Alterations in tissue oxygen consumption and extraction after trauma and hemorrhagic shock

Objectives: Although trauma and hemorrhage are associated with tissue hypoperfusion and hypoxemia, changes in oxygen delivery (D over dot O-2), oxygen consumption (V over dot O-2), and oxygen extraction at the organ level in a small animal (such as the rat) model of trauma and hemorrhage have not been examined. Therefore, the objectives of this study were to determine whether blood flow, D over dot O-2, V over dot O-2, and oxygen extraction ratio in various organs are differentially altered after trauma-hemorrhagic shock and acute resuscitation in the rat. Design: Prospective, randomized animal study. Setting: A university research laboratory. Subjects: Male Sprague-Dawley rats (n = 6-7 animals/group) weighing 275-325 g. Interventions: Male rats underwent laparotomy (i.e., soft tissue trauma) and were bled to and maintained at a blood pressure of 40 mm Hg until 40% of shed blood volume was returned in the form of lactated Ringer's solution. They were then resuscitated with four times the volume of shed blood with lactated Ringer's solution for 60 mins. At 1.5 hrs postresuscitation, cardiac output and blood flow were determined by using strontium-85 microspheres. Blood samples (0.15 mt each) were collected from the femoral artery and vein and the hepatic, portal, and renal veins to determine total hemoglobin and oxygen content. Systemic and regional D over dot O-2, V over dot O-2, and oxygen extraction ratio were then calculated. Measurements and Main Results: Both the systemic hemoglobin and systemic arterial oxygen content in hemorrhaged animals at 1.5 hrs postresuscitation were >50% lower as compared with sham-operated controls. Cardiac output and blood flow in the liver, small intestine, and kidneys decreased significantly, but blood flow in the brain and heart remained unaltered after hemorrhage and resuscitation. Systemic D over dot O-2 and V over dotO(2) were 73% and 54% lower, respectively, than controls at 1.5 hrs after resuscitation. Similarly, regional D over dot O-2 and V over dot O-2 in the liver, small intestine, and kidneys decreased significantly under such conditions. In addition, the liver had the most severe reduction in V over dot among the tested organs. However, the oxygen extraction ratio in the liver of sham animals was the highest (72%) and remained unchanged after hemorrhage and resuscitation. Conclusion: Because the liver experienced the most severe reduction in V over dot O-2 associated with an unchanged oxygen extraction capacity, this organ appears to be more vulnerable to hypoxic insult after hemorrhagic shock. (Crit Care Med 2000; 28: 2837-2842).