Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
  • A61B5/083—Measuring rate of metabolism by using breath test, e.g. measuring rate of oxygen consumption
  • A61B5/0836—Measuring rate of CO2 production
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
  • A61B5/091—Measuring volume of inspired or expired gases, e.g. to determine lung capacity

Definitions

• the present invention relates generally to techniques for determining functional residual capacity (FRC), the volume of gases that remain within a subject's lungs following exhalation, or, more broadly, the effective lung volume (ELV) of the subject, which includes gases that have diffused into the lung tissues.
• FRC functional residual capacity
• ELV effective lung volume
• the present invention relates to techniques for noninvasively determining FRC or ELV.
• Functional residual capacity is the volume of gases, including carbon dioxide (CO 2 ), that remains within the lungs of a subject at the end of exhalation, or expiration.
• FRC usually comprises about 40% of total lung capacity, and typically amounts to about 1.8 liters to about 3.4 liters.
• FRC buffers against large breath-to-breath changes in the amount of carbon dioxide in the alveoli of the subject's lungs,-which may be measured in terms of partial pressure of CO 2 (P AC0 2) or as a fraction of gases that comprise CO 2 (f ⁇ co2)- With normal tidal volumes, P ACO2 and f A co 2 typically fluctuate by only about 2 rnrnHg or about 0.25%, respectively.
• V A or FRC gas volume
• the lung tissue and pulmonary blood compartments are often represented in terms of their equivalent gas volumes (i.e., scaled by their effective storage capacity) and denoted V t i s and Vbi o od- While FRC only accounts for the volume of gases (including CO 2 ) in the alveoli, effective lung volume (ELV) includes FRC, as well as gases that remain diffused within the tissues of the lungs of the subject at the end of exhalation and, therefore, accounts for gases in all three compartments.
• ELV is typically a slightly larger volume than FRC, these terms may be used interchangeably in the ensuing description for purposes of simplicity.
• Each compartment equilibrates with changes in CO 2 at a different rate.
• Gedeon, A., et al. "Pulmonary blood flow (cardiac output) and the effective lung volume determined from a short breath hold using the differential Fick method," J. CLIN. MONIT.
• Gadeon 2002 teaches that V A equilibrates instantly with changes in end tidal CO 2 (p e tco2 when measured in terms of partial pressure and f et co 2 when measured in terms of the fraction of gases that comprise CO 2 ) and slowly (e.g., in about ten to about twenty seconds) with changes in p AC02 and content of CO 2 in arterial blood (c a co2), while it takes less time for Vti S and Vbiood to equilibrate when PACO2 and c a co2 change.
• the relationship between a subject's chest wall and lungs and the elastic recoil of the lungs defines FRC and, thus, ELV.
• FRC determinations may be useful in accurately diagnosing such conditions. FRC determinations are also useful in diagnosing and treating respiratory failure and hypoxemia.
• V/Q mismatch or V T /V Q mismatch.
• That technique includes measuring the V M CO 2 and f e tco2 of a subject
• V M CO 2 For the first breath following the breath-hold, f et co2 increases and V M CO 2 , which is calculated over the duration of the breath hold and the subsequent breath, decreases. Assuming, due to buffering by the CO 2 stores of the
• V B CO 2 i.e., CO 2 passing from the pulmonary capillary blood into the alveoli of the lung
• Equations, Q of the subject to the subject's ELV. Two of these equations compare the pre-breathhold conditions to the post-breathhold conditions and the pre-breathhold conditions to the recovery conditions.
• the ELV of an ELV and PCBF data pair that satisfies both of these equations is considered to be the subject's actual ELV.
• Gedeon 2002 is believed to provide inaccurate data, as it is based on the assumption that "CO 2 inflow [may] not [be] significantly affected” by breath-holding, while breath-holding will cause a change in P ACO2 - This assumption
• V B CO 2 changes linearly with P ACO2 while PCBF and the amount of CO 2 in the venous blood (c v co2 or, as Gedeon2002 refers to it, p ven ) remain constant.
• the present invention includes methods for noninvasively measuring, or estimating, FRC or ELV, as well as apparatus and systems for obtaining FRC and ELV measurements with minimal invasiveness.
