Microvascular Blood Flow and Oxygenation in the Rat Mesentery during Hemorrhagic Hypotension (HH)

Microvascular observations and measurements of the oxygen partial pressure (PO 2 ) distribution are essential to understand tissue oxygenation. Longitudinal PO 2 gradients have been reported previously, but few attempts have been made to investigate factors that can alter such a gradient. Despite its importance in hemorrhagic shock, the mesenteric microvascular PO 2 distribution has never been evaluated during HH. We tested the hypothesis that the diverse hemodynamic responses of arterioles to HH would lead to changes in longitudinal PO 2 gradients. The mesenteric microcirculation of anesthetized rats was studied under normovolemia (baseline) and during HH (mean arterial pressure = 50 mmHg). Diameter and blood flow of arterioles and venules of different orders were studied using intravital videomicroscopy. PO 2 was determined using the phosphorescence quenching technique. During HH, blood flow was uniformly reduced in arterioles and venules, while PO 2 showed a wider distribution. In arterioles, the small longitudinal PO 2 gradient recorded during baseline was enhanced during HH. Two groups of vessels were observed with regard to PO 2 : large vessels showed PO 2 levels similar to or above baseline, while small vessels showed reduced PO 2 . Changes in blood flow did not explain this variability, since the reduction in flow was similar for all vessels, irrespective of vessel order or type. Support: CNPq and NIH.