The American journal of physiology
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Comparative Study
Cerebral blood flow and oxygen delivery during hypoxemia and hemodilution: role of arterial oxygen content.
To determine the role of arterial O2 content (CaO2) in the cerebral blood flow (CBF) responses to hypoxemia and hemodilution, CaO2 was progressively reduced from approximately 18 to approximately 6 ml O2/dl in normocapnic, normothermic, pentobarbital-anesthetized rabbits. This was done either by reducing PaO2 (hypoxemia, minimum PaO2 approximately 26 mmHg) or arterial hematocrit (isovolemic hemodilution with hetastarch, minimum hematocrit approximately 14%) while CBF was measured with radioactive microspheres. As CaO2 decreased, CBF increased in both groups but was greater in hypoxemic animals at CaO2 values < or = 9 ml O2/dl. ⋯ By contrast, the small increase in oxygen extraction ratio seen with hypoxemia did not achieve significance. These results suggest that there are different adaptive responses to acute hypoxemia or hemodilution. They also imply that at similar CBF and CaO2 values, tissue O2 availability may be greater during hemodilution than during hypoxemia.
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Comparative Study
Capillary hemodynamics in hemorrhagic shock and reperfusion: in vivo and model analysis.
A computer network model and in vivo measurements of microcirculatory blood flow in skeletal muscle were used to study the mechanisms responsible for low flow in hemorrhagic shock and reperfusion, with focus on the potential importance of capillary diameters and leukocyte rheology. Model flows were determined by the network pressure gradient, systemic hematocrit and leukocrit, leukocyte cytoplasmic viscosity, and vessel dimensions. ⋯ Ringer-lactate (RL) reperfusion only partially restored control LDF flow, whereas a small-volume bolus of hypertonic saline-dextran followed by RL gave complete LDF flow recovery. The model predicted these flows for moderate hemodilution states, with the added insight that low-flow conditions are exacerbated by leukocytes only if they become activated, which is often a complication in ischemia/reperfusion.