WO2010143027A1

WO2010143027A1 - A device for obtaining metabolic parameters

Info

Publication number: WO2010143027A1
Application number: PCT/IB2010/000190
Authority: WO
Inventors: Riaan Conradie; Franco Bauer Du Preez
Original assignee: Stellenbosch University
Priority date: 2009-06-11
Filing date: 2010-02-02
Publication date: 2010-12-16
Also published as: ZA201108719B

Abstract

A device (10) for obtaining metabolic parameters of a subject by determining the oxygen consumption and carbon dioxide production of the subject is provided. The device includes a portable body (12) defining an air flow path (20) between a mouthpiece (14) and a vent-hole (16). Inhaled and exhaled air is passed through the mouthpiece along the air flow path. Air flow meters (26, 30, 34) measure the flow rate of air exhaled through the mouthpiece, inhaled through the mouthpiece and expired into the atmosphere, respectively. At least one removable cartridge (24) is provided in the body, the cartridge having carbon dioxide sequestering material therein for sequestering both the inhaled and exhaled air of carbon dioxide.

Description

A DEVICE FOR OBTAINING METABOLIC PARAMETERS

FIELD OF THE INVENTION

This invention relates to indirect calorimetry in which measurements of carbon dioxide production and oxygen consumption permit the metabolism of a subject to be characterized.

BACKGROUND TO THE INVENTION

Indirect calorimeters are available that accurately measure the carbon dioxide production and oxygen consumption by a person over time using sophisticated gas sensors, enabling the respiratory quotient and metabolism of that person to be determined. These devices are, however, expensive machines that require operation by a trained technician, limiting their use to hospitals and clinics.

To the applicant's knowledge, only one hand-held indirect calorimeter is currently available in the marketplace - the BodyGem™ calorimeter available from HealtheTech™. This device, however, relies on a sophisticated oxygen concentration sensor - a ruthenium based oxygen probe - and ultrasonic flow meters, and is consequently still prohibitively expensive to most users.

Other devices have been described but these suffer from various disadvantages. United States patent 4,917,108 to Mault discloses an oxygen consumption meter that measures the difference in flow between inhaled and exhaled gas to determine the oxygen consumption of a subject. This device, however, does not determine carbon dioxide production of the subject, and any estimation of metabolic parameters of the subject must therefore assume, rather than directly measure, the respiratory quotient of the subject.

Typically, the respiratory quotient is estimated to be 0.85. In other devices, for example US patent 5,1579,958 also to Mault, the carbon dioxide gas concentration is determined by using a relatively expensive gas concentration sensor called a capnometer, which determines the concentration of carbon dioxide by measuring the absorption of infrared light.

One device that determines both the oxygen consumption and the carbon dioxide production by differential flow measurements alone is disclosed in United States patent 2,630,798 to White. The device disclosed in White suffers from several drawbacks. Firstly, compensation for ambient carbon dioxide levels must be made mathematically. Since ambient carbon dioxide levels are not constant in the atmosphere, this introduces error into the estimation of the respiratory quotient. Furthermore, the inhaled and exhaled air travelling into and out of the device along the different paths have different temperatures and water vapour content, which introduce further inaccuracies into the measurements. No disclosure is made as to the kind of gas meter that would be used to provide the required accuracy. The applicant believes that these drawbacks account for the fact that, to the applicant's knowledge, all commercially available devices that determine respiratory quotient by means of differential air flow measurements also utilize relatively expensive capnometers for direct measurement of oxygen and/or carbon dioxide concentrations.

