Flow behavior of fetal, neonatal and adult RBCs in narrow (3-6 µm) capillaries--Calculation and experimental application.

BACKGROUND AND OBJECTIVES: In capillaries with diameters less than those of resting RBCs, the cells have to deform to pass through such narrow vessels. Since RBCs of fetuses, neonates and adults differ in their geometrical properties the flow behavior of RBCs with different sizes in uniform tubes with diameters of 3 to 6 µm were studied by means of a micropipette system and a mathematical model. Assumptions in this model include an RBC flow velocity of 1 mm/s, axisymmetric cell shape and a gap between the RBC and the vessel wall that allows sufficient lubrication. The flow resistance depends on the surface area and volume of RBCs, the plasma viscosity and the vessel diameter. METHODS: Surface area and volume of different RBC populations (20 fetuses, 20 preterm neonates, 10 term neonates and 10 adults) were determined by means of a micropipette system and plasma viscosity was measured using a capillary tube viscometer. The flow behavior of RBCs with different volumes (61, 83 and 127 fl) was studied by direct microscopic observation using a micropipette system. The micropipettes had diameters of 3.5, 4.1, 5.1, and 6.0 µm. The flow velocity of the RBC in the pipettes was 1 mm/s and the calculated and measured cell lengths were compared. RESULTS: Volume and surface area of RBCs were 140 ± 29 fl and 172 ± 20 µm2, respectively, in the fetuses, decreased with increasing maturity (term neonates: 110 ± 20 fl and 149 ± 18 µm2) and reached the lowest values in adults (93 ± 14 fl and 136 ± 12 µm2). Plasma viscosities increased with increasing maturity due to rising plasma protein concentrations. The flow model leads to the following conclusions: During the passage of 3- to 6-µm vessels, the large fetal and neonatal RBCs are more elongated than the smaller adult RBCs. The critical vessel diameter, i.e., when the rear of the RBC becomes convex during passage of a narrow capillary, was 4.1 µm for fetal RBCs, 3.6 µm for neonatal RBCs and 3.3 µm adult cells. Suspended in the same medium, fetal and neonatal RBCs require 27% (term neonates) to 100% (fetuses) higher driving pressures than adult RBCs to achieve the necessary elongation for passing through a 4.5-µm capillary. However, the different RBCs require similar driving pressures if the cells are suspended in the corresponding autologous plasma. Cell lengths of the RBCs with different geometry determined during the flow experiments agreed with the predicted values. CONCLUSION: We conclude that the large size of fetal and neonatal RBCs may cause impaired flow in narrow vessels with diameters below the critical values of 3.6 to 4.1 µm. In vessels with diameters above the critical diameter (Dcr), the disadvantage of the large size of neonatal and fetal RBCs appears to be completely compensated for by the lower plasma viscosity.

The role of dual energy CT in differentiating between brain haemorrhage and contrast medium after mechanical revascularisation in acute ischaemic stroke.

OBJECTIVES: To assess the feasibility of dual energy computed tomography (DE-CT) in intra-arterially treated acute ischaemic stroke patients to discriminate between contrast extravasation and intracerebral haemorrhage. METHODS: Thirty consecutive acute ischaemic stroke patients following intra-arterial treatment were examined with DE-CT. Simultaneous imaging at 80 kV and 140 kV was employed with calculation of mixed images. Virtual unenhanced non-contrast (VNC) images and iodine overlay maps (IOM) were calculated using a dedicated brain haemorrhage algorithm. Mixed images alone, as "conventional CT", and DE-CT interpretations were evaluated and compared with follow-up CT. RESULTS: Eight patients were excluded owing to a lack of follow-up or loss of data. Mixed images showed intracerebral hyperdense areas in 19/22 patients. Both haemorrhage and residual contrast material were present in 1/22. IOM suggested contrast extravasation in 18/22 patients; in 16/18 patients this was confirmed at follow-up. The positive predictive value (PPV) of mixed imaging alone was 25 %, with a negative predictive value (NPV) of 91 % and accuracy of 63 %. The PPV for detection of haemorrhage with DE-CT was 100 %, with an NPV of 89 % and accuracy improved to 89 %. CONCLUSIONS: Dual energy computed tomography improves accuracy and diagnostic confidence in early differentiation between intracranial haemorrhage and contrast medium extravasation in acute stroke patients following intra-arterial revascularisation. KEY POINTS: • Contrast material and haemorrhage have similar density on conventional 120-kV CT. • Contrast material hinders interpretation of CT in stroke patients after recanalisation. • Iodine and haemorrhage have different attenuation at lower kVs. • Dual energy CT improves accuracy in early differentiation of haemorrhage and contrast extravasation. • Early differentiation between iodine and haemorrhage helps to initiate therapy promptly.

Blood viscosity and optimal hematocrit in preterm and full-term neonates in 50- to 500-micrometer tubes.

Blood viscosity is an important determinant of blood flow resistance. Because a substantial part of flow resistance arises in small arteries and arterioles with diameters of 100 microns and less, rheologic properties of blood from preterm infants (24 to 36 wk of gestation), full-term neonates, and adults were measured in glass tubes with diameters of 50, 100, and 500 microns for a wide range of adjusted feed hematocrits (0.15-0.70). At each of the feed hematocrits, blood viscosity decreased when going from a 500-microns tube to a 50-microns tube. The viscosity reduction increased with increasing hematocrit. Moreover, the viscosity reduction was more pronounced in the neonates than in the adults. At a hematocrit of 0.70, the viscosity reduction averaged 56% in preterm infants, 50% in full-term neonates, and 39% in adults (p less than 0.005). However, the viscosity reductions at a hematocrit of 0.30 were only 35, 29, and 19%, respectively (p less than 0.05). In all four groups, blood viscosity increased exponentially with increasing hematocrit. The steepness of the hematocrit-viscosity curves decreased with decreasing tube diameter and with decreasing maturity of the infants. Erythrocyte transport efficiency (hematocrit/blood viscosity) was calculated to estimate the optimal hematocrit (i.e. hematocrit with maximum erythrocyte transport). In 500-microns tubes, the optimal hematocrit was about 0.40 in all of the groups. In 100-microns tubes, the optimal hematocrit was 0.44 +/- 0.05 in the adults and 0.52 +/- 0.04 in the neonates (p less than 0.05). In 50-microns tubes, the optimal hematocrit was 0.51 +/- 0.04 in adults and 0.60 +/- 0.05 in the neonates.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood viscosity and optimal hematocrit in narrow tubes.

Blood viscosity in normal adults was measured in glass tubes with diameters of 50, 100 and 500 microns for a wide range of adjusted feed hematocrits (15-70%). Blood viscosity decreased at each of the adjusted feed hematocrits when going from a 500-micron tube to a 50-micron tube. The viscosity reduction increased with increasing hematocrit. The steepness in the hematocrit-viscosity curves decreased with decreasing tube diameter. Erythrocyte transport efficiency (hematocrit/blood viscosity) was calculated to estimate the optimal hematocrit for oxygen transport. Optimal hematocrit averaged 38% in 500-micron tubes, 44% in 100-micron tubes and 51% in 50-micron tubes. Our results suggest that the strong Fåhraeus-Lindqvist effect at high hematocrits may help to maintain oxygen transport in polycythemic patients as long as the driving pressure is sufficient.