A comparison of direct and indirect measurements of oxygen delivery and consumption: the effects of prostacyclin in two human volunteers

Abstract

To examine the relation of oxygen delivery to uptake in normals, we have measured cardiac index, oxygen delivery and oxygen consumption directly and indirectly in two human volunteers before and during a prostacyclin (PGI2) infusion (5 ng/kg/min). We have also investigated the relation of direct and indirect measurements of delivery to consumption in two critically ill patients over a more prolonged study period of 24 hours. Overall, there were close correlations between both cardiac index measured by thermodilution with that calculated from the Fick equation (r = 0.97 p less than 0.001) and oxygen consumption measured directly by analysis of inspired gases (V O2) with that calculated by the reverse Fick method (OUI) (r = 0.95 p less than 0.001). Nevertheless, the limits of agreement between the two methods were wide (1.6 L/min.m2 for the cardiac index and 70.5 ml 02/min.m2 for oxygen consumption, 95% confidence limits). In the two human volunteers, PGI2 produced substantial increases in oxygen delivery but there was no change in oxygen consumption measured directly or indirectly; V O2 and OUI were unrelated and independent of oxygen delivery. However, in the two patients studied over 24 hours, there were close correlations between delivery and both V O2 (r = 0.93 p less than 0.001) and OUI (r = 0.94 p less than 0.001). These results suggest that the derivation of oxygen consumption by the reverse Fick method (OUI) is a reasonable substitute for direct measurements of V O2 but occasionally the absolute values so obtained may be somewhat different.(ABSTRACT TRUNCATED AT 250 WORDS)