Abstract:
The invention presents techniques for identifying and guiding treatment for medical conditions, based upon the carbon dioxide concentration in the patient&#39;s breath. In an exemplary application, the techniques of the invention may be used to distinguish obstructive lung disease from restrictive lung disease, even though the symptoms of the conditions are similar. The techniques of the invention may further be used to monitor the effectiveness of the treatment.

Description:
TECHNICAL FIELD 
     The invention relates to medical devices, and in particular, to medical devices used to guide diagnosis, monitoring and/or treatment of respiratory conditions. 
     BACKGROUND 
     Every day, patients with difficulty breathing seek medical help. In such cases, the patients may complain of shortness of breath, but may have no idea as to the cause of the condition. Many cases of shortness of breath fall into two general categories of respiratory disorders. 
     One category of respiratory disorder that may cause shortness of breath is obstructive lung disease. A patient with obstructive lung disease suffers from a narrowing of the airways leading to the alveoli in the lungs. This narrowing, often caused by inflammatory reactions, results in a reduction of the patient&#39;s ability to ventilate the alveoli, because the narrowed airways reduce the maximum velocity of flow through the airways. Chronic obstructive pulmonary diseases such as asthma, bronchitis and emphysema, are some of the disorders that can cause narrowing of the airway. 
     A second category of respiratory disorder that may cause shortness of breath is restrictive lung disease. Restrictive lung disease is characterized by a reduction of the overall gas-exchange area in the lungs. A restrictive lung disease may be temporary, such as a short-term filling of the alveoli with fluid, or more long-lasting, such as fibrosis that prevents the alveoli from expanding during inhalation. A restrictive lung disease may also be caused by congestive heart failure leading to pulmonary edema. 
     When a patient complains of difficulty breathing, it is difficult for health care professionals to rapidly determine whether the problem is due to obstructive or restrictive origins. The symptoms caused by both conditions are similar. The patient&#39;s medical history may be of no help, or the patient may be incapable of giving a medical history due to age or a language barrier. 
     To make a reliable diagnosis of obstructive lung disease or restrictive lung disease, physicians often employ a spirometer. A spirometer is a device that measures the flow and volume of air breathed in and out. The patient breathes into the device at the direction of a health professional. The measurements recorded in a spirogram can be used to distinguish obstructive lung disease from restrictive lung disease. 
     There are, however, drawbacks to spirometry. First, spirometers are rarely available to health professionals treating a patient away from a hospital. Many emergency medical professionals are not trained in spirometry. Getting the patient to a spirometer and to a health professional trained in spirometry often takes time, and the patient&#39;s need for treatment may be urgent. Breathing difficulties can be life-threatening if not diagnosed accurately and treated promptly. 
     Second, a proper spirogram requires the patient to exert effort to follow the directions of the health professional, such as directions to inhale as much air as possible, to exhale as hard as possible and to expel as much breath as possible. Patients that are short of breath may be incapable of following the directions. Young children also have difficulty with the effort-dependent system. 
     Because obstructive lung disease and restrictive lung disease are treated with different methods and different medicines, distinguishing the conditions is important. Risks associated with making an incorrect diagnosis are dire. A patient who suffers from congestive heart failure but is misdiagnosed as suffering from chronic obstructive pulmonary disease, for example, may be mistakenly treated with a beta agonist. Beta agonist therapy can significantly increase myocardial oxygen consumption and worsen ischemia for that patient. 
     SUMMARY 
     In general, the invention is directed to techniques for rapidly and reliably distinguishing obstructive lung disease from restrictive lung disease. In addition, the invention is directed to techniques for monitoring the response of the patient to treatment for the condition. 
     To distinguish obstructive lung disease from restrictive lung disease, the invention employs measurements of the concentration of carbon dioxide in the breath of the patient. A device such as a capnograph can be used to take these measurements, and the measurements taken by the capnograph are called a capnogram. The capnograph tracks the concentration of carbon dioxide during each exhaled breath. 
