Passive, proportional measurement of oxygen and carbon dioxide consumption for assessment of metabolic parameters
Inventors
MCCLUNG, Holly • Hoyt, Reed Wasson • Candell, Lawrence M • MAHAN, Joseph M. • Shaw, Gary A • Siegel, Andrew M • STANDLEY, Robert Linton • Thompson, Kyle
Assignees
Massachusetts Institute of Technology • United States Department of the Army
Publication Number
US-11464426-B2
Publication Date
2022-10-11
Expiration Date
2039-05-16
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Abstract
A conventional flow tube for a metabolic cart is usually a straight length of pipe whose inner diameter is fixed by the respiratory burden imposed by the flow tube on the user, with a smaller diameter imposing a higher respiratory burden. The ratio of the straight flow tube's length to diameter is fixed by fluid dynamics, so increasing the flow tube's diameter causes the flow tube's length to increase. As the flow tube gets longer, it exerts more torque on the user's neck and jaw, creating discomfort. Reducing the flow tube's length causes an undesired increase in the respiratory burden but increasing the flow tube's diameter to reduce the respiratory burden makes the flow tube less comfortable, making the flow tube unconformable, hard to breathe through, or both. Bending the flow tube, e.g., in an L shape, makes it possible to increase the flow tube's propagation length without increasing the flow tube's lever arm length.
Core Innovation
Indirect calorimetry is a well-established method for measuring in vivo gas exchange, specifically the volume of oxygen consumed (VO2) and carbon dioxide produced (VCO2) by an individual, to estimate substrate utilization and energy metabolism. Traditional systems, such as whole room calorimeters and metabolic carts, either impose practical constraints limiting the types of activities conducted or present challenges in temporal resolution, size, cost, and mobility. Whole room calorimeters allow free-living conditions but have coarse temporal resolution and high cost, whereas metabolic carts offer higher temporal resolution but are bulky and impractical for mobile or field use.
Breath-by-breath systems address mobility and size issues by measuring flow rates and gas concentrations rapidly and minimally, but face challenges with time alignment of measurements due to separate sensor placements and response times. Additionally, conventional flow tubes used in metabolic carts are typically straight with fixed diameter, imposing respiratory burdens on users and discomfort due to their length and exerted torque leading to a trade-off between respiratory burden and comfort.
The invention addresses these problems by introducing a flow-rate proportional passive side-stream sampling system with a bent flow tube, mixing chamber, and sensors, enabling accurate metabolic measurements without valves or moving parts. The bent flow tube, with bends of about 75 to 105 degrees, enables increasing the tube’s propagation length without increasing lever arm length, reducing the torque and making the device more comfortable. The system passively diverts a proportional fraction of exhaled breath into a mixing chamber for analysis, and the bend creates asymmetric pressure profiles that prevent inhaled air from entering the mixing chamber, thus avoiding dilution of the sample.
This arrangement allows for a smaller mixing chamber and use of slower, less expensive sensors while preserving measurement fidelity. The proportional passive sampling maintains a constant fraction of exhalation sampled, reducing respiratory burden and increasing comfort compared to traditional straight tubes. The system can be completely passive without pumps or mechanical valves, enhancing reliability, reducing costs, and enabling portability for field or mobile metabolic measurements previously impractical.
Claims Coverage
The patent includes multiple independent claims covering the main inventive features of a flow-rate proportional passive side-stream sampling system, a method of proportional passive flow-rate sampling, and a bent flow tube for a metabolic cart.
Flow-rate proportional passive side-stream sampling system
A system comprising a bent flow tube with a bend between its inlet and outlet configured for bidirectional breath flow, a first port between the inlet and bend fluidly connected to a mixing chamber that receives a fraction of exhaled breath proportional to instantaneous exhale flow, and at least one sensor in fluid communication with the mixing chamber to measure volumetric flow rate, oxygen and/or carbon dioxide content or partial pressures of the sampled breath fraction.
Bend angle and port arrangement in the flow tube
The bent flow tube has a bend of about 75 to 105 degrees between inlet and outlet and may include a second port between the bend and the outlet connected to the mixing chamber and lumen to convey gas back.
Valves and saliva trap integration
The flow tube can include at least one valve disposed between the inlet and bend to relieve pressure during inhalation, and a saliva trap fluidly connected to the lumen to collect saliva excreted by the person.
Removable mouthpiece
The system may have a removable mouthpiece connected to the inlet of the bent flow tube for user comfort and hygiene.
Mixing chamber design
The mixing chamber may include perforated baffles and/or a perforated circuit board to foster mixing of multiple breath fractions and may house the sensor(s) measuring gas parameters.
Proportional passive flow-rate sampling method
A method comprising receiving exhaled breath at the inlet of a rigid bent flow tube, conveying a fraction of exhaled breath from a port between the inlet and a J-shaped bend to a mixing chamber proportional to instantaneous exhale flow rate, and measuring gas parameters of the fraction with at least one sensor.
Bent flow tube for metabolic cart
A flow tube with a proximal end, distal end, and a bend between them defining a lumen to convey exhalations. It includes a first port between distal end and bend and a second port between bend and proximal end fluidly connected to a mixing chamber for bidirectional conveyance of breath portions, with optional integrated valves, saliva trap, and removable mouthpiece. The bend is typically about 75 to 105 degrees, e.g., about 90 degrees.
The claims collectively cover a passive, valve-less, flow-rate proportional breath sampling system featuring a bent flow tube with specific port and valve configurations, integrated saliva trapping, a specialized mixing chamber for gas measurement, and methods for proportional passive flow rate sampling that enhance comfort, reduce respiratory burden, and improve measurement fidelity in metabolic assessments.
Stated Advantages
The device is small, inexpensive, simple to use, and completely passive, with no moving parts or electronics.
The bent flow tube reduces torque on the user's neck and jaw, improving comfort during use.
The system maintains accurate, proportional sampling of exhaled breath without dilution by ambient air, preserving measurement fidelity.
Smaller mixing chamber size allows for slower, less expensive sensors with longer service life.
The device enables portability and mobile use for field metabolic measurements, overcoming size and weight limitations of traditional metabolic carts.
Because the system is passive and valveless, it reduces manufacturing cost and maintenance while allowing easy sanitization.
Documented Applications
Measurement of metabolic rate and energy expenditure in laboratory and field settings.
Use in athletic and work-related activities requiring mobile metabolic measurements, including resting, walking, jogging, running, rowing, and cycling.
Applications requiring breath-by-breath gas exchange measurements with high temporal resolution.
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