Multiple propane gas burn rates procedure to determine accuracy and linearity of indirect calorimetry systems: an experimental assessment of a method

Abstract

Objective: Indirect calorimetry (IC) systems measure the fractions of expired carbon dioxide (F_eCO₂), and oxygen (F_eO₂) recorded at the mouth to estimate whole-body energy production. The fundamental principle of IC relates to the catabolism of high-energy substrates such as carbohydrates and lipids to meet the body's energy needs through the oxidative process, which are reflected in the measured oxygen uptake rates (V̇O₂) and carbon dioxide production rates (V̇CO₂). Accordingly, it is important to know the accuracy and validity of V̇O₂and V̇CO₂ measurements when estimating energy production and substrate partitioning for research and clinical purposes. Although several techniques are readily available to assess the accuracy of IC systems at a single point for V̇CO₂ and V̇O₂, the validity of such procedures is limited when used in testing protocols that incorporate a wide range of energy production (e.g., basal metabolic rate and maximal exercise testing). Accordingly, we built an apparatus that allowed us to manipulate propane burn rates in such a way as to assess the linearity of IC systems. This technical report aimed to assess the accuracy and linearity of three IC systems using our in-house built validation procedure.

Approach: A series of trials at different propane burn rates (PBR) (i.e., 200, 300, 400, 500, and 600 mL min^-1) were run on three IC systems: Sable, Moxus, and Oxycon Pro. The experimental values for V̇O₂ and V̇CO₂ measured on the three IC systems were compared to theoretical stoichiometry values.

Results: A linear relationship was observed between increasing PBR and measured values for V̇O₂and V̇CO₂ (99.6%, 99.2%, 94.8% for the Sable, Moxus, and Jaeger IC systems, respectively). In terms of system error, the Jaeger system had significantly (p < 0.001) greater V̇O₂(mean difference (M) = -0.057, standard error (SE) = 0.004), and V̇CO₂(M = -0.048, SE = 0.002) error compared to either the Sable (V̇O₂, M = 0.044, SE = 0.004; V̇CO₂, M = 0.024, SE = 0.002) or the Moxus (V̇O2, M = 0.046, SE = 0.004; V̇CO₂, M = 0.025, SE = 0.002) IC systems. There were no significant differences between the Sable or Moxus IC systems.

Conclusion: The multiple PBR approach permitted the assessment of linearity of IC systems in addition to determining the accuracy of fractions of expired gases.

Keywords: Accuracy; Energy production; Indirect calorimetry; Linearity; Propane gas.

Figure 1. Schematic representation of the in-house built propane gas device utilized to assess the accuracy and linearity of indirect calorimetry systems.

The propane gas device encompasses a tank of chemically pure propane gas with a two-stage Western Medical gas regulator and its gas hose connected to a one-way gas mass-flow transducer subsequently connected to a gas mass-flow controller, and to a Bunsen burner. The burner is located into a 2.4 L glass canopy that flows into a 0.4 L glass tubing. From there, the entire system is connected to IC by a 1.4-inch diameter hose.

Figure 2. Mean rates of oxygen uptake (VO₂, L min⁻¹.

(A) Carbon dioxide production (VCO₂, L min⁻¹; (B) and respiratory exchange ratio (RER; (C) measured during propane gas combustion tests with three metabolic carts: Sable system (dashed line), Moxus system (solid line), and Jaeger system (dotted line). Values are means SD.

Figure 3. Mean difference (delta) between the stoichiometric theoretical values and the actual experimental values of oxygen uptake (V̇O₂, L min⁻¹.

(A) Carbon dioxide production (V̇CO₂, L min⁻¹; (B) and respiratory exchange ratio (RER; C) measured during propane gas combustion tests with three indirect calorimetric systems: Sable system (dashed line), Moxus system (solid line), and Jaeger system (dotted line). Values are means.