage III (WBV at 90 1/s: 6.02 mPas) than stage I (WBV at 90 1/s: 4.51 mPas). Our results show that there is a correlation between hemorheological parameters and fundus appearance in hypertensives, and this suggests that hemorheological factors may playa role in the development of hypertensive retinopathy.