OBJECT OF THE INVENTION

It is an object of this invention to provide a device for obtaining metabolic parameters of a subject which is relatively simple in construction, permits determination of respiratory quotient and is cost effective to produce. SUMMARY OF THE INVENTION

In accordance with the invention there is provided a device for obtaining metabolic parameters of a subject by determining the oxygen consumption and carbon dioxide production of the subject, comprising: a portable body defining an air flow path between a mouthpiece and a vent-hole, the mouthpiece to be supported in contact with the subject's mouth so as to permit inhaled and exhaled air to be passed though the body along the air flow path; a first flow meter mounted in the air flow path in proximity to the mouthpiece for measuring the full flow rate of air exhaled through the mouthpiece; a second flow meter mounted in the air flow path in proximity to the mouthpiece for measuring the full flow rate of air inhaled through the mouthpiece; a first non-return valve associated with the first flow meter to permit only exhaled air to move through the first flow meter; a second non-return valve associated with the second flow meter to permit only inhaled air to move through the second flow meter; at least one removable cartridge provided in the body along the air flow path, the cartridge having carbon dioxide sequestering material therein for sequestering both the inhaled and exhaled air of carbon dioxide; and a third flow meter mounted in the air flow path in proximity to the vent- hole for measuring the flow rate of expired air that has been sequestered of carbon dioxide.

Further features of the invention provides for the body to include a single removable cartridge through which both inhaled and exhaled air is directed; for a third non-return valve to be associated with the third flow meter to direct exhaled air through the third flow meter; and for a fourth non-return valve to be provided in proximity to the vent-hole to permit inhaled air to be directed into the cartridge. Still further features of the invention provide for the non-return valves to be mounted on the removable cartridge; for the flow meters to be mounted on the body; and for the cartridge to be disposable.

Yet further features of the invention provide for the flow sensors to be piezoelectric or thermal flow sensors.

Further features of the invention provide for the carbon dioxide sequestering material to be soda lime that includes calcium hydroxide.

Still further features of the invention provide for mouthpiece to be detachable from the body and to be disposable after use.

Yet further features of the invention provide for temperature sensors to be included in the body for measuring the temperature of air flowing through the body; and for water vapour sequesterers to be provided adjacent the first and third flow meters to remove water vapour from the air before it travels through the cartridge.

Further features of the invention provides for the body to be portable and of unitary construction. According to one aspect of the invention, the body includes a microprocessor mounted in the body and a digital readout display, the microprocessor being configured to receive measurements from the flow meters and to calculate both the oxygen consumption and carbon dioxide production of the subject by computing the difference between the flow rates measured by the flow meters, for the microprocessor to be configured to compute the metabolic parameters, and to display the metabolic parameters on the digital readout display. According to a different aspect of the invention, the body includes a microprocessor and a wired or wireless connection means for communicating with a computer, the microprocessor being configured to receive measurements from the flow meters and output the measurements to the computer for computation and display of metabolic parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only with reference to the accompanying representations in which:

Figure 1 is a schematic illustration of a device for obtaining the metabolic parameters of a subject in accordance with the invention; and

Figure 2 Figure 2 is a illustration of an embodiment of the device of Figure 1.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Figure 1 is a schematic illustration of a device (10) for obtaining the metabolic parameters of a subject (not shown), which in this embodiment may be a human but could also be an animal or other organism or process involving respiration or combustion. The device includes a portable body (12) which has hollow interior that defines an airflow path between a mouthpiece (14) provided at one end of the device, and at least one vent-hole (16) provided at the opposite end of the device.

The mouthpiece (14) is shaped so as to be supported in contact with the subject's mouth (not shown) so as to permit inhaled and exhaled air to be passed through the mouthpiece (as indicated by arrows 18) and along the air flow path (as indicated by arrows 20). The mouthpiece may be made a mask that covers the nose and mouth of a subject or may instead fit into or around a user's mouth in which case a nose clip may be required to ensure that all inhaled and exhaled air is directed through the mouthpiece. The mouthpiece may also be detachable from the body for disposal after each use. Such mouthpieces are well known and will not be further described herein.

