1. Field of Invention
The field of the currently claimed embodiments of this invention relates to apparatuses and methods for quantifying inspiratory and expiratory airflow and characterizing respiratory disorders.
2. Discussion of Related Art
The gold standard assessment of ventilation is the measurement of airflow with a pneumotachograph (American Sleep Disorders Assoc. 1995; Kushida et al. 2005; American Academy of Sleep Medicine 1999), although it is not routinely used, instead semi-quantitative measures of airflow are utilized. Pneumotachographs measure airflow through a tube that imposes a small resistance to airflow, which is not a problem during wakefulness in normal healthy individuals. During sleep or in patients at risk for respiratory failure airflow, the added resistance or any deadspace is a problem primarily for two reasons. First, the added resistance might change respiratory pattern and ventilation particularly when flow is at a minimum (Hlavac et al. 2007; Morrell, Browne, & Adams 2000 ; Pillar et al. 2000; Tun et al. 2000; Hudgel, Mulholland, & Hendricks 1987). Second, the use of a pneumotachograph may exacerbate respiratory failure in patients who cannot adapt their respiratory pattern in response to the added load. Another reason why pneumotachographs are rarely used in clinical practice during sleep.
In addition, measuring inspiratory and expiratory airflow limitation requires simultaneous measures of airflow and airway pressures, because of uncertainties of the absolute zero of airflow measurements. Currently, defining the absolute zero is a major problem in measuring airflow since both electrical and mechanical signals drift over time, leading to inaccuracies of measuring airflow. Current methods of defining the absolute zero use either software or hardware algorithms that can have at least two disadvantages: 1) Software algorithms distort the airflow signal thereby affecting the airflow contour; and 2) Hardware algorithms can detect the absolute zero but they do not correct the electrical or mechanical drifts in the airflow signal. There thus remains a need for improved apparatuses for quantifying respiratory and inspiratory airflow.
An apparatus for quantifying a user's expiratory and inspiratory airflow according to an embodiment of the current invention includes an air tube adapted to be sealed over at least one of the nose or mouth of a user, a pressure sensor configured to be selectively fluidly connected with one of the air tube or an ambient environment external to the air tube, a valve assembly arranged between the air tube and the pressure sensor to switch between a measuring configuration in which the pressure sensor is fluidly connected with the air tube while fluid connection with the ambient environment is excluded, and a reference configuration in which the pressure sensor is fluidly connected with the ambient environment while fluid connection with the air-tube is excluded, and a data processing unit arranged to communicate with the pressure sensor and the valve assembly. The data processing unit is configured to provide instructions to the valve assembly to switch between the measuring and the reference configurations. The data processing unit is further configured to determine an absolute zero of expiratory and inspiratory airflow based on signals from the pressure sensor obtained while the valve assembly is in the reference configuration and to measure at least one of expiratory and inspiratory airflow while the valve assembly is in the measuring configuration. The processing unit is further configured to determine at least one of expiratory airflow limitation or inspiratory airflow limitation relative to the absolute zero airflow.
A method of quantifying a subject's respiratory and inspiratory airflow according to an embodiment of the current invention includes measuring at least one of respiratory airflow or inspiratory airflow of the subject, measuring an absolute zero airflow in a local environment of the subject, and determining at least one of expiratory airflow limitation or inspiratory airflow limitation based on the measuring at least one of respiratory airflow or inspiratory airflow relative to the absolute zero airflow.
Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.
Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.
Some embodiments of the current invention provide methods and devices that allows quantifying on a breath by breath basis the degree of inspiratory and expiratory flow limitation and dynamic hyperinflation. As described in more detail below, this approach utilizes the absolute zero and deviation in measured airflow at specific time points from the zero line. The degree of inspiratory airflow limitation can provide a marker for the degree of upper airway obstruction and can be obtained by measuring the level of airflow during inspiration. Similarly, by using the expiratory flow contour some embodiments of the current invention allows one to determine the degree of expiratory airflow limitation and magnitude of dynamic hyperinflation, both of which are hallmarks for the severity of asthma and chronic obstructive lung disease. Current airflow sensors miss these markers of inspiratory upper airway obstruction, COPD and Asthma.
Some embodiments of the current invention obviate measuring airway pressures by determining the absolute zero from the airflow signal. Thus, some embodiments of the current invention allow for the quantification of inspiratory and expiratory airflow limitation and dynamic hyperinflation from the airflow signal alone. An apparatus according to an embodiment of the current invention measures repeatedly the absolute zero and corrects electrical and mechanical drifts.
Some embodiments of the current invention can solve several problems. First, it defines absolute zero without distorting the airflow signal and it can automatically correct the airflow signal based on the measured absolute zero. Second, by knowing the absolute zero, the new apparatus would also prevent an overestimation and/or underestimation of inspiratory airflow, which accrue in existing methods due to the problems mentioned above. Third, the repeated re-zeroing of any electrical drift and automated calibration by referencing the airflow to atmosphere allows accurate airflow measurements over limitless time periods. Therefore, embodiments of the current invention are suited to accurately quantify and monitor inspiratory and expiratory disorders of breathing.