• carbon dioxide and flow measurements may be obtained at or near the mouth of a subject. Such measurements are obtained during baseline, or "normal,” breathing, as well as during and shortly after inducement of a change in the subject's effective ventilation. For example, measurements may be obtained during or shortly following a rebreathing maneuver, in which a subject inhales gases including an above-normal amount of CO 2 . Continuing with the rebreathing example, the obtained measurements are evaluated to determine the amount of time required for exhaled CO 2 levels to return to normal — effectively an evaluation of CO 2 "washout" from the subject's lungs.
• CO 2 and flow measurements may be evaluated to determine the amount of time it takes CO 2 to "wash in,” or reach peak levels within, the lungs of the subject following rebreathing.
• amounts of CO 2 or another appropriate gas may be measured.
• the ELV of the subject may be substantially noninvasively determined, or estimated.
• a noninvasive ELV estimation apparatus that incorporates teachings of the present invention is configured ⁇ e.g., programmed) to evaluate CO 2 and flow data from a subject and process the same in such a way as to calculate ELV.
• a system of the present invention includes such an apparatus, as well as CO 2 and flow sensors, which obtain CO 2 and flow measurements in as noninvasive a manner as possible (with the possible exception of an endotracheal tube) and communicate data representative of the measured CO 2 and flow levels to the noninvasive ELV estimation apparatus.
• Fig. 1 is a schematic representation of an alveolus of an individual, illustrating the locations at which various respiratory and blood gas parameters may be determined;
• Fig. 2 is a graph that illustrates the volume of gases in the carbon dioxide stores of a respiratory tract of an individual (VA) during a series of respiratory cycles, or breaths;
• Fig. 3 is a plot of the transformed VMCO 2 data points against c A co 2 data points, in which the plotted points are substantially in-line with one another;
• Fig. 4 is a schematic representation of an example of a monitoring system incorporating teachings of the present invention.
• Fig. 5 is a line graph showing the correlation between two sets of ELV calculations that have been made in accordance with teachings of the present invention.
• FRC or ELV may be determined by evaluating a respiratory gas, such as carbon dioxide, and respiratory flow. Respiratory gas and flow signals may be used to determine a variety of parameters and, along with a mathematical model of the subject's lung, used to determine FRC or ELV.
• a respiratory gas such as carbon dioxide
• Respiratory gas and flow signals may be used to determine a variety of parameters and, along with a mathematical model of the subject's lung, used to determine FRC or ELV.
• the ensuing description includes a discussion of the manner in which one or more exemplary algorithms are derived, as well as reasoning to support such derivation, to facilitate substantially a noninvasive determination of the subject's FRC or ELV.
• FRC and ELV may be determined while the respiratory and cardiovascular, or hemodynamic, performance of a subject are being determined in a substantially noninvasive manner.
• Exemplary measures of the cardiovascular performance of a subject include, but are not limited to, pulmonary capillary blood flow and cardiac output.
• the carbon dioxide Fick equation has long been used to determine both pulmonary capillary blood flow and cardiac output.
• One form of the carbon dioxide Fick equation follows:
• PCBF VCO 2 /(c vC o2 - CAC 02 ), (1) where PCBF represents pulmonary capillary blood flow, VCO 2 is carbon dioxide elimination, c v co 2 is carbon dioxide content of the venous blood of the monitored individual, and C ACO2 is the carbon dioxide content of the alveolar ⁇ i.e., pulmonary capillary) blood of the monitored individual.
• VCO 2 The most accurate way to measure VCO 2 would be to directly measure the flow of CO 2 from the blood within the pulmonary capillaries that surround the
• cardiac output (Q) may be substituted for PCBF in equation (2).
• equation (3) takes the form of the standard equation for a line in a
• Equations (2) and (3) are based on the rate at which carbon dioxide leaves, or
• V B CO 2 carbon dioxide excretion
• V CO 2 is not measured directly at the alveoli. It is measured in a less direct manner — at or near the subject's mouth. Carbon dioxide signals that originate at or near the mouth of a subject are typically obtained and processed, along with respiratory flow signals, to facilitate such measurements.
• K ⁇ ck U.S. Patent Publication US 2002/0183643 Al of K ⁇ ck et al.
• V M CO 2 V M CO 2
• V B CO 2 when considered in terms of flow
• K ⁇ ck explains that such miscorrelation is caused by the CO 2 stores of a subject's lungs, specifically by the buffering capacity of the CO 2 stores.