The body (12) includes a compartment (22) along the air flow path that is filled with a carbon dioxide sequestering material, typically Ca(OH)2 (calcium hydroxide) which is colloquially referred to as soda lime and commercially available as SodaSorb™, MediSorb™, or the like. Calcium hydroxide reacts with carbon dioxide to form insoluble CaCO₃ (calcium carbonate) and water. When air, which has approximately 20% oxygen, 80% nitrogen and 0.06% carbon dioxide, moves through a calcium hydroxide filled compartment it is thus cleared of substantially all carbon dioxide. In this embodiment, the calcium hydroxide is provided in the form of a removable cartridge (24) which can be separated from the body (12) and replaced once the calcium hydroxide has been used up. It is envisaged that the cartridge (24) will thus be disposable. In the illustrated embodiment, a single cartridge is provided through which both inhaled and exhaled air is passed, but it will be appreciated that more than one cartridge could be provided so that, for example, the exhaled air moves through a first cartridge and the inhaled air through a second cartridge.

A first flow meter (26) is mounted in the air flow path inside the body and in proximity to the mouthpiece (14), and is arranged to measure the full flow rate of air exhaled through the mouthpiece by the subject. A first non-return valve (28) is associated with the first flow meter (26) to permit only exhaled air to move through the first flow meter. The first flow meter is preferably a piezoelectric flow sensor with an air laminizer, such as those available from Marulatech, which is is able to accurately determine the volume of air that travels through the flow meter over a given time interval. Thermal flow sensors may also be used.

Adjacent to the first flow meter, a second flow meter (30) is arranged to measure the full flow rate of air inhaled through the mouthpiece by the subject. A second non-return valve (32) is associated with the second flow meter to permit only inhaled air to move through the second flow meter. The second flow meter is also preferably a piezoelectric or thermal flow sensor.

At the opposite end of the device and in proximity to the vent-hole or holes (16), a third flow meter (34) is mounted in the air flow path. The third flow meter is arranged to measure the flow rate of expired air that has been sequestered of carbon dioxide by the calcium hydroxide-filled cartridge (24). In this embodiment, because a single cartridge is used through which both inhaled and exhaled air is directed, a third non-return valve (36) is associated with the third flow meter to direct only exhaled air through the third flow meter, and a fourth non-return valve (38) is provided in proximity to the vent hole and through which inhaled air is directed into the body (12). It will be appreciated, however, that in the event that two separate and cartridges are provided which have separate flow paths, then the second and third nonreturn valves will not be required since the air flow though those cartridges would already be unidirectional due to the first and second non-return valves (28,32) provided adjacent the mouthpiece.

In a preferred embodiment of the invention, the non-return valves are mounted on the removable cartridge and the flow meters are mounted on the body. The non-return valves are preferably of low-cost construction so as to make them disposable together with the cartridge.

In a yet further preferred embodiment of the invention, water vapour scrubbers are provided adjacent flow meters A and C so as to ensure that the air travelling through the cartridge does not have an appreciable amount of water vapour in it, so as to improve the accuracy of the measurements. In addition, in a still further preferred embodiment of the invention, temperature sensors may be provided in the housing adjacent the flow meters to enable the correction of flow measurements due to local variations in temperature (the pressure measured by the flow meters is extremely sensitive to temperature).

It is possible to determine both the rate of oxygen consumption and the rate of carbon dioxide production by the subject using the device (10). For ease of illustration, the flow rate measured by the first flow meter (26) is indicated as "A", the flow rate measured by the second flow meter (30) as "B" and the flow rate measured by the third flow meter (34) as "C". The composition of the air at each of points A, B, and C is as follows:

A = atmospheric O2 - O₂ inhaled, atmospheric N₂, CO₂ exhaled

B = atmospheric O₂, atmospheric N₂, zero CO₂

C = atmospheric O₂ - O₂ inhaled, atmospheric N₂, zero CO₂

The oxygen consumption and carbon dioxide production of the subject can then be determined as follows:

Carbon dioxide production (CO₂ exhaled) = A - C Oxygen consumption (O₂ inhaled) = B - C

Knowledge of the carbon dioxide production and oxygen consumption information enables the subject's metabolic rate to be determined by indirect calorimetry. Firstly, the subject's respiratory quotient (RQ) can be determined as follows:

Where ^^ and ^ are the carbon dioxide production rates and oxygen consumption rates respectively. Although RQ specifies the number of molecules of carbon dioxide produced per molecule of oxygen consumed, at standard temperature and pressure, the volume of a gas is equivalent to the number of molecules, therefore the volume flow measurements made by the device can be used to very closely approximate the actual RQ.