     In a typical capnogram, the carbon dioxide concentration in the breath rises as a patient begins to exhale. The carbon dioxide concentration plateaus, then drops as the patient concludes exhalation. The shape of the curve that follows the carbon dioxide concentration is correlated to the ventilatory status of the patient. In particular, measurements of carbon dioxide concentration can be used to distinguish obstructive lung disease from restrictive lung disease. 
     In one embodiment, the invention is directed to a method comprising measuring a concentration of carbon dioxide in a breath expired by a patient and using this measurement to determine the presence of obstructive lung disease or restrictive lung disease. The method may take into consideration, for example, the duration of a steady rise of the concentration of carbon dioxide in the breath or the rate of increase of the concentration of carbon dioxide, as measured by the initial angle and slope of the capnogram. The method may also compare the carbon dioxide concentration in the breath with a characteristic curve. The method may further include monitoring the condition of the patient following treatment. 
     In another embodiment, the invention presents a device comprising a gas sensor that measures the concentration of carbon dioxide in a breath expired by a patient and a processor that determines the presence of obstructive lung disease or restrictive lung disease as a function of the measurement. The device usually includes an output device that reports the determination. 
     In a further embodiment, the invention presents a method comprising measuring a concentration of carbon dioxide in a breath expired by a patient and guiding treatment as a function of the measurement. Guiding treatment may include determining the presence of lung conditions, determining the severity of the conditions, and selecting medications to treat the conditions. 
     The invention may provide a number of advantages. For example, the invention quickly provides information to a health professional to guide treatment of the patient. In an exemplary usage, the invention rapidly and reliably distinguishes obstructive lung disease from restrictive lung disease without the need for a spirometer. Moreover, unlike a spirometer, the techniques of the invention may benefit patients that are incapable of following breathing directions. Furthermore, the invention may be small and easily portable, and may be brought to the patient by an emergency medical professional. As a result, the ventilatory status of the patient may be assessed quickly. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1A includes a chart and a diagram of a capnogram and a respiratory condition of a normal patient, for comparison to FIGS. 1B and 1C. 
     FIG. 1B includes a chart and a diagram of a capnogram and a respiratory condition of a patient with an obstructive lung disease. 
     FIG. 1C includes a chart and a diagram of a capnogram and a respiratory condition of a patient with a restrictive lung disease. 
     FIG. 2A includes a chart of a capnogram of a patient with an obstructive lung disease. 
     FIG. 2B includes a chart of a capnogram of a patient with a restrictive lung disease. 
     FIG. 3 is a block diagram of an apparatus that is one embodiment of the invention. 
     FIG. 4 is a flow diagram illustrating techniques for using capnography to analyze respiratory conditions. 
     FIG. 5 is a flow diagram illustrating techniques for using capnography to monitor respiratory conditions following treatment. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A,  1 B and  1 C show a series of three charts  10 ,  20  and  30 , each chart accompanied by a diagram of an alveolus  14 . Chart  20  shows a representative capnogram of a patient with obstructive lung disease, and chart  30  shows a representative capnogram of a patient with restrictive lung disease. Capnograms  20  and  30  are shown in reference to a capnogram  10  for a normal patient, i.e., a patient with no substantial lung disease. 
     The alveoli accompanying capnograms  10 ,  20  and  30  illustrate the nature of the condition of the patient. Each alveolus  14  includes a thin-walled inflatable sac  18  and a conducting airway  16 . The alveolus accompanying capuogram  20  shows obstructions  24  in airway  16 . Sac  18  may be able to expand and perform gas exchange, but expulsion of gas from sac  18  is hampered by obstructions  24 , which narrow the lumen of airway  16 . Obstructions  24  are characteristic of obstructive lung disease. 
     The alveolus accompanying capnogram  30  shows restriction  34  in sac  18 , characteristic of restrictive lung disease. Restriction  34  may prevent sac  18  from expanding, or may limit the gas exchange performed by sac  18 . Airway  16  is clear, allowing unimpeded expulsion of breath, but restriction  34  limits the volume of gas in the breath. 