• V M CO 2 includes both V B CO 2 and CO 2 that has flowed
• V STOREsCO 2 CO 2 stores
• VBCO 2 VMCO 2 - V S TORESCO 2 . (4)
• the CO 2 stores of an individual's lungs may be evaluated by use of a model of the lung, such as the simple model of the lung depicted in Fig. 1, in which a single alveolus 100 and a corresponding pulmonary capillary 102 represent the lung.
• the direction in which blood flows through pulmonary capillary 102 is represented by arrows 103.
• the mouth of an individual is represented at reference 106.
• the carbon dioxide stores of the lung are depicted, for the purpose of simplicity, as comprising the physical gas volume 104 of the alveolus (VA).
• VA is related to tidal volume (Vx), as well as to the functional residual capacity (V FRC ) of the lung.
• Vx tidal volume
• V FRC functional residual capacity
• CO 2 maybe distributed within other stores, such as the alveolar tissues and other tissues of the lung (collectively the ELV).
• the lung model shown in Fig. 1 also omits VT/V Q mismatch and shunting of blood (i.e., the portion of cardiac output that does not flow through the pulmonary arteries and capillaries, or that is not PCBF).
• V A * The effective volume of the CO 2 stores of an individual's lungs are denoted herein as "V A *.”
• a model of the lung such as that depicted in Fig. 1, may be evaluated on a breath-by-breath basis.
• a breath (n) may be delineated as the period from the end of one inspiration to the end of the next inspiration, as illustrated in Fig. 2.
• Fig. 2 depicts an example of the effective volume of CO 2 stores in the subject's respiratory tract (e.g., lungs) during the course of respiration.
• the amount of CO 2 that flows into and out of the CO 2 stores from one breath to the next may be expressed as a change in alveolar CO 2 fraction (f ⁇ CO 2 ) (i.e., the fraction of gases in the alveolus that comprise CO 2 ), or the difference between f ⁇ CO 2 for a particular breath (f ⁇ CO 2 (n)) and f ⁇ CO 2 for the previous breath
• V STORES CO 2 the volume of the CO 2 stores (V STORES CO 2 ) for a particular breath (n) may be determined by multiplying the effective volume in which the CO 2 stores are located (VA*) by the change in f ⁇ C02 from the previous breath (n-1) to the current breath (n) and by the subject's respiratory rate (RR). Equation (4) then becomes:
• V B CO 2 (n) V M C0 2 (n) + V A * (n) [ f A CO 2 (n) - f A CO 2 (n-l) ] RR. (5)
• Equation (5) is particularly useful for estimating V B CO 2 from V M CO 2 measurements that are obtained during the transition from "normal" breathing (e.g. , nonrebreathing) to a change in the effective ventilation of the subject (e.g.,
• V B CO 2 may be substituted for V B CO 2 in equation (5).
• V M CO 2 and RR may be measured directly, the alveolar CO 2 fraction (f AC0 2) and VA* cannot. It is known, however, that fAC02 is proportional to PACO2, which is proportional to p et co2, which may be measured directly (e.g., by use of a capnometer). The p et co 2 measurement may then be used, as known in the art, to obtain an fA C 02 value for each breath.
• V A may be adaptively estimated, such as by using the linear correlation
• V A may, therefore, be determined as the value that
• V B CO 2 (n) V M CO 2 (n) + V A * (n) [fMx»(n)-f ⁇ co2(n-l)] RR. (6)
• equation (6) is used, as such noise may result in an inaccurate estimation of V B CO 2
• equation (7) may be simplified to:
• V B CO 2 (n-l) - VBC ⁇ 2(n) , i m fACO2(n) - f A co2(n-l) ⁇ — (10)
• V B CO 2 (n) V M C0 2 (n) + ⁇ J ⁇ L L V B C0 2 (n-l) - V B CO 2 (n) J (11)
• V B C0 2 (n) (1- ⁇ ) V M C0 2 (n) + ⁇ V B C0 2 (n-l), (13)
• the transformation coefficient ( ⁇ ) in equations (13) and (14) may be determined iteratively, by using an initial ⁇ value, then progressively increasing and/or decreasing the ⁇ value to
• an optimal correlation coefficient (r 2 ) between the V B CO 2 values and the p et co2 or Cco2 values Other methods for determining an optimal ⁇ value include, without limitation, rote searching, global searching, gradient searching (e.g., use of a gradient descent search algorithm), use of a least mean squares algorithm, use of other predetermined equations or sets of predetermined equations, use of a truly adaptive filtering technique, and use of other techniques to determine the optimal ⁇ value, as known in the art.
• equation (13) Use of an optimal transformation coefficient ( ⁇ ) (equation (14)) in equation (13) provides a relatively accurate, simple mathematical model of the lung of a subject.
• the algorithm of equation (13) may be used to calculate the amount of CO 2 that flows into and out of the carbon dioxide stores of the lungs on a "breath-to- breath" basis.