The apparatus 100 further includes a valve assembly 112 arranged between the air tube 104 and the pressure sensor 110 to switch between a measuring configuration in which the pressure sensor 110 is fluidly connected with the air tube 104 while fluid connection with the ambient environment is excluded, and a reference configuration in which the pressure sensor 110 is fluidly connected with the ambient environment while fluid connection with the air tube 104 is excluded. The apparatus 100 further includes a data processing unit 114 arranged to communicate with the pressure sensor 110 and the valve assembly 112. The data processing unit 114 is configured to provide instructions to the valve assembly 112 to switch between the measuring and the reference configurations. The data processing unit 114 is further configured to determine an absolute zero of respiratory and inspiratory airflow based on signals from the pressure sensor 110 obtained while the valve assembly is in the reference configuration and to determine a net difference in respiratory and inspiratory flow with respect to the absolute zero. The valve assembly 112 can include a solenoid actuator 116 for switching the valve between the measuring and reference configurations. (See, also,
In some embodiments of the current invention, the data processing unit 114 can be further configured to output information to a user-output-component based on the net difference in respiratory and inspiratory flow with respect to the absolute zero. In some embodiments, the user-output-component can include at least one of an audio or video alarm, for example. In some embodiments, the user-output-component can include a video display adapted to display at least one of alphanumeric or graphical information, for example.
In some embodiments, the apparatus 100 can further include a data storage unit in communication with the data processing unit 114. The data storage unit can be adapted to store at least one of signals from the pressure sensor or calculated values from the data processing unit for later retrieval. The data storage unit can include a removable data storage medium, for example. In some embodiments, the apparatus 100 can further include a data interface to at least retrieve data stored in the data storage unit.
The apparatus according to some embodiments of the current invention can provide solutions for detecting inspiratory and expiratory flow limitation and dynamic hyperinflation, for example. In an embodiment, the apparatus has four parts (see
In an example, the ‘Pitot flowmeter’ is a polyethylene lightweight (1.5 grams), low dead-space (˜10 cm3) flowmeter (KeyFlow™, Key Technologies Inc, Baltimore, USA) that uses the Pitot tube principal to determine midstream airflow rate flowing through a wide bore flow tube (
The major technical difference of the Pitot flowmeter to standard pneumotachograhs is that it measures midstream airflow rather than side stream pressure. The theoretical principle used in the Pitot flowmeter's measurement of airflow is derived from application of the Pitot tube approach and is based on the Bernoulli Equation:
Δp+ 1/2ρV2+ρgh=constant equation 1
where: Δp=differential pressure, ρ=density, V=velocity, g=gravity, h=elevation. This ideal equation is valid for measures at any point along a stream line for steady fluid flows with constant density and for which friction is negligible. Furthermore, both gravity and elevation are also constant and negligible in this application of equation 1. The differential pressure measured between the Pitot tube ports relates to the fluid velocity as follows:
p
up
−p
dn=½ρV2 equation 2
where, V is the velocity in the centerline of the flow sensor. The airflow rate is then calculated from the centerline velocity as follows:
Q=C·V·A equation 3
where, Q=flow rate, A=cross sectional area of the flow sensor, C=velocity profile pressure head correction factor in the flow sensor. During turbulent flow, the velocity profile of the pressure head is effectively flat, thus C is very close to 1.0. In fully developed laminar flow, the velocity profile is parabolic and C is closer to 0.5. In the Pitot flowmeter, the flow rate algorithm is empirically determined to account for variation of the velocity profile for changes in flow rate.
An apparatus and methods according to some embodiments of the current invention can allow for quantification of inspiratory and expiratory airflow for extended time periods without performing repeated manual calibration or correction procedures. By knowing the absolute zero, one can detect the magnitude of inspiratory and expiratory airflow limitation and the degree of dynamic hyperinflation. Dynamic hyperinflation occurs when inspiration starts prematurely while expiratory airflow is still present. The knowledge of an absolute zero can discriminate whether airflow immediately prior to an inspiration has ceased (e.g. it approaches zero) or not (e.g. if it exceeds the zero) (
Apparatuses according to some embodiments of the current invention can be applied in clinical or institutional settings and in the home environment for patients and subjects who need monitoring of airflow for diagnostic and therapeutic purposes or to control the efficacy of a given treatment that would affect ventilation.
Some applications can include the following:
The magnitude of inspiratory and expiratory airflow limitation and the degree of dynamic hyperinflation can be determined on a breath-by-breath basis independently of additional pressure measurements.
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.
This application claims priority to U.S. Provisional Application No. 61/450,985 filed Mar. 9, 2011, the entire contents of which are hereby incorporated by reference.
This invention was made with U.S. Government support of Grant No. P50 HL084945, awarded by SCCOR. The U.S. Government has certain rights in this invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US12/28562 | 3/9/2012 | WO | 00 | 9/9/2013 |
Number | Date | Country | |
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61450985 | Mar 2011 | US |