Patent Application
20200121222
The present invention is directed to measurement of gas concentrations, and in particular is directed to systems and methods for measuring and comparing concentration of gaseous components within different gaseous mixtures.
A clinician might measure the concentration of a specific type of gas within different gaseous mixtures and use this information to determine how the concentrations vary between the gas mixtures. This measured comparison of gas concentration might be useful for patient diagnosis or a clinical study. However, obtaining an accurate measurement of gas concentration in biological samples is very challenging. Condition of the gas, such as temperature, pressure, humidity, and density, can be a cause of measurement errors. These errors might be compounded when attempting to measure the concentration difference between two different gas flows because the conditions of the gases might differ from one another.
Humidity has a major effect on accuracy of the concentration measurement because humidity not only changes gas concentration, but also affects gas density, heat conduction, and heat capacity, and therefore sensor accuracy. For example, during expiration, the body breathes out a gas in which the partial pressure of water vapor is 46 mmHg at 37 degrees centigrade at ambient pressure. If the gas temperature is cooled down to the ambient temperature (20 degrees centigrade), condensation occurs and the partial pressure of water vapor becomes 18 mmHg. Most gas analyzers capture the water drops caused by condensation using a water reservoir and measure the gas concentration under a condition in which the relative humidity is controlled to a range of 30-100%.
A mechanical ventilator is typically connected to a gas supplying system. Therefore, the inspiration gas does not normally have humidity (0% relative humidity). This humidity difference between inspiration (0%) and expiration (30-100%) is a significant obstacle in achieving an accurate measurement of the gas concentration differences between inspiration and expiration in a patient who is mechanically ventilated.
An accurate measurement of comparison between concentration of a gaseous component during exhalation and concentration of the gaseous component during inhalation can be of value in a number of ways. For example, such a measurement is useful in determining dosage of a gaseous substance delivered to a patient, such as, for example, anesthetics applied during surgery or inhalation gas (e.g., NO, H2, and H2S) used for the study of some emerging therapies. It is also possible that, if the concentration difference of oxygen between inspiration and expiration is measured accurately, O2 consumption of the body can be estimated. O2 consumption is used in multiple disciplines, such as nutrition support, sport medicine, and cardiopulmonary functional studies, because the O2 consumption represents global metabolic alternations in the body. However, most conventional measurement devices fail to provide an adequate real-time measurement of O2 consumption, which is mainly due to inaccurate measurement of the concentration difference between inhalation and exhalation.
An object of the present invention is to provide a system and method for accurate measurement of the gas concentration difference between inhalation and exhalation of a patient.
A system according to an exemplary embodiment of the present invention comprises: a first sampling portion configured to sample inhalation gas made up of a first gaseous mixture; a second sampling portion configured to sample exhalation gas made up of a second gaseous mixture; a gas analyzer configured to measure gas concentrations; a switching valve that controls flow of the sampled inhalation gas and the sampled exhalation gas to the gas analyzer so as to alternately measure concentration of gaseous components within the first and second gaseous mixtures; a humidity control system that maintains humidity within the first and second gaseous mixtures to a predetermined humidity level; and a calculation section configured to calculate concentration differences of the gaseous components between the first and second gaseous mixtures.
In an exemplary embodiment, the first gas sampling portion is connected to an inhalation gas circuit of a mechanical ventilator, and the second gas sampling portion is connected to an exhaust port or an exhalation gas circuit of the mechanical ventilator.
In an exemplary embodiment, the first gas sampling portion is connected to an inhalation gas circuit of a breathing mask with a first one-way valve, and the second gas sampling portion is connected to an exhalation gas circuit of the breathing mask with a second one-way valve.
In an exemplary embodiment, the humidity control system comprises at least one of a membrane dryer unit, a refrigerating dryer unit or a desiccant.
In an exemplary embodiment, the humidity control system is a membrane dryer unit, and purge gas used for the membrane dryer unit is at least one of dry gas from a gas supplying system, dry gas from a gas tank or dry gas as dried by a desiccant.
In an exemplary embodiment, the humidity control system is a refrigerating unit, and the refrigerating unit lower temperature of the first and second gaseous mixtures to −20° C. or below.
In an exemplary embodiment, the gas analyzer measures at least oxygen concentration and carbon dioxide concentration, and the first calculation section is configured to calculate oxygen concentration differences and carbon dioxide concentration differences.
In an exemplary embodiment, the calculation section is further configured to calculate respiratory quotient (RQ) based on the calculated concentration differences.
In an exemplary embodiment, the system further comprises a flow sensor that measures flow rate of at least one of the inhalation gas or the exhalation gas, wherein the calculation section is further configured to calculate oxygen consumption (VO2) and carbon dioxide generation (VCO2) based on the measured concentration differences and the flow rate.
