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Calibration of Metabolic SystemsIntroduction
Nearly all metabolic measurement systems are based on analysis of respiratory gas exchange at the mouth, thus, the calibration of these systems should consist in controlled simulation of the process of breathing. This simulation should incorporate essential physiological characteristics such as the respiration-like pattern of gas flows and the dynamic temporal profiles of O2 and CO2 concentrations, reflecting the tidal nature of their within-breath fluctuations. Consequently, the essence of metabolic calibration is generation of a predetermined mass flow of gases intended to be detected by the measurement system under calibration. The calibration apparatus would, therefore, consist of two functionally distinct subsystems, namely, the pump, which is essentially a motorized syringe and the mass flow control kit enabling titration of the calibration gas (21% CO2, 79% N2) mixture and its subsequent mixing with room air during the inspiratory phase of pumping to convert the continuous "metabolic flow" of titration to the intermittent pattern of respiratory flows generated by the pump. The resulting separation of metabolic rates from the level of ventilation constitutes the fundamental tenet of stringent calibration (see Huszczuk et al, 1990).
A Motorized "Syringe" Pump
Click here to see the Motorized Syringe Pump
Mean Flow = VT x f/30
Peak Flow = Mean Flow x square root of 2
In practical application, it creates a unique opportunity to slowly scan the entire range of pumping (breathing) frequencies at any chosen stroke (tidal) volume while your experimental data acquisition system (i.e. flow sensor - electronic processing - computer - software storage and display, etc.) is computing tidal volume, which is equal to the area under the flow curve. Most of existing flow transducers distort this flow curve at least in one of the practically relevant regions, which will be reflected in a deviation of the computed tidal volume from a selected value. In most cases a 1% deviation is acceptable, although it does not mean that other problems do not exist. These usually concern chemical composition of gases, which nearly always is different than air typically used to calibrate respiratory flow meters and is known to greatly affect the transduction characteristics of nearly all flow transducers. Therefore, generation of respiratory flows must be complemented with means of varying their chemical composition to enable comprehensive calibration of flow transducers. <P>
Generation of Metabolic Mass Flows
Metabolic mass flow is accomplished by supplementing the respiratory pump with a metabolic kit consisting of the following devices:
In essence, then, the respiratory flows generated by the pump become the transport vehicle for the metabolic output, much like in mammalian physiology, as soon as the calibration gas is allowed to flow (i.e., the equivalent of the right ventricular output).
The Calibration Gas Mixture
A mixture of 21% CO2, balance N2, has been chosen due to the following rationale.
Hypothetically, the ideal metabolizing medium would extract all 21% of O2 available in air and, with respiratory exchange ratio R = 1, would exhale 21% CO2 in N2. The choice of R = 1 is important, as it corresponds to equality of inspired and expired tidal volumes, thus obviating potential errors in computing O2 uptake (VO2), that may result from unidirectional (generally expiratory) measurement of respired volumes. Consequently, the simulated O2 uptake and CO2output (VCO2) are approximately equal and amount to about 21% of the calibration gas flow, e.g., if the calibration gas flow was set to 10 l/min, then, the VO2 and VCO2 rates will be 2.1 l/min. The exact rates will slightly vary depending on ambient humidity, temperature and the actual CO2 content in the calibration gas. The cal gas flow controls are calibrated in ATPD (ambient temperature, pressure dry), so to derive the STPD (standard pressure temperature dry) values, the actual barometric pressure and room temperature have to be known.
Traditional calibration procedures utilize a large syringe to displace a known volume of air through the gas flow sensor, while gas analyzers are subjected to a two-point calibration (usually room air and a calibration gas mixture from a tank) which software uses to force a regression line.
This can be satisfactory only when the linearity of all sensors involved can be established across the whole span of potential signal amplitudes, which is questionable at best. The overall software computing routine is actually never scrutinized, because in addition to the input variables, proper entry of parameters such as barometric pressure, humidity, temperature and transport delays etc. is also required. This can only be achieved "off-line" with the Douglas bag method - a tedious and also error-prone procedure.
The only alternative is generation of the ventilatory and metabolic variables, which can be fed to the metabolic measurement system and controlled at will, to either scan the whole expected measurement range, or focus on the one of particular interest (e.g. resting metabolism).
The fundamental advantage of the metabolic calibrator is that it utilizes the mass preservation law. This in practice means that any chosen mass flow (metabolic rate) can be delivered to the system under calibration using a range of the ventilatory flows resulting in a range of dilution rates as illustrated in figure 1, where delta O2 = 21-Mixed exhaled O2. Setting pumping rates (f) to deliver VE1, VE2 and VE3 at constant tidal volume VT will result in the respective gas concentrations of C1, C2 and C3. In all three settings the computed variables VT, VO2 and VCO2 should be the same and reflect chosen tidal volumes and metabolic rate as areas of the three rectangles (1, 2, 3) must be equal. Note that, consequent to the rules of dilution, the low ventilation regions correspond to high gas concentration ones and vice versa. Therefore the performance of all three sensors in a metabolic measurement system as well as the accuracy of computing (software) can be quickly and accurately scrutinized.