Increased tissue oxygen extraction and acidosis with progressive severity of sepsis

Background. Lactic acidosis and increased production of CO₂ are common in septic shock. Presumably, both acidosis and CO₂ enhance the release of oxygen from hemoglobin. The purpose of this study was to assess the relationship of oxygen utilization, CO₂ production, acidosis, and hemoglobin oxygen (Hgb-O₂) dissociation with progressive severity of sepsis to shock. Materials and methods. Femoral arterial and vein, hepatic vein, portal vein, and pulmonary artery catheters were placed in 16 anesthetized swine. Organ blood flow was determined by timed injections of colored microspheres. After baseline measurements, Pseudomonas aeruginosa was infused in eight animals. This bacterial slurry was continued inciting a progression of sepsis to shock. Eight animals served as instrumented controls. Results. With sepsis and shock, there was a progressive decrease in pH and an increase in pCO₂ in plasma with all sampling sites (P < 0.01 septic shock versus baseline versus control). Blood flow to the liver and intestines increased with sepsis (P < 0.01) but then returned to near baseline control values during shock. VO₂ and/or percent O₂ extraction increased with sepsis and septic shock for the whole body and for the liver, intestine and leg (P < 0.01). There was a strong correlation between venous O₂ saturation, acidosis, and pCO₂ to percent O₂ extraction (r > 60; P < 0.0001). However, calculated P₅₀ values for Hgb-O₂ dissociation remained unchanged. Conclusions. This study demonstrates that increased oxygen extraction in severe sepsis is related to a fall in tissue oxygen availability and not related to any allosteric change in Hgb-O₂ dissociation. Therefore, acidosis and hypercapnia do not have a demonstrable effect on altering oxygen availability during sepsis.