The Science Journal of the American Association for Respiratory Care

1997 OPEN FORUM Abstracts

USE OF CCO, SPO2, AND SVO2 TO DETERMINE ENERGY EXPENDITURE: HOW RELIABLE IS IT?

D. Shaltis, BAS, RRT, S. Bailey, RRT, H. Bautista, K. Wilkie, BS, R. Dechert, MS, RRT- Critical Care Support Services- University of Michigan Medical Center

Introduction: Currently, indirect calorimetry is the most accurate and useful technique for determination of energy expenditure (EE) in the ICU population. Indirect calorimetry is preferred over the circulatory calculation of EE in the critically ill because the Fick equation requires parameters that involve sampling of arterial and mixed venous blood and thermal bolus injection for cardiac output determination. With the introduction of pulse oximetry and oximetric pulmonary artery catheters that employ continuous cardiac output (CCO) technology, however, the parameters needed for calculation of the Fick equation can be easily obtained without such costly and time consuming analysis. If the circulatory calculation of EE through the use of CCO, pulse oximetry, and mixed venous oximetry can provide accurate patient results, use of indirect calorimetry in the ICU could be diminished extensively. Methods: All mechanically ventilated patients admitted to the Trauma Burn Emergency Service who required pulmonary artery catheterization (Baxter Edwards Swan-Ganz CCO/SvO2/VIP TD, models 7468F and 746H8F, Baxter Healthcare Corporation, Edwards Critical Care Division, Irvine, CA) were considered for inclusion in this comparative study. Comparison data points were collected whenever EE through indirect calorimetry was warranted per ICU protocol. Each data point consisted of the following information: CCO, SvO2, SpO2, VO2 and EE values obtained from the metabolic cart (Sensormedic Delta Trac, Sensormedics, Yorba Linda, CA.), and Hgb obtained from the daily SvO2 calibration. Fick VO2(mL/min) was calculated as [CCO x (CaO2-CvO2) x 10] with CaO2 = Hgb x 1.34 x SpO2 and CvO2 = Hgb x 1.34 x SvO2. The portion of dissolved oxygen in plasma was not added to the equation since no PaO2 was obtained. EE was calculated from VO2 results. Bias and error were determined using the technique described by Bland and Altman. Results: 19 patients were included in this comparative study. A total of 42 data points were collected. EE calculated from CCO, SpO2, and SvO2 underestimated the values obtained from our reference standard with bias = -325 kcals/24 hours and error = 871 kcals/24 hours. We did not observe a significant correlation between the calculated EE and measured EE (r^{2}=0.19).

Conclusion: Use of CCO, SpO2, and SvO2 to calculate energy expenditure produced a relative error of 30-35% when compared against our clinical standard. Due to the significant relative error and lack of appropriate correlation, we cannot recommend the use of these continuous monitors to obtain energy expenditure in critical care patients. Determination of energy expenditure in this patient population must be obtained from dedicated metabolic carts.

OF-97-173

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