Concept and Working Principles

 

 

The concept of the chamber system is illustrated in the figure above. Fresh air (in our case, medical grade air supplying the hospital) is sent at a known rate into the sealed room and expired room air is passively exhausted. Inside the sealed room, a single human subject staying in the chamber breathes in the fresh air and consumes the O2 in the fresh air, and expires the air CO2, thus changing the concentration of O2 and CO2 in the room air. In other words, the gas concentration in the chamber will be different from the gas concentration of the fresh air, because of the existence of the subjectSo by monitoring the gas concentration of the expired air and compare it with the gas concentration of the original air, the volume of the chamber, and how fast the air goes in and out of the chamber, we can deduce the oxygen consumption (VO2) and carbon dioxide production (VCO2) of the subject [1].

 

VO2 =(-Inflow Rate×(O2^Out× H-O_2^In )-Chamber Volume×dO2)    
VCO2 =(Inflow Rate×(CO^2Out×H+CO_2^In )+Chamber Volume×dCO2) 

Knowing VO2 and VCO2, we can further extrapolate energy expenditure or metabolic rates, which is energy expenditure per unit time, thanks to an equation called Weir’s equation [2]:

MR=(3.941 ×VO2+1.104 ×VCO2) – Weir’s Equation

 

In general, the chamber system monitors critical variables such as inflow air rate, O2 concentration in inflow air and room air, CO2 concentration in inflow and room air, room temperature, room pressure, and room humidity. All these variables are logged by a computer in the control lab via integrated software developed in LabVIEW for post-processing in Matlab®. To measure the changes in gas concentration levels in the chambers, the gas concentrations of the inflow air and the outflow air are constantly monitored at a sampling rate of once per minute. This is done by first drawing air at a rate of 2 l/min from the inflow air and outflow air, drying the air samples to a level below 1000ppm using a gas sample dryer, and then measuring the air samples’ gas concentrations using gas analyzers (manufactured by Siemens, model: Ultramat/Oxymat 6). In order to maximize the resolution of the gas analyzers, the range of the O2 gas analyzers is set to 20% ~ 21%, and the range of the CO2 gas analyzers is set to 0% ~ 1%, which are the normal ranges of room air concentration with a human subject in it.

 

References:

[1] George, A. Bartholomew, David Vleck, and Carol M. Vleck. “Instantaneous measurements of oxygen consumption during pre-flight warm-up and post-flight cooling in sphingid and saturniid moths.” Journal of Experimental Biology 90, no. 1 (1981): 17-32.

[2] Weir, JB de V. “New methods for calculating metabolic rate with special reference to protein metabolism.” The Journal of physiology 109, no. 1-2 (1949): 1-9.