Critical hematocrit in intestinal tissue oxygenation during severe normovolemic hemodilution

Background: A critical point in oxygen supply for microvascular oxygenation during normovolemic hemodilution has not been identified. The relation bet ween organ microvascular oxygen partial pressure (mu Po-2) and organ oxygen consumption ((V) over dot O-2) during a decreasing oxygen delivery (Do(2)) is not well understood. The present study was designed to determine the sy stemic hematocrit and organ Do(2) values below which organ mu Po-2 and (V) over dot O-2, cannot be preserved by regulatory mechanisms during normovole mic hemodilution. Methods: Eighteen male Wistar rats were randomized between an experimental group (n = 12), in which normovolemic hemodilution was performed with paste urized protein solution (PPS), and a control group (n = 6). Systemic hemody namic and intestinal oxygenation parameters were monitored. Intestinal mu P o-2 was measured using the oxygen-dependent quenching of palladium-porphyri n phosphorescence. Results: Baseline values in hemodilution and control group were similar, He modilution decreased hematocrit to 6.2 +/- 0.8% (mean +/- SD). Constant cen tral venous pressure measurements suggested maintenance of isovolemia. Desp ite an increasing mesenteric blood flow, intestinal Do(2) decreased immedia tely. Initially, mu Po-2 was preserved, whereas mesenteric venous Po-2 (P(m v)o(2)) decreased; below a hematocrit of 15%, mu Po-2 decreased significant ly below P(mv)o(2). Critical Do(2) was 1.5 +/- 0.5 ml . kg(-1) . min(-1) fo r (V) over dot O-2, and 1.6 +/- 0.5 ml . kg(-1) . min(-1) for mu Po-2. Crit ical hematocrit values for (V) over dot O-2 and mu Po-2 were 15.8 +/- 4.6% and 16.0 +/- 3.5%, respectively. Conclusions: Intestinal mu Po-2 and (V) over dot O-2 were limited by a crit ical decrease in Do(2) and hematocrit at the same time. Beyond these critic al points not only shunting of oxygen from the microcirculation could be de monstrated, but also a significant correlation between intestinal mu Po-2 a nd (V) over dot O-2.