embodiments of the present invention will be more completely understood through the following detailed description , which should be read in conjunction with the attached drawings . in this description , like numbers refer to similar elements within various embodiments of the present invention . pulsus paradoxus represents a quantifiable , exaggerated decrease in arterial blood pressure during inspiration . although frequently described in the medical literature as merely a