• VA*(n) of equation (14) is equivalent to ELV and flow may be converted to volume, which results in elimination of RR, allowing ⁇ to be expressed more simply as:
• equation (15) were multiplied through with ⁇ f ⁇ co2 (i-e., f ⁇ co2(n) - f ⁇ co2(n-l), the expression could be viewed as calculating the relative amount of CO 2 stored in ELV over the total change in the amount of CO 2 from a change in the effective ventilation of a subject (e.g., rebreathing or another change in effective ventilation). If PCBF/RR is calculated from data obtained before and during a change in the effective ventilation of the subject (e.g., rebreathing or another change in effective ventilation), equation (15) may be rewritten as follows:
• Equation (16) may be rearranged as follows:
• VA* — (PCBF/RR) s C0 2 pBaro (17)
• Equation (17) may be used to substantially noninvasively determine ELV when virtually any change in the effective ventilation of the subject (e.g., rebreathing, change in respiratory rate, change in respiratory volume, etc.) has occurred, whether or not the subject continues to breathe as data is collected, with data obtained during "normal” breathing being compared with data obtained once the change in effective ventilation has occurred.
• a change in the effective ventilation of the subject e.g., rebreathing, change in respiratory rate, change in respiratory volume, etc.
• Equation (6) does not take into account the possibility, or even likelihood
• V STORES CO 2 the amount of CO 2 stored within the lungs
• V STORES CO 2 the amount of CO 2 stored within the lungs
• V B CO 2 (n) V M CO 2 (H) + (V A * (n) + V ⁇ (n)) x (f A co 2 (n) - f A co 2 (n-l)) + (V ⁇ (n) - V ⁇ (n-1)) x f A co2(n), (18)
• the present invention includes use of an algorithm that corrects ELV for possible changes in V CO2STORES and combines the ELV correction with the CO 2 form of the differential Fick equation:
• V M CO 2 is the average breath-to-breath volume, not flow, of carbon dioxide eliminated from the subject's lungs, as measured at the mouth, during breaths that precede and effective change in the ventilation of the subject (e.g., rebreathing or another change in effective ventilation).
• the ELV value of equation (19) includes tidal volume (V T ).
• the inspiratory tidal volume should be subtracted from ELV, as estimated for use in equation (19).
• V M CO 2 (n) V M C0 2 ex pi red (n- 1) - V M CO 2 i nSp i red (n)) .
• Equation (19) can be solved for ELV ( VA * ):
• PCBF can be determined through some other method, be it invasive (e.g., thermodilution), or noninvasive (e.g., electrical bioimpedance).
• Parts of equation (21) may be used in at least two embodiments of the present invention, one of which includes use of the first part of equation (21) to
• ELV Emitter LV . More specifically, if one could assume that V B CO 2 is constant even though the PA CO2 is changing due to a change in the effective ventilation of the subject ⁇ e.g., rebreathing or another change in effective ventilation), ELV may be determined as follows:
• VA* [VMCO 2 PRE - V M C ⁇ 2(n)l (22) - 1)
• Equation (22) may be used to evaluate ELV when a change in the effective ventilation of the subject (e.g., rebreathing, change in respiratory rate, change in respiratory volume, etc.) has been effected, and while the subject continues to breathe (i.e., not during maneuvers which require breath-holding or which otherwise temporarily terminate breathing).
• a change in the effective ventilation of the subject e.g., rebreathing, change in respiratory rate, change in respiratory volume, etc.
• data obtained during "normal" breathing may be compared with data obtained once the change in effective ventilation has occurred.
• breath (n-1) may represent a normal breath
• (n) may represent the first breath in which the change in effective ventilation has occurred.
• the second of these embodiments employs both the first part of equation (21) (i.e., equation (22), as well as the second part of equation (21): DUR
• the ELV calculation of equation (22) may be modified to compensate for changes in pA C02 during a breath, or continuously changing p AC02 - Specifically, the ratio of the change in VCO 2 to the change in PA CO2 in equation (23)
• Equation (24) represents the slope of the line that describes the amount of CO 2 that exits the CO 2 stores through the mouth as CO 2 exiting the blood is added to the CO 2 stores, or the "sensitivity" with which changes in p et co 2 represent changes in PA C 02 as CO 2 from the blood flows into the CO 2 stores, which in turn provides an indication of buffering capacity of the CO 2 stores.
• (pA CO 2(n)-p AC02) provides an indication of the magnitude of the PACO2 change to be scaled when p e tco2 is measured at or near the mouth.