The RQ value is representative of the molecular structure of the matter that is being combusted. For example, fats are much more reduced (contain fewer internal oxygen atoms) than carbohydrates and have an RQ of approximately 0.7 compared to an RQ of 1.0 for carbohydrates. The energy produced by a person can be calculated using the following formula:

Q = 1.33 IH (J t- 3,692 (2)

where Q is the Energy (in Kcal) produced per litre of oxygen for a resting person. This equation is described in Blanc, S. et al., Journal of Clinical Endocrinology and Metabolism (1998), 83(12): 4289-4297.

The Resting Metabolic Rate (RMR) of a person (in Kcal per day) can then be determined by simply multiplying the energy produced per litre of oxygen (Q) with the amount of oxygen (in litres) consumed per day (S).

RMR = QS (3a)

Many existing devices assume a predetermined average for the RQ value and calculate the metabolic rate of individuals using only oxygen production rates. In the present invention, direct determination of the RQ value enables other metabolic parameters to be determined - such as the incidence of catabolic metabolism and diabetes that may be indicated by low RQ values (close to 0.7) or the effects of high glycemic index diets indicated by high RQ values (close to 1.0). A wide range of RQ values for humans have been observed but the value usually ranges between a high of 1.0, indicating that the subject is using only carbohydrates as fuel, and a low of 0.7, indicating that the subject is using only fat as fuel. The following table shows this relationship:

Once the Resting Metabolic Rate has been determined, it is also possible to determine a person's fat free mass (FFM) using the Katch-McArdle equation:

SMR -37ύ

FFM = (3b)

21 ≤

Using the FFM and the weight of an individual, the fat percentage of the individual can be determined as follows:

% Body Fat = 100 * ^^T^I^~^M ₍₄₎

Vret^βhtTata i

The oxygen consumption rate

can also be used in determining a person's heart stroke volume. Heart stroke volume is often determined by placing a measurement device directly in the heart by means of a catheter. However, if the venous (coming from the lungs) and the arterial (returning to the lungs) blood oxygen saturation levels and heart rate are known (both of which can be measured non-invasively), it becomes possible to non- invasively determine heart stroke volume by solving the following two equations:

(5a)

Circulation rae = Heart rate X Heart stroke valwne (5b)

The device of the invention is used by the subject breathing through the device until the breathing pattern stabilises. The oxygen consumption and carbon dioxide production rates can then be measured by the device and the measurements used in the determination of the respiratory quotient, energy production, resting metabolic rate, fat free mass, body fat percentage and heart stroke volume of a subject.

Figure 2 is a schematic illustration of one embodiment of a portable device (100) constructed according to the invention. The device is hand-held and includes a digital readout display (102), which may include touch-screen functionality to permit various parameters (such as the weight of the subject) to be entered. The device is connectable to a mouthpiece (not shown) by means of which a subject can breathe through an airflow path that extends through the device. The body includes a microprocessor (not shown) which is configured to receive measurements from the flow meters and calculate both the oxygen consumption and carbon dioxide production of the subject by computing the difference between the flow rates measured by the flow meters. In the illustrated embodiment, the microprocessor computes the various metabolic parameters and displays these on the digital readout display (102). An alternative to this arrangement, however, is a device that includes a microprocessor with wired (e.g. USB) or wireless connection means for communication with a computer. Then the microprocessor could send the measurements to the computer, which would compute the various metabolic parameters and display the result on a monitor by means of a software application or a web interface. Such a software application or web interface could be used to track progress or measurements over time as well as to provide a platform to track calorie intake.