In an exemplary embodiment, the flow sensor comprises a thermal sensor, a hygrometer and a pressure sensor.
In an exemplary embodiment, the first gas sampling portion comprises a first gas mixing chamber and the second gas sampling portion comprises a second gas mixing chamber.
A method for measuring biological information according to an exemplary embodiment of the present invention comprises: sampling inhalation gas made up of a first gaseous mixture; sampling exhalation gas made up of a second gaseous mixture; controlling flow of the sampled inhalation gas and the sampled exhalation gas to a gas analyzer so as to alternately measure concentration of gaseous components within the first and second gaseous mixtures; maintaining humidity within the first and second gaseous mixtures to a predetermined humidity level prior to measurement of the concentration of the gaseous components; and calculating concentration differences of the gaseous components between the first and second gaseous mixtures.
In an exemplary embodiment, the method further comprises the steps of: connecting the first gas sampling portion to an inhalation gas circuit of a mechanical ventilator; and connecting the second gas sampling portion to an exhaust port or an exhalation gas circuit of the mechanical ventilator.
In an exemplary embodiment, the method further comprises the steps of: connecting the first gas sampling portion to an inhalation gas circuit of a breathing mask with a first one-way valve; and connecting the second gas sampling portion to an exhalation gas circuit of the breathing mask with a second one-way valve.
In an exemplary embodiment, the humidity is maintained by a humidity control system comprising at least one of a membrane dryer unit, a refrigerating dryer unit or a desiccant.
In an exemplary embodiment, the humidity control system is a membrane dryer unit, and purge gas used for the membrane dryer unit is at least one of dry gas from a gas supplying system, dry gas from a gas tank or dry gas as dried by a desiccant.
In an exemplary embodiment, the humidity control system is a refrigerating unit, and the refrigerating unit lower temperature of the first and second gaseous mixtures to −20° C. or below.
In an exemplary embodiment, the gas analyzer measures at least oxygen concentration and carbon dioxide concentration, and the method further comprises the step of calculating oxygen concentration differences and carbon dioxide concentration differences.
In an exemplary embodiment, the method further comprises the step of calculating respiratory quotient (RQ) based on the calculated concentration differences.
In an exemplary embodiment, the method further comprises the steps of: measuring flow rate of at least one of the inhalation gas or the exhalation gas; and calculating oxygen consumption (VO2) and carbon dioxide generation (VCO2) based on the measured concentration differences and the flow rate.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example principles of the invention.
The features and advantages of exemplary embodiments of the present invention will be more fully understood with reference to the following, detailed description when taken in conjunction with the accompanying figures, wherein:
A gas concentration measurement system according to an exemplary embodiment of the present invention controls the humidity of gases in real-time to a predetermined humidity level (e.g., 1 mmHg water pressure, 2 mmHg water pressure, 3 mmHg water pressure, 20 mmHg water pressure) before the gases from different sources, or streams, are alternately passed through to a sensor for measurement of the gas component concentrations. Calculation of the difference between gas concentration values obtained by one sensor decreases the error propagation that would otherwise occur with multiple sensors. This allows for a more accurate gas concentration measurement as compared to any other currently available gas concentration measurement system.
In order to reduce the errors associated with the duplicate sensor systems described above, a system for measuring gas concentration difference in accordance with an exemplary embodiment of the present invention includes the use of only one gas analyzer so that the gas concentrations within different gaseous mixtures can be measured under the same humidity conditions.
The humidity control system 22 may include components that increase and/or decrease the humidity of the gas flowing through the gas concentration measurement module 20 based on feedback measurements to maintain the humidity at a specific level or within a predetermined range. For example, the humidity control system 22 may include a membrane dryer unit, a refrigerating dryer unit or a desiccant.
As mentioned, a desiccant may also be used to dry the sampling gas flowing into the gas concentration measurement module 20. Examples of suitable desiccants include montmorillonite clay, silica gel, molecular sieve, calcium oxide and calcium sulfate.
The gas analyzer 23 is configured to measure at least the oxygen concentration and the carbon dioxide concentration in the inhalation and exhalation gases. The gas analyzer 23 might be, for example, an infrared gas analyzer, and more specifically, in the case of O2 sensing, might be a paramagnetic sensor, electro-galvanic oxygen sensor, zirconia sensor, fiber-optic sensor based on fluorescence quenching, or use laser absorption spectroscopy, and in the case of CO2 sensing, might be a chemical sensor, or use nondispersive infrared spectroscopy (NDIR) or Fourier-transform infrared spectroscopy (FTIR).
