Microvascular blood flow and oxygenation during hemorrhagic hypotension

Understanding microvascular oxygen transport requires the knowledge of microvessel topology and geometry, blood flow and oxygen levels. Microvascular hemodynamic responses to hemorrhagic hypotension (HH) such as size-dependent vasoconstriction and blood flow reduction could lead to increased longitudinal oxygen partial pressure (PO2) gradients. However, the mesenteric microvascular PO2 has never been evaluated during HH. Therefore, we studied hemodynamic variables and PO2 distribution in 165 mesenteric microvessels from 39 anesthetized rats to investigate whether HH-induced vasoconstriction and blood flow reduction were associated with changes in longitudinal PO2 gradients. Vessels were analyzed according to their position in the network, as well as a few interstitial PO2 areas. We found that during baseline a small PO2 gradient exists, but HH is accompanied by more pronounced microvascular longitudinal PO2 gradients. Decreased blood flow did not seem to completely explain these findings, since blood flow was uniformly diminished in arterioles and venules, independent of diameter and position in the network. During HH, some microvessels presented higher PO2 than during baseline despite blood flow reduction, possibly due to a combination of systemic hyperoxia and low oxygen consumption of mesentery. The data suggest that blood flow measurements may be a poor indicator of the oxygenation status in some regions of the mesentery. The enhanced mesenteric longitudinal PO2 gradient may lead to regions with different levels of other physiologically active compounds.