The invention therefore provides a device for obtaining metabolic parameters of a subject which is relatively simple in construction, permits accurate determination of respiratory quotient and is cost effective to produce. The device makes use of differential flow measurements without the need for relatively expensive oxygen or carbon dioxide sensors. Various other advantages are apparent from the aforegoing description.

It will be appreciated that the device could also be used to determine the caloric content and macromolecular composition of a foodstuff, rather than the metabolic parameters of a living subject. This can be achieved by blowing air into a combustion chamber at a fixed rate and measuring the influx of air into the chamber, combusting the foodstuff in the chamber, and measuring the outflow of air from the chamber. The RQ determined by the device will then be indicative the macromolecular composition of the foodstuff (e.g. lipids, carbohydrates and proteins) as well as the caloric content of the foodstuff.

While the invention has been described in detail with reference to various embodiments, it will be appreciated the invention is not limited to the embodiments described and that various modifications may be made without departing from the scope of the invention defined in the accompanying claims.

Claims

1. A device (10) for obtaining metabolic parameters of a subject by determining the oxygen consumption and carbon dioxide production of the subject, comprising: a portable body (12) defining an air flow path (20) between a mouthpiece (14) and a vent-hole (16), the mouthpiece to be supported in contact with the subject's mouth so as to permit inhaled and exhaled air to be passed though the body along the air flow path; a first flow meter (26) mounted in the air flow path in proximity to the mouthpiece for measuring the full flow rate of air exhaled through the mouthpiece; a second flow meter (30) mounted in the air flow path in proximity to the mouthpiece for measuring the full flow rate of air inhaled through the mouthpiece; a first non-return valve (28) associated with the first flow meter to permit only exhaled air to move through the first flow meter; a second non-return (32) valve associated with the second flow meter to permit only inhaled air to move through the second flow meter; at least one removable cartridge (24) provided in the body along the air flow path, the cartridge having carbon dioxide sequestering material therein for sequestering both the inhaled and exhaled air of carbon dioxide; arid a third flow meter (34) mounted in the air flow path in proximity to the vent-hole for measuring the flow rate of expired air that has been sequestered of carbon dioxide.

2. The device as claimed in claim 1 wherein the body includes a single removable cartridge through which both inhaled and exhaled air is directed, a third non-return valve (36) is associated with the third flow meter to direct exhaled air through the third flow meter, and a fourth non-return valve (38) is provided in proximity to the vent-hole to permit inhaled air to be directed into the cartridge.

3. The device as claimed in claim 1 or claim 2 in which the non-return valves are mounted on the removable cartridge and the cartridge is disposable.

4. The device as claimed in claim 3 in which the flow meters are mounted on the body. '

5. The device as claimed in any one of the preceding claims in which the flow sensors are piezoelectric or thermal flow sensors.

6. The device as claimed in any one of the preceding claims in which the carbon dioxide sequestering material is soda lime.

7. The device as claimed in any one of the preceding claims in which the mouthpiece is detachable from the body and disposable after use.

8. The device as claimed in any one of the preceding claims in which temperature sensors are provided in the body for measuring the temperature of air flowing through the body.

9. The device as claimed in' any one of the preceding claims in which water vapour sequesterers are provided adjacent the first and third flow meters to remove water vapour from the air before it travels through the cartridge.

10. The device as claimed in any one of the preceding claims in which a microprocessor is provided in the body and is configured to receive measurements from the flow meters and calculate the oxygen consumption and carbon dioxide production of the subject by computing the difference between the flow rates measured by the flow meters.

11. The device as claimed in claim 10 in which a digital readout display is provided on the body and the microprocessor is configured to compute the metabolic parameters of the subject using the calculated oxygen consumption and carbon dioxide production of the subject and to display the metabolic parameters on the digital readout display. '

12. The device as claimed in claim 10 in which wireless or wired connection means are provided by which the microprocessor is able to communicate with a computer, and the microprocessor is configured to output the calculated oxygen consumption and carbon dioxide production of the subject for computation and display of the metabolic parameters of the subject by the computer.