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 correations between both cardiac index measured by ther-modilution with that calculated from the Fick equation (r=0.97 p <0.001) and oxygen consumption measured directly by analysis of inspired gases (V 02) with that calculated by the reverse Fick method (OUI) (r=0.95 p <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 <0.001) and OUI (r = 0.94 p <0.001).

These results suggest that the derivation of oxygen consumption by the reverse Fick method (OUI) is a reasonable substistute for direct measurements of VO2 but occasionally the absolute values so obtained may be somewhat different. Acute vasodilatation with PGI2 does not produce a significant increase in oxygen consumption, however measured, in normals in whom consumption is independent of delivery. Calculating OUI from the cardiac index is unlikely to account for correlations between oxygen delivery and uptake observed in the critically ill which in this study probably reflected changes in metabolic rate.

Key-words:

• Oxygen consumption
• arterial oxygen delivery
• prostacyclin
• delivery dependent oxygen consumption
• metabolic rate