Tissue oxygenation after exchange transfusion with ultrahigh-molecular-weight tense- and relaxed-state polymerized bovine hemoglobins

Abstract

Hemoglobin (Hb)-based O(2) carriers (HBOCs) constitute a class of therapeutic agents designed to correct the O(2) deficit under conditions of anemia and traumatic blood loss. The O(2) transport capacity of ultrahigh-molecular-weight bovine Hb polymers (PolybHb), polymerized in the tense (T) state and relaxed (R) state, were investigated in the hamster chamber window model using microvascular measurements to determine O(2) delivery during extreme anemia. The anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After an initial moderate hemodilution to 18% hematocrit, animals were randomly assigned to exchange transfusion groups based on the type of PolybHb solution used (namely, T-state PolybHb and R-state PolybHb groups). Measurements of systemic parameters, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O(2) levels were performed at 11% hematocrit. Both PolybHbs were infused at 10 g/dl, and their viscosities were higher than nondiluted blood. Restitution of the O(2) carrying capacity with T-state PolybHb exhibited lower arterial pressure and higher functional capillary density compared with R-state PolybHb. Central arterial O(2) tensions increased significantly for R-state PolybHb compared with T-state PolybHb; conversely, microvascular O(2) tensions were higher for T-state PolybHb compared with R-state PolybHb. The increased tissue Po(2) attained with T-state PolybHb results from the larger amount of O(2) released from the PolybHb and maintenance of macrovascular and microvascular hemodynamics compared with R-state PolybHb. These results suggest that the extreme high O(2) affinity of R-state PolybHb prevented O(2) bound to PolybHb from been used by the tissues. The results presented here show that T-state PolybHb, a high-viscosity O(2) carrier, is a quintessential example of an appropriately engineered O(2) carrying solution, which preserves vascular mechanical stimuli (shear stress) lost during anemic conditions and reinstates oxygenation, without the hypertensive or vasoconstriction responses observed in previous generations of HBOCs.

Fig. 1.

Hemodilution was attained by means of a progressive, stepwise, isovolemic hemodilution protocol in which the red blood cell (RBC) volume was continuously decreased and the plasma volume was increased while the total blood volume constant was maintained (dotted line). The extreme anemic state was achieved by two hemodilution exchanges (first 40% followed by 35% of blood volume) and an exchange transfusion step (35% of the blood volume) using either tense-state or relaxed-state bovine hemoglobin (bHb) polymers (T-PolybHb and R-PolybHb, respectively; at 10 g/dl).

Fig. 2.

Relative changes from baseline in mean arterial pressure (MAP) and heart rate (HR) after moderate hemodilution and exchange transfusion with T-PolyHb and R-state PolybHb. The dotted line represents the baseline (BL) level. †P < 0.05 relative to BL; ‡P < 0.05 compared with moderate hemodilution; §P < 0.05 among groups. Baseline MAP values (means ± SD) for each group were as follows: 114 ± 6 mmHg for T-PolybHb and 111 ± 7 mmHg for R-PolybHb. Baseline HR values (means ± SD) for each group were as follows: 434 ± 29 beats/min for T-PolybHb and 426 ± 27 mmHg for R-PolybHb.

Fig. 3.

Relative changes from baseline in arteriolar and venular hemodynamics after moderate hemodilution followed by an exchange transfusion with either T-PolybHb or R-PolybHb. The dotted line represents the BL level. †P < 0.05 relative to BL; ‡P < 0.05 compared with moderate hemodilution; §P < 0.05 among groups. Diameters (means ± SD) in each animal group were as follows: arterioles, 61.7 ± 7.9 μm, n = 34, and venules, 69.1 ± 8.3 μm, n = 36, for BL T-PolybHb; and arterioles, 63.5 ± 8.9, n = 32, and venules, 67.8 ± 9.2, n = 33, for BL R-PolybHb, where n is the number of vessels studied. RBC velocities (means ± SD) in each animal group were as follows: 4.2 ± 0.8 mm/s in arterioles and 1.9 ± 0.8 mm/s in venules for BL T-PolybHb; and 4.3 ± 0.9 mm/s in arterioles and 1.8 ± 0.9 mm/s in venules for BL R-PolybHb. Flow values (means ± SD) in each animal group were as follows: 13.9 ± 4.8 nl/s in arterioles and 7.2 ± 3.2 nl/s in venules for BL T-PolybHb and 14.7 ± 4.4 nl/s in arterioles and 7.3 ± 3.0 nl/s in venules for BL R-PolybHb.

Fig. 4.

Functional capillary density (FCD) relative to BL after moderate hemodilution followed by an exchange transfusion with either T-PolybHb or R-PolybHb. The dotted line represents the BL level. †P < 0.05 relative to BL; ‡P < 0.05 compared with moderate hemodilution; §P < 0.05 among groups. FCD at BL was 122 ± 16 capillaries/cm for T-PolybHb and 119 ± 17 capillaries/cm for R-PolybHb.

Fig. 5.

Intravascular and extravascular Po₂ after hemodilution followed by an exchange transfusion with either T-PolybHb or R-PolybHb. §P < 0.05 among groups.

Fig. 6.

A: arterial O₂ supply (total O₂ transported by RBCs and PolybHb). B: arterial to arteriolar O₂ extraction (O₂ released from RBCs and PolybHb before the blood arrives to the microcirculation). C: arteriolar O₂ supply [O₂ transported by RBCs and PolybHb that is available (i.e., arrives) to microvascular tissues]. D: arterial to venular O₂ extraction (O₂ released at the microvascular level from RBCs and PolybHb). E: venular O₂ reserve (O₂ remaining in RBCs and PolybHb and returned to the central circulation). †P < 0.05, total O₂ RBCs plus PolybHb; ‡P < 0.05, O₂ from RBCs; §P < 0.05, O₂ from PolybHb.