Subcutaneous microvascular responses to hemodilution with a red cell substitute consisting of polyethyleneglycol-modified vesicles encapsulating hemoglobin

Abstract

Phospholipid vesicles encapsulating purified hemoglobin [Hb vesicles (HbV); diameter 259 +/- 82 mm; oxygen affinity 31 mm Hg; [Hb] 5 and 10 g/dL] were developed to provide oxygen-carrying capacity to plasma expanders. Their function as a blood replacement was tested in the subcutaneous microvasculature of awake hamsters during severe hemodilution in which 80% of the red blood cell mass was substituted with suspensions of the vesicles in 5% human serum albumin (HSA) solution. Vesicles were tested with membranes that were unmodified (HbV/HSA) or conjugated with polyethyleneglycol (PEG) on the vesicular surface (PEG-HbV/HSA). The viscosity of 10 g/dL HbV/HSA was 8 cP at 358 s-1 owing to the intervesicular aggregation, while that of 10 g/dL PEG-HbV/HSA was 3.5 cP, since PEG chains inhibit aggregation. Both materials yielded normal mean arterial pressure, heart rate, and blood gas parameters at all levels of exchange, which could not be achieved with HSA alone. Subcutaneous microvascular studies showed that PEG-HbV/HSA significantly improved microhemodynamic conditions (flow rate, functional capillary density, vessel diameter, and oxygen tension) relative to unmodified HbV/HSA. Even though the enhancement of PEG modification did not achieve the functional characteristics of the blood-perfused microcirculation, PEG reduced vesicular aggregation and viscosity, improving microvascular perfusion relative to the unmodified type. These results highlight the significance of microvascular analysis in the design of red cell substitutes and the necessity of surface modification of HbV to prevent aggregation.

Systemic and microvascular responses to hemorrhagic shock and resuscitation with Hb vesicles

“…The viscosities of HbV(3.8)/HSA and HbV(7.6)/HSA at 150 s Ϫ1 are 1.8 and 3.0 cP, respectively, being lower than the viscosity of blood (4.5 cP) and that of previously reported HbV(10)/HSA (4 cP) (30). Higher viscosities should lead to higher shear stress, the consequent release of vasorelaxation factors, and higher FCD, even though we did not find a related response of microvascular diameters.…”

“…Moreover, molecular O 2 carriers and HbV, being much smaller than RBCs, release O 2 in closer proximity to the arteriolar wall (17,31), significantly augmenting the flux O 2 to the arteriolar walls inducing vascular autoregulatory responses aimed at maintaining tissue oxygenation constant through vasoconstriction and the reduction of blood flow (7,41). When these two factors are taken into consideration, Hb concentration can be adjusted to be ϳ5 g/dl provided that the level of blood exchange does not exceed 80%, in which case the tissue can become hypoxic (30). Lower Hb concentrations such as 3.8 g/dl may still be useful but only applicable to a 50-60% level of blood exchange.…”

“…Lowered blood viscosity and blood flow does not transmit adequate pressure to the capillaries, causing the decrease of FCD (37,39). The significant decrease in Hct could also be a contributing factor to decrease FCD because the reduction of Hct increases the plasma layer and the resulting plasma skimming, leading to an underestimation of the number of perfused capillaries (30). Semitransparent elements presumed to be HbV particles were visible in the capillaries of the HbV/HSA groups, and because FCD was estimated on the basis of number of capillaries through which RBCs were flowing, FCD values might have underestimated the total number of functioning capillaries for the HbV/HSA groups.…”

Oxygen delivery and consumption in the microcirculation after extreme hemodilution with perfluorocarbons

“…Hct was measured from centrifuged arterial blood samples taken in heparinized capillary tubes (Readacrit Centrifuge, Clay Adams, Division of Becton-Dickinson; Parsippany, NJ). Hb content was determined spectrophotometrically from a single fluid drop (B-Hemoglobin, Hemocue; Stockholm, Sweden) (18).…”

Normovolemic hemodilution with Hb vesicle solution attenuates hypoxia in ischemic hamster flap tissue

“…HbV solutions caused significantly less vasoconstriction and hypertensive response than the conjugated or polymerized hemoglobins currently in experimental and clinical investigation (21). HbV solutions maintained microvascular function and oxygen delivery to the tissue even at relatively high levels of blood exchange (23). Another advantage of HbV is that P 50 can be modified by selecting the appropriate amount of coencapsulated allosteric effector (pyridoxal 5Ј-phosphate) (24), which may be essential for achieving maximal oxygen supply to the jeopardized tissue.…”