     Capnograms  10 ,  20  and  30  include tracings  12 ,  22  and  32 , which plot the measured concentration of carbon dioxide in the breath as a function of time. Each tracing  12 ,  22  and  32  shows the concentration of carbon dioxide rise, reach a plateau and drop. The shapes of tracings  12 ,  22  and  32 , however, are different. As will be shown in more detail below, analysis of the shapes of tracings  22  and  32  may be used to distinguish obstructive lung disease from restrictive lung disease. 
     Tracing  22  from a patient with obstructive lung disease shows a more gradual rise in the ascending slope of the carbon dioxide concentration, as compared with tracings  12  and  32  from a normal patient and a patient with restrictive lung disease, respectively. The more gradual rise is caused by the inability of the patient to exhale rapidly due to obstructions  24 . The patient ventilates adequately because sac  18  is clear, but the patient is not able easily to expel the contents of sac  18  through airway  16 . 
     The ascending slope of tracing  32  from a patient with restrictive lung disease shows a rapid rise in carbon dioxide concentration when compared with tracing  22 , but a nearly normal rise in carbon dioxide concentration when compared with tracing  12 . A patient with restrictive lung disease has restriction  34  in sac  18  but no obstructions to prevent exhalation of carbon dioxide, so the rise in carbon dioxide concentration is initially normal, or nearly so. The carbon dioxide concentration in tracing  32 , however, plateaus at a lower concentration when compared to tracings  12  and  22 , indicating that the patient is less adequately ventilated than the normal patient and the patient with obstructive lung disease. 
     FIGS. 2A and 2B provide a more detailed analysis of capnograms  20  and  30 . When a patient first begins to exhale, the carbon dioxide concentration in the first part of the breath is negligible. The first exhaled gases generally carry air from so-called “dead space,” i.e., the trachea, bronchi and other structures in the brigs in which no gas exchange takes place. In a typical patient, the volume of the dead space is approximately 150 mL. As gases from alveoli are expelled with air from the dead space, the concentration of carbon dioxide in the breath rises. When the dead space gases are mostly expelled, the concentration of carbon dioxide begins to reach a plateau. The plateau is typically not flat. 
     Once the concentration of carbon dioxide in the breath begins to rise, the rise in concentration may be approximated by a straight line. The straight line may form the hypotenuse of a right triangle. In tracing  22 , the rise of carbon dioxide concentration is approximated by hypotenuse  42  of right triangle  40 , and in tracing  32 , the rise of carbon dioxide concentration is approximated by hypotenuse  52  of right triangle  50 . 
     Base  46  of triangle  40  represents the duration of the rise of carbon dioxide concentration, i.e., the approximate time it takes for the carbon dioxide concentration in the breath of a patient with obstructive lung disease to reach a plateau. Height  44  of triangle  40  represents the concentration of carbon dioxide when the patient reaches the plateau. Likewise, for a patient with restrictive lung disease, base  56  represents the duration of the rise of carbon dioxide concentration, and height  54  represents the concentration of carbon dioxide when the patient reaches the plateau. 
     Many of the quantities are related, and other quantities can be derived, by the application of trigonometry. For example, the areas of triangles  40  and  50  can be computed and the lengths of hypotenuses  42  and  52  can be determined. The rate of increase of carbon dioxide concentration can also be determined by taking the derivative of the beginning of tracings  22  and  32 , which gives the slope. 
     Moreover, take-off angles  48  and  58  can be found. Take-off angles  48  and  58  are one measure of the slope of hypotenuses  42  and  52 , and are a function of how rapidly carbon dioxide concentration in the breath rises. Although take-off angles  48  and  58  can be derived by trigonometry from other measurements, take-off angles  48  and  58  can also be measured directly, independent of other parameters. 
     As shown by tracing  22 , a patient with obstructive lung disease takes a longer time than a patient with restrictive lung disease to expel dead space air. This is shown by the more gradual slope of hypotenuse  42 , as compared to hypotenuse  52 . The gradual slope of hypotenuse  42  is indicative of obstructive lung disease because the gradual slope represents that it takes longer for the patient to move carbon dioxide-rich gas from his alveoli. 
     By contrast, the slope of hypotenuse  52  is considerably steeper than hypotenuse  42 . The steep slope of hypotenuse  52  is not indicative of obstructive lung disease because it suggests a rapid expulsion of carbon dioxide-rich gas from the alveoli. The extent of hypotenuse  52 , height  54  and base  56  are small, however, when compared to the counterparts of triangle  40 . Another measure of the difference is the area of triangle  50 , which is considerably smaller than the area of triangle  40 . The smaller area of triangle  50  is indicative of restrictive lung disease because the patient suffers from restricted gas exchange, and cannot expel as large a volume of carbon dioxide-rich gas from the alveoli. 
     Applying analysis techniques such as those described above, the initial carbon dioxide concentration in the exhalation of a patient can be used to distinguish obstructive lung disease from restrictive lung disease. A patient with obstructive lung disease expels carbon dioxide more slowly, but in greater volume, than a patient with restrictive lung disease. 
     Importantly, capnograms  20  and  30  need not be effort-dependent. Unlike spirograms, in which the patient must follow a set of instructions, capnograms  20  and  30  may be taken while the patient is breathing as comfortably as he is able, without requiring the patient to follow any breathing instructions. The clarity of tracings  22  and  32  may be improved if the patient is able to follow simple breathing instructions from a health professional, but following the instructions is not essential to the invention. 
     FIG. 3 is a block diagram of a system  70  that may be used to practice the invention. System  70  includes intake apparatus  72 . The patient exhales into intake apparatus  72 , which may be an apparatus such as a nasal cannula or a mask. The exhalation from the patient passes through tube  74  to gas sensor  76 , which measures the concentration of carbon dioxide in the breath. Gas sensor  76  may be part of a capnograph. Gas sensor  76  may measure carbon dioxide concentration using techniques such as infrared detection, which can track changes in concentration in real time. 
     Gas sensor  76  passes measurements  90  to low-pass filter  78 , which prevents aliasing. Filter  78  passes filtered measurements  92  to analog-to-digital converter  80 , which converts filtered analog measurements  92  to digital measurement data  94 . Processor  82  receives digital measurement data  94 . Digital measurement data  94  may be stored in random access memory (RAM)  84 . 
     Based upon digital measurement data  94 , processor  82  evaluates the carbon dioxide concentration in the patient&#39;s breath over time. Processor  82  may, for example, construct tracings such as tracings  22  or  32  shown in FIGS. 2A and 2B, and derive triangles such as triangles  40  or  50 . Processor  82  may find quantities such as duration of the rise of carbon dioxide concentration or take-off angle. Using quantities such as these, processor  82  may determine whether the data support a diagnosis of obstructive lung disease or restrictive lung disease. 
     Processor  82  may, for example, measuring the duration of a steady rise of the concentration of carbon dioxide. A long duration is indicative of obstructive lung disease and a short duration is indicative of restrictive lung disease. Accordingly, processor  82  may determine that the patient probably suffers from obstructive lung disease when the duration is longer than a threshold duration, and may determine that the patient probably suffers from restrictive lung disease when the duration is shorter than the threshold duration. 
     In addition or in the alternative, processor  82  may measure the rate of increase of the concentration of carbon dioxide. The rate of increase may be quantified by, for example, the steepness of the hypotenuse of the ascending slope, or by the magnitude of the take-off angle, or both. Processor  82  may determine that the patient probably suffers from obstructive lung disease when the rate of increase is lower than a threshold rate, and may determine that the patient probably suffers from restrictive lung disease when the rate of increase is higher than the threshold rate. 
     As an alternative to or in addition to this analysis, processor  82  may compare digital measurement data  94  to one or more characteristic curves. Memory such as read-only memory (ROM)  86  may store data that are characteristic of obstructive lung disease and data that are characteristic of restrictive lung disease. Processor  82  may correlate the measurements of the concentration of carbon dioxide from the patient with the characteristic curves. When the correlation exceeds a preselected threshold value, processor  82  may determine that the data support a diagnosis of obstructive lung disease or restrictive lung disease. 
     In addition to determining whether the patient more likely suffers from obstructive lung disease or restrictive lung disease, processor  82  may also gauge the severity of the condition. Processor  82  may report a severe case of obstructive lung disease, for example, when take-off angle  48  is below a particular value, indicating that the patient has extreme difficulty pushing out his breath. Degrees of severity may also be reported, such as “critical,” “moderate” and “mild.” 
     Processor  82  reports the results of the analysis to the health professional via input/output (I/O) device  88 . I/O device  88  may include, for example, a display screen that displays text or graphics, or a collection of light emitting diodes. Processor  82  may report an analysis, such as “Patient&#39;s exhaled carbon dioxide concentration indicates a greater likelihood of obstructive lung disease than restrictive lung disease,” or “Patient&#39;s exhaled carbon dioxide concentration indicates a high probability of obstructive lung disease.” Processor  82  may further report on the severity of the condition, and/or may display the tracing of the carbon dioxide concentration. Furthermore, processor  82  may suggest an appropriate treatment based upon the analysis. 
     In contrast to a spirometer, system  70  may be small and easily portable. Accordingly, system  70  may be included in first aid packages in public venues such as airports and health clubs, or may be carried to the patient by an emergency medical professional. Furthermore, unlike a spirometer, system  70  may provide guidance for treatment of the patient very quickly, and need not be effort-dependent. 
     The organization of system  70  is an example of one system that may be used to practice the invention, and the invention is not limited to the system shown. For example, digital measurement data  94  may be supplied to RAM  84  via a direct memory access module (not shown in FIG.  3 ), rather than via processor  82 . ROM  86  may include erasable programmable read-only memory (EPROM). I/O device  88  may be one of several input and/or output devices. The invention encompasses all of these variations. 
     FIG. 4 is a flow diagram illustrating an embodiment of the invention in an exemplary application, such as the case of a patient suffering from a shortness of breath. System  70  receives expired breath from the patient via intake apparatus  72  ( 100 ). Gas sensor  76  measures the carbon dioxide concentration ( 102 ) and reports the measurements to processor  82 . 
     In addition to making measurements of carbon dioxide concentration, system  70  helps in determine the nature of the condition and further helps guide treatment of the patient. In a typical application, processor  82  analyzes the measurements over time ( 104 ) using techniques such as those described above and ascertains whether the data support a determination that lung disease is present ( 106 ). When the data support a determination that obstructive lung disease is present, processor  82  may so report via I/O device  88  ( 108 ). Similarly, when the data support a determination that restrictive lung disease is present, processor  82  may so report ( 110 ). In some circumstances, the data may support neither case, and processor  82  may so report ( 112 ). 
     Processor  82  may also report additional information ( 114 ) that may guide the treatment of the patient. For example, processor  82  may report the severity of the condition, or may suggest a medicine for the condition, or may recommend that the measurements be repeated, or may suggest that the patient be instructed to breathe in a particular manner. 
     FIG. 5 is a flow diagram showing how the invention may be implemented to monitor the effectiveness of treatment. In some circumstances, such as treatment of some forms of asthma, proper treatment produces a prompt improvement in the condition of the patient, and this improvement can be monitored. System  70  receives expired breath from a patient via intake apparatus  72  ( 120 ), gas sensor  76  measures the carbon dioxide concentration ( 122 ) and processor  82  analyzes the measurements ( 124 ). Instead of reporting a determination of lung disease, however, processor  82  monitors changes in the condition of the patient, and reports the changes via I/O device  88 . In this way, the invention may be used to observe the responsiveness of the patient to treatment. 
     Various embodiments of the invention have been described. These embodiments are illustrative of the practice of the invention. Various modifications to the apparatus or methods may be made without departing from the scope of the invention. For example, the invention need not be embodied in a standalone apparatus, but may be combined with an apparatus that performs other diagnostic or treatment functions. Similarly, the invention need not be embodied in a method that analyzes only carbon dioxide concentration in the breath, but may include other diagnostic measurements such as measurements of heart rate, respiration rate, blood pressure, electrocardiogram and blood oxygenation. 
     Other embodiments may employ capnograms from a plurality of breaths, and may process the capnograms by techniques such as averaging. These and other embodiments are within the scope of the following claims.