The calculation section 24 may be a computer hardware component that includes a memory unit and a processor, where the memory has stored thereon processor readable code that, when read by the processor, carries out operations to calculate one or more of the following parameters: oxygen concentration difference (between inhalation and exhalation), carbon dioxide concentration difference (between inhalation and exhalation), respiratory quotient (RQ), oxygen consumption (VO2) and carbon dioxide generation (VCO2). In this regard, respiratory quotient (RQ) may be calculated based on the measured CO2 and O2 concentration differences, and oxygen consumption (VO2) and carbon dioxide generation (VCO2) may be calculated based on the concentration differences and flow rate of at least one of the inhalation gas or the exhalation gas. In order to calculate oxygen consumption (VO2) and carbon dioxide generation (VCO2), in accordance with another exemplary embodiment illustrated in
Also, as shown in
The following Example and Comparative Example illustrates advantages of the present invention.
A Douglas Bag method was used to measure concentration of O2 and CO2 in inhaled and exhaled air of a test lung (QuickLung Breather, IngMar Medical, Pittsburgh, USA) operated by a mechanical ventilator (AVEA ventilation system, CareFusion, Yorba Linda, Calif., USA). A 50 L PVC bag (Douglas Bag, Harvard Apparatus, Holliston, Mass., USA) and a 3.8 L polyvinylidene fluoride (PVDF) bag (Dual-Valve Kynar PVDF Bag, Cole-Parmer Kynar, Vernon Hills, Ill., USA) were prepared. The exhaled and inhaled gases were continuously collected into the 50 L PVC bag and the 3.8 L PVDF bag, respectively. The 50 L PVC bag was connected to the exhaust port of the mechanical ventilator so that all the exhaled gas was collected in the bag. A part of the inhaled gas (10-15% of the ventilation volume) was collected into the 3.8 L PVDF bag from an adapter port that was added into the ventilator circuit. The exhaled and inhaled gases were collected simultaneously and a total gas collection time was 7-8 minutes for each experiment. After the gas collection was completed, the two sealed bags were frozen inside a freezer (FUM 21SVCRWW, General Electric, Louisville, Ky., USA). The temperatures of the bag surface and the sample gas were monitored and the gas concentration measurements were started after the temperatures reached below −20 degrees centigrade. Two sampling tubes were prepared and disposed in the freezer for each bag, and a gas analyzer, which was setup outside the freezer, obtained the exhaled and inhaled gas separately through the tubes from outside. A 3-way stop connector was connected to the two sampling tubes and the gas analyzer so that the analyzer always obtained one of the gas samples, with the input to the gas analyzer switched by changing the gas flow direction of the 3-way stop connector. The gas analyzer used to measure the gas concentrations was a GF-210R model, produced by Nihon Kohden Corporation of Tokyo, Japan.
The same test lung, mechanical ventilator and gas analyzer from the Comparative Example were used to measure CO2 and O2 concentrations in inhaled and exhaled air, but instead of using the Douglas Bag method, the test was performed using a system including two chambers (one for each of inhaled gas and exhaled gas), a humidity control system for equilibrating humidity levels within each chamber, a single gas analyzer and a switching valve to control gas flow from the chambers to the gas analyzer. This was a conceptual proof experiment of a real-time measurement with humidity control. Two separate 250 mL glass bins were prepared. These glass bins were frozen inside a dry ice container. The exhaled gas and the inhaled gas were sampled directly from the mechanical ventilation circuit through a sampling port added into the ventilation circuit. The exhaled (exhaust) gas was passed through one of the bins and the inhaled gas was passed through another bin. The gas analyzer was connected downstream of the gas flows so that the gas analyzer obtained a gas after the humidity was controlled by the icy bins. Gas sampling rate was controlled by the gas analyzer at 200 mL/min and a gas flow direction was switched by a 3-way stop connector so that the analyzer obtained one of the gas samples. The inhaled gas was humidified by adding a water reservoir into the mechanical ventilation circuit. In the experimental setting, the humidity level measured at the inhalation port was <0% (under detection limit) and the humidity level at the exhaust port was 40-70%. The humidity in both gases became the same level of <0% after the gas samples were passed through each icy bin.
Table 1 below shows the results of the Comparative Example and the Example (three Trials were performed in each example), where FIO2 indicates the O2 concentration (%) in the inhalation gas; FEO2 indicates the O2 concentration (%) in the exhalation gas; FECO2 indicates the CO2 concentration (%) in the exhalation gas; and RQ is calculated by FECO2/(FIO2-FEO2).
The data in Table 1 shows that the results of Example 1 are close to the values obtained using the Douglas Bag method in the Comparative Example, and therefore may be considered accurate results. This demonstrates that the real-time measurements of gas samples directly collected from the ventilator circuits have equal values with the measurements in a steady state, where the gas concentration was measured by the traditional Douglas Bag method.
Now that embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be construed broadly and not limited by the foregoing specification.
This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/746,641, filed Oct. 17, 2018, the contents of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62746641 | Oct 2018 | US |
