FAHRAEUS AND FAHRAEUS-LINDQVIST EFFECTS FOR NEONATAL AND ADULT RED-BLOOD-CELL SUSPENSIONS

In blood vessels with diameters less than 500 mum, both the hematocrit and viscosity decrease with decreasing tube diameter [Fahraeus effect (FE) and Fahraeus-Lindqvist effect (FLE)]. Because both effects may b e influenced by red blood cell (RBC) volume and osmolality, the effect s of RBC type and suspending medium osmolality (216, 294, and 473 mosm ol/kg) on tube hematocrit (H(T)) and relative viscosity (eta(r)) in na rrow tubes (32 to 145 mum diameter) were studied for 0.40 L/L (40%) he matocrit suspensions of human neonatal and adult RBC in buffer. Osmola lity of 473 mosmol/kg caused shrinkage of RBC by 20% so that neonatal RBC assume the volume of adult RBC in isotonic buffer. The FLE and FE were present for both neonatal and adult RBC suspensions regardless of osmolality. The viscosity reduction when going from a 145- to a 32-mu m tube was greatest for the hypertonic neonatal and adult RBC: changes were -44% (473 mosmol/kg) and -31% (294 mosmol/kg) for neonatal RBC, and -39% (473 mosmol/kg), -34% (294 mosmol/kg), and -21% (216 mosmol/k g) for adult RBC. The eta(r) were significantly lower (7% on average) for isotonic neonatal RBC compared with adult cells in 32-mum (p < 0.0 25), 46-mum, and 146-gm tubes (p < 0.001). In contrast, HT and thus th e FE were less affected by RBC type or osmolality (only 13% change ove r entire range of osmolality and diameter): relative H(T) values were systematically lower (p < 0.02), and the FE greater, for isotonic neon atal versus adult RBC. At each osmolality, eta(r) increased linearly w ith increasing H(T). However, the viscosity-H(T) relations differed at various osmolalities. A semiempirical model indicated that 1) H(T), r elative cell volume and thus mean cellular Hb concentration, and tube diameter are primary determinants of eta(r), and 2) eta(r), can be pre dicted for neonatal and adult RBC in hypertonic and isotonic media ove r a wide range of tube diameters. We conclude that the FLE and FE are more pronounced for neonatal RBC than for adult RBC. This cannot be ex plained by the increased volume of neonatal RBC, because shrunk RBC sh owed higher FLE than RBC in isotonic solution. Increased FLE and FE of neonatal RBC may facilitate blood flow in narrow vessels, particularl y in vessels with high osmolality (i.e. renal vasa recta).