1995 OPEN FORUM Abstracts
In Vitro Validation Of The Puritan-Bennett 7250 Metabolic Monitor
F Wayne Johnson RCP CRTT RPFT RCPT, Kathy Jacobson RDA, K Knaus Kinninger RCP RPFT, Ronda Pinnell CVT, David Burns MD. UC-San Diego Medical Center, San Diego, California
Introduction: We validated a new commercially available open circuit Metabolic Monitor (PB7250, Puritan-Bennett, Carlsbad, CA) designed as an accessory to the PB7200ae ventilator. The microprocessor-controlled PB7250 Monitor interfaces pneumatically and electrically with the PB7200ae ventilator for the clinical application of indirect calorimeter measurements. In laboratory simulations we examined whether changes in PEEP, FIO2, VE, and Vt influence the accuracy of the measurement of VO2 and VCO2 with the PB7250 Monitor.
Methods: The simulation of VO2 and VCO2 was achieved by infusing known flows of N2 and CO2 into a constructed lung model, (Damask, Anesth. 1982;57:213) (Kappagoda Cardiovas. Res. 1972;6:589). The infusion rates of the N2 and CO2 were kept constant to assure a known predicted value through a series of precision needle valves (Nupro Co., Willoughly, OH), rotameters (Fisher-Porter, Warminster, PA) and pressure gauges (Dwyer Instruments, Michigan City, IN). The N2 and CO2 gas flows were verified volumetrically before and after each trial by a dry rolling spirometer (OHIO Medical Products, Madison, WI). The predicted gold standards for VO2 and VCO2 were calculated using the following equations: VO2= (VN2 added)(FIO2/1-FIO2)(STPD) VCO2= (VCO2 added)(STPD). The independent measurement of FIO2 is essential to establish a true predicted value for VO2. The FIO2 was analyzed with a 1100A mass spectrometer (Perkin-Elmer, Pasadena, CA). All simulations were performed with the PB7200ae ventilator with humidification of inspired gas (35ºC), disposable heated wire circuit, SCT heated humidifier. Our selected test matrix utilized modes of ventilation which included CMV, PC-IRV and Flow-By with a predicted VO2 and VCO2 of .300 L/min (STPD). The CMV settings included: flow waveforms of ramp, square and sine; peak flow of 30, 40, 50, 60, L/min.; PEEP of 5, 15 cm H_2O; Vt .700 L; f of 12, 18, and FIO2 of .21, .40, .60 .80. The PC-IRV settings included: inspiratory pressure of 25 and 40 cm H_2O;I/E ratio of 1:1 and 2:1; PEEP of 0, 5, 15 cm H_2O; f of 18 and 22 and FIO2 of .21, .40, .60, .80. The Flow-By settings in a ramp flow waveform included: base flow/sens. of 10/5, 5/1 L/min; peak flow of 40, 60 L/min; PEEP of 0, 5, 15 cm H_2O; Vt .250, .800 L; f of 10, 25 and FIO2 of .40, .60.
Results: The performance evaluation compares the PB7250 results with a "gold standard" or predicted value simulated by the lung model. The number and the values of metabolic measurements are presented in the table. The mean difference between methods (bias) and the standard deviation of the differences (precision) show statistical assessment of agreement and variability (Bland, Lancet 1986; Feb:307-310).
MODE CMVPC-- IRV FLOW--BY
Predicted .300 L/min .300 L/min .300 L/min
VO2VCO2VO2 VCO2VO2 VCO2
BIAS %1.77 -1.51 3.04 2.421.63 -.91
PRECISION 2.92 1.924.73 2.843.80 3.42
# of MEASUREMENTS 720720 400 400 560 560
Conclusion: The Puritan-Bennett 7250 Metabolic Monitor provides accurate measurements of VO2 and VCO2 over a wide range of mechanical ventilator settings encountered by the critical care patient. Measurements of bias and precision were comparable and within acceptable limits of measurements obtained by devices currently utilized in clinical practice.