Assessment of Three Methods to Reduce the Influence of Mathematical Coupling on Oxygen Consumption and Delivery Relationships

¹ From the Division of Cardiac Surgery, and Program of Critical Care, Pulmonary Research Laboratory, St. Paul's Hospital, University of British Columbia, Vancouver, BC, Canada

Study objectives: To determine if oxygen consumption/oxygen delivery (VO₂/DO₂) relationships derived using calorimetry (which are not influenced by shared measurement error) agreed with those obtained using the pulmonary artery (PA) catheter alone. To evaluate three strategies to reduce the influence of shared measurement error to determine if agreement between the two methods could be improved.

Methods: Twenty-seven patients were studied following coronary artery bypass surgery. Calorimetric VO₂, six thermodilution cardiac outputs (COs), and arterial and mixed venous oxygen content measurements were made at baseline and were repeated 30 min following dobutamine administrations of 3 µg/kg/min and 5 µg/kg/min.

Results: Dobutamine produced a dose-dependent increase in DO₂, from 378±65 mL/min/m² to 446±78 mL/min/m² (p<0.01) and in both PA catheter and calorimetric-derived VO₂, from 104±18 mL/min/M² to 114±22 mL/min/m² (p<0.05) and from 117±15 mL/min/m² to 126±19 mL/min/m² (p<0.01), respectively. Agreement was poor (bias=12%, SD=21%) between the calorimetric and PA catheter methods of determining VO₂/DO₂ slope. When three CO measurements were used to calculate VO₂, and three separate CO measurements were used to calculate DO₂, the level of agreement between the two methods improved (bias=2%, SD=15%). Increasing the number of COs resulted in a similar improvement in the level of agreement between the two methods. Weighting the slope to the observed change in DO₂ was the best method to improve the level of agreement (bias=2%, SD=6% for three COs).

Conclusions: To reduce the influence of shared measurement error, the best strategy to improve the measurement of VO₂/DO₂slope is to maximize the change in DO₂ (optimally over 100 mL/min/m²).

Key Words: calorimetry • cardiac output • oxygen consumption • oxygen delivery • pathologic supply dependence

Submitted on January 3, 1997
Accepted on November 3, 1997

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