New Metabolic Cart
I continue to develop improved hardware and software for use in expired gas analysis indirect calorimetry (EGAIC). This work extends my prior research at the University of New Mexico, where I received a patent for the improved hardware and software (US Patent 6,942,623), and later completed the publication of the validation of the system against the Parvomedics (Validation of new mixing chamber system for indirect calorimetry).
gic here is that a large mixing chamber at the end of a relatively long segment of expired tubing is dysfunctional for breath-by-breath EGAIC due to multiple breath mixing in the tubing and the mixing chamber. Of course, the lower the exercise intensity and therefore the smaller the ventilation, the greater the mixing and compromised internal validity of the system for breathe-by-breathe, or breath or time averaged data acquisition and analysis.The other side of the problem also concerns simple airway function. The current accepted methods of EGAIC are flawed because they do not account for the presence of the air in the conducting zone of the lung. During inhalation, this air is end-tidal air, and effectively alters the inspired gas partial pressures (lowers net PiO2 and raises PiCO2). Conversely, during exhalation, the air in the conducting zone is room air, which effectively raises the integral or mixed average of PeO2 and lowers PeCO2. Do not be complacent with this, for even though the changes for inspiration and expiration oppose each other for a given gas, they do not cancel out. Rather, they compound the problem. I show this in the attached Excel file with some fictitious data. This data is by no means perfect, as you can see the extreme RER values for the lowest and highest corrected data set values that are designed to represent low vs. high intensity exercise conditions. Regardless, the point is very clear that mixing of inspired and expired air during the different phases of ventilation exerts physiologically meaningful changes to the gas fractions used in computations of EGAIC. Note that the major assumption I used to derive true end tidal gas fraction conditions was for a 0.9 vs. 1.1 correction of the mixed expired gas fractions for oxygen and carbon dioxide, respectively. In reality these would not be constants based on the changing proportionality between the ADS volume and the tidal volume as exercise intensity increased.
My new method of expired gas mixing and sampling partially removes all these problems, as I sample expired air at the end of exhalation from a compliant mixing bag connected to the expired side of the mouthpiece. Thus, there is negligible added dead space for air mixing, and the relatively small compliant and elastic flow through mixing bag allows initial exhaled air (that most contaminated with the ADS air) to be flushed through the mixing bag so that more true end-tidal gas is sampled. Alignment of ventilation and expired gas fraction signals are based on the measurement of the system delay time (time for gas samples to be read by the rapid response analyzers).
I am about the start a study to quantify the discrepancy in expired gas fractions based on different mixing chamber configurations, expired tube lengths, and data acquisition configurations between my new system and the Parvomedics Tue One system.