• the in equation (24) may be substituted with a different value that
• RR represents the time interval between the start of a change in effective ventilation (e.g., rebreathing) and the time when the measured PA CO2 left the alveoli.
• Diagnostic system 1 includes, among other things, an airway 52 in communication with the airway A of an individual I, as well as a flow meter 72 and a carbon dioxide sensor 74 positioned along airway 52.
• Flow meter 72 and carbon dioxide sensor 74 communicate signals to corresponding monitors 73 and 75, which communicate electronically with a processor 82 of a respiratory monitor 80 (e.g., the processor and respiratory monitor of a NICO ® monitor available from Novametrix Medical Systems (Wallingford, CT, division of Respironics, Inc).
• Processor 82 is programmed to determine at least VCO 2 and p et co 2 based on signals communicated thereto from flow meter 72 and carbon dioxide sensor 74.
• processor 82 may be programmed to use signals from one or both of flow meter 72 and carbon dioxide sensor 74 or calculated parameters (e.g., VCO 2 and p e tco2) in the above-described algorithms (i.e., one or more of equations (1) — (24)) to facilitate the substantially noninvasive and accurate determination of individual Fs ELV. Alternatively, such calculations may be made manually.
• VCO 2 and p et co 2 values are obtained during both a baseline, or first, ventilatory state, and when a change in the effective ventilation of individual I has been effected, or a second ventilatory state.
• a first ventilatory state may be effected under substantially "normal" breathing conditions.
• the baseline ventilatory state may be defined under a first set of other, selected breathing conditions.
• the second ventilatory state occurs when one or more respiratory control parameters are manipulated to achieve breathing conditions differ from those present during the first ventilatory state to a degree that effect a measurable change in minute ventilation.
• the second ventilatory state may be induced, for example, by altering the value of a limit variable, e.g., inspiratory pressure, tidal volume, flow rate or time, from a value of the limit variable during the first ventilatory state.
• a change in effective ventilation may be induced by altering the threshold value of a cycle variable from the threshold level of the cycle variable during the first ventilatory state.
• a change in effective ventilation may be induced by altering the threshold triggering value of a triggering variable, such as inspiratory pressure or flow rate.
• a change in effective ventilation may be induced by delivering to the individual a series of at least three "sigh breaths," which are deeper than normal breaths. Changes in effective ventilation may also comprise periods of unsteady, or "noisy,” breathing.
• VCO 2 and p et co 2 values that are obtained are then processed in accordance with one or more of equations (1) - (24)) to substantially noninvasively and accurately determine individual Fs ELV.
• ELV was calculated for various breaths using equation (18). ELV values that were calculated when a sufficient fAco2(n) - f A co 2 (n-l) threshold was present and during certain breaths (e.g., the second breath into rebreathing, the first breath of recovery, etc.) were considered valid and are included as data points in the graph of Fig. 5. Notably, the plotted data points represent ELV minus inspiratory tidal volume. ELV values that were calculated from data obtained during transition from normal breathing into rebreathing are shown as diamond-shaped points. ELV values that were calculated from data obtained during the transition from rebreathing to recovery are shown as squares.

Landscapes

• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Biomedical Technology (AREA)
• Heart & Thoracic Surgery (AREA)
• Pulmonology (AREA)
• Physics & Mathematics (AREA)
• Obesity (AREA)
• Biophysics (AREA)
• Pathology (AREA)
• Engineering & Computer Science (AREA)
• Emergency Medicine (AREA)
• Physiology (AREA)
• Medical Informatics (AREA)
• Molecular Biology (AREA)
• Surgery (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Veterinary Medicine (AREA)
• Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=38121383

Family Applications (1)

Country Status (3)

Families Citing this family (3)

Family Cites Families (2)

• 2005
  • 2005-10-20 JP JP2007538028A patent/JP2008517655A/ja active Pending
  • 2005-10-20 EP EP05809890A patent/EP1805681A1/de not_active Withdrawn
  • 2005-10-20 WO PCT/US2005/037735 patent/WO2006047212A1/en active Application Filing

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events