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

Abstract

Microvascular observations and measurements of the oxygen partial pressure (PO2) distribution are essential to understand tissue oxygenation. Longitudinal PO2 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 PO2 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 PO2 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. PO2 was determined using the phosphorescence quenching technique. During HH, blood flow was uniformly reduced in arterioles and venules, while PO2 showed a wider distribution. In arterioles, the small longitudinal PO2 gradient recorded during baseline was enhanced during HH. Two groups of vessels were observed with regard to PO2: large vessels showed PO2 levels similar to or above baseline, while small vessels showed reduced PO2. 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.