The relevance of measuring O2 supply and O2 consumption for assessment of regional tissue oxygenation.

Septic shock and ARDS are associated with disturbed tissue oxygenation. It has been suggested to increase O2 supply (DO2) above the normal level (> 600 ml/min/m2) to compensate for the tissue hypoxia. The lack of a rise in O2 consumption (VO2) after increases of DO2 has been presumed to indicate adequate tissue oxygenation (negative O2 flux test). We were interested in whether a negative O2 flux test precludes an improvement of regional tissue oxygenation. The pH value of the gastric mucosa (pHi) is considered to be a sensitive marker for hypoxia in the splanchnic region. We measured pHi as well as DO2 and VO2 in 10 patients with hyperdynamic septic shock to assess the effect of volume substitution on tissue oxygenation. The initial therapeutic approach (volume substitution and catecholamines) led to a DO2 of 717 +/- 187 ml/min/m2. However, all patients had pHi values < 7.35 indicating regional tissue hypoxia. An additional increase of DO2 by colloidal volume substitution caused a significant rise of pHi from 7.20 +/- 0.05 to 7.25 +/- 0.05 but did not change VO2. We conclude that a negative O2 flux test does not rule out regional tissue hypoxia, and second, an increase in DO2 may improve tissue oxygenation without measurable changes in VO2. Furthermore, adequate volume substitution is an important step in the treatment of septic shock to increase total body blood flow and more specifically regional blood flow.