A Laboratory Evaluation of the Biergy VVR Calorimeter

Abstract

We evaluated the accuracy of the Biergy VVR Calorimeter in the laboratory. METHODS: To evaluate the accuracy of oxygen consumption (Vo₂) measurements, we created a controlled volumetric leak of oxygen from a closed circuit and studied the VVR's Vo₂ value determinations at seven volumes from 100 to 400 ml. We also compared tidal volumes (VT) measured by the VVR to those measured by pneumotachography, at four known volumes. To assess the effects of the VVR on mechanical ventilation when placed in line with a ventilator, we measured VT from a ventilator with and without the VVR in line, using four volume settings and three levels of compliance. We determined the CO2 absorber's functional life by simulating a CO2 production of 250 ml/min and noting how many minutes the absorber maintained FECO₂ at 0%. We measured the absorber's resistance to flow prior to use, after 30 min, and at the end of its functional life. RESULTS: Correlation between true Vo₂ and VVR-measured VO₂ was excellent (r = 0.991). However, Vr measured by the VVR was approximately 20% less than actual VT. Placing the VVR between a ventilator and test lung (to simulate use) caused falls in delivered VT commensurate with peak inspiratory pressure. The mean ± SD functional life of the CO2 absorber was 78 ± 4 min, and changes in the absorber's resistance to flow were not statistically significant. CONCLUSIONS: In the laboratory, the VVR accurately measured Vo, simulated by loss of oxygen volume, but VT measurements by the VVR were inaccurate. Appropriate use of the device during mechanical ventilation requires that delivered Vr be adjusted for changes caused by an increase in compressible volume. Our results suggest that accurate calculations of resting energy expenditure can be made, provided appropriate principles are understood and taken into account. (Respir Care 1988;33:341-347.)