Closed-loop control of air supply to whole-room indirect calorimeters to improve accuracy and standardize measurements during 24-hour dynamic metabolic studies.

Abstract

The aim of this study was to test proportional-integral-derivative (PID) control of air inflow rate in a whole-room indirect calorimeter to improve accuracy in measuring oxygen (O₂ ) consumption ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>O</mi> <mn>2</mn></msub> </mrow> </math> ) and carbon dioxide (CO₂ ) production ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>CO</mi> <mn>2</mn></msub> </mrow> </math> ). A precision gas blender infused nitrogen (N₂ ) and CO₂ into the calorimeter over 24 hours based on static and dynamic infusion profiles mimicking <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>O</mi> <mn>2</mn></msub> </mrow> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>CO</mi> <mn>2</mn></msub> </mrow> </math> patterns during resting and non-resting conditions. Constant (60 L/min) versus time-variant flow set by a PID controller based on the CO₂ concentration was compared based on errors between measured versus expected values for <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>O</mi> <mn>2</mn></msub> <mo>,</mo> <mspace></mspace> <mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>CO</mi> <mn>2</mn></msub> <mo>,</mo></mrow> </math> respiratory exchange ratio, and metabolic rate. Compared with constant inflow, the PID controller allowed both a faster rise time and long-term maintenance of a stable CO₂ concentration inside the calorimeter, resulting in more accurate <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>CO</mi> <mn>2</mn></msub> </mrow> </math> estimates (mean hourly error, PID: -0.9%, 60 L/min = -2.3%, p < 0.05) during static infusions. During dynamic infusions mimicking exercise sessions, the PID controller achieved smaller errors for <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>CO</mi> <mn>2</mn></msub> </mrow> </math> (mean: -0.6% vs. -2.7%, p = 0.02) and respiratory exchange ratio (mean: 0.5% vs. -3.1%, p = 0.02) compared with constant inflow conditions, with similar <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow><mover><mi>V</mi> <mo>̇</mo></mover> <msub><mi>O</mi> <mn>2</mn></msub> </mrow> </math> (p = 0.97) and metabolic rate (p = 0.76) errors. PID control in a whole-room indirect calorimeter system leads to more accurate measurements of substrate oxidation during dynamic metabolic studies.