The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

VALIDATION OF THE QUINTON METABOLIC CART DURING MECHANICALLY VENTILATION

K. Knaus Kinninger RCP RPFT, F. Wayne Johnson RCP RPFT, Kathy Jacobson RDA, David M. Burns MD University California San Diego Medical Center, San Diego California

Introduction: Metabolic testing of the critically ill ventilator dependent patients is of particular concern to the practitioner. It is important to accurately measure the patients metabolic requirements because of the link between a patients nutritional needs and ventilator dependency. The accuracy of indirect calorimeters may deteriorate when measurements are made on patients mechanically ventilated with high levels of PEEP or high concentrations of FIO2, the very patients for which the determination of oxygen consumption (VO2) and carbon dioxide production (VCO2) would be most useful. The Quinton Metabolic Cart (QMC) (Quinton Instrumentation, Bothell, WA.) is a non-invasive portable monitor, employing an open circuit technique of indirect calorimetry. The system was designed to operate with the spontaneously breathing or mechanically ventilated patient. We examined the performance characteristics of the QMC in a laboratory simulation to determine whether changes in PEEP, FIO2, VT and rate influence the measurement of VO2 and VCO2 during mechanical ventilation. Methods: Our laboratory simulation of VO2 and VCO2 was modeled after an established technique of infusing known flows of N2 and CO2 into a constructed lung model. The N2 and CO2 gas flows were controlled by precision rotometers and verified volumetrically before and after each trial. The predicted values for VO2 and VCO2 to be used as the method for comparison measurements, were calculated using the following equations:

VO2 = (VN2 added)(FIO2/1-FIO2)(STPD)

VCO2 = (VCO2 added)(STPD)

Independent FIO2 measurements were analyzed with a 1100A mass spectrometer (Perkin-Elmer, Pasadena, CA). All simulations were performed with a PB7200ae ventilator with a heated wire circuit, humidification at 35 C, and the following settings; CMV; ramp waveforms; peak flow of 60 l/min; PEEP of 0,15 cmH20; FIO2 of .21, .40, .60; Vt .500 and .700 L; f of 12, 18, and 20. Our test matrix established a predicted VO2 and VCO2 of .300 L/min. Results: The mean difference between methods (Bias) and the standard deviation (Precision) were determined to express agreement between data sets (Bland & Altman, Lancet, Feb. 1986)

VO2 VCO2

# of measurements 24 24

Bias % -2.43 0.25

Precision % 1.95 2.40

Conclusion: The QMC provides stable and accurate measurements of VO2 and VCO2 over a range of ventilator settings typically seen in the ICU critical care patient. Measurements of bias and precision were comparable and within acceptable limits of measurements obtained by devices currently utilized in clinical practice.

Reference: OF-96-023

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