Fick-Derived Hemodynamics: Oxygen Consumption Measured Directly vs Oxygen Consumption Calculated from CO2 Production under Steady State and Dynamic Conditions

Indirect calorimetry is being used increasingly as a tool for hemodynamic monitoring via the Fick equation. This investigation was undertaken to examine the use of carbon dioxide elimination ( V ˙ C O 2A ) and related respiratory quotient (RQ A ) to calculate oxygen uptake ( V ˙ o 2A ) and estimate oxygen consumption ( V ˙ O 2 ) during steady-state and dynamic hemodynamic conditions. Nine patients undergoing abdominal aortic surgery were studied intraoperatively and Fick-derived hemodynamic measurements were made using a monitoring system employing indirect calorimetry, pulse oximetry, and pulmonary artery oximetry. Comparisons were made between measured V ˙ o 2A and calculated V ˙ o 2A derived from the V ˙ C O 2A and the initial RQ A (RQ i ), which is assumed not to change. Prior to aortic crossclamping (steady state), there were no significant differences between the measured and calculated methods with respect to oxygen consumption (184 ±24 ml/min vs 185 ±17 ml/min), oxygen delivery (753 ± 141 ml/min vs 769 ± 178 ml/min), and cardiac output (4.7 ±0.6 L/min vs 4.7 ±0.7 L/min). However, immediately following aortic unclamping (dynamic state), the RQ A changed precipitously from the baseline RQ i . Consequently, significant differences between the measured and calculated methods were noted in oxygen uptake (213 ±41 ml/min vs 193 ±25 ml/min, p<0.001), oxygen delivery (780 ±297 ml/min vs 716 ±296 ml/min, p<0.001), and cardiac output (5.8 ±2.2 L/min vs 5.3 ±1.8 L/min, p<0.001). Additionally, following unclamping, the peak V ˙ o 2A was 242 ±49 compared with a c V ˙ o 2A of only 198 ±22 (p<0.01). We conclude that the use of V ˙ C O 2A to calculate V ˙ o 2A may lead to erroneous measurements under dynamic conditions, such as unclamping of the abdominal aorta.