[ 0026 ] fig1 a , 1b 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 illustrates 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 capnogram 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 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 . [ 0032 ] fig2 provides 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 lungs 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 . [ 0041 ] fig3 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 fig2 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 . 1 / 0 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 fig3 ), 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 . [ 0051 ] fig4 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 . [ 0054 ] fig5 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 of 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 .
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 .