Objective: To evaluate different methods and markers for assessing adequacy of tissue oxygenation in shock.
Design: Prospective, controlled animal trial. Two groups of six pigs, subjected to either a superior mesenteric artery occlusion shock or a hemorrhagic shock. A third group of five pigs served as controls.
Setting: Hospital animal research laboratory.
Subjects: Anesthetized, ventilated, juvenile, domestic pigs.
Interventions: Clamping of the superior mesenteric artery for 5 hrs, followed by reperfusion or withdrawal of blood to achieve a mean arterial pressure of 50 mm Hg for 3 hrs was performed, followed by resuscitation using the withdrawn whole blood. Invasive hemodynamic monitoring with arterial and pulmonary artery catheters was done. A tonometer was placed in the terminal ileum.
Measurements and main results: Ileal intramucosal pH, systemic base excess (or deficit), lactate concentration in systemic venous and arterial blood as well as in portal blood, ascitic fluid, and thoracic duct lymph, hemodynamics, and oxygen-related variables were measured. Five hours of intestinal ischemia caused no significant changes compared with the control group with regard to base excess or any of the hemodynamic or oxygen-related variables measured. However, lactate concentrations in the ascitic fluid and intramucosal pH were significantly altered within 1 hr of regional ischemia. Lactate concentration in the thoracic duct lymph was significantly increased after 2 hrs of ischemia, while lactate concentrations in the portal, systemic, and arterial blood were significantly increased after 2 hrs of regional ischemia. Reperfusion was associated with a high mortality rate, and only one animal survived the reperfusion period. In the hemorrhagic shock group, cardiac output and mean arterial pressure were significantly (intentionally) decreased 60 mins after the hemorrhage, while the heart rate, base excess, and systemic and portal blood lactate concentrations were significantly increased after 2 hrs of general hypoperfusion compared with those values in the control group. Ileal intramucosal pH, mixed venous oxygen saturation, oxygen delivery, oxygen extraction, and lactate concentrations in the arterial blood and thoracic duct lymph were significantly different from those values in the control group 3 hrs after the onset of hemorrhagic shock. Reperfusion induced a normalization of the hemodynamic and metabolic status of the animals.
Conclusion: Many conventional markers of tissue hypoxia are useful when assessing general hypoperfusion, whereas intestinal intramucosal pH is the only reliable and clinically useful indicator of inadequate regional intestinal tissue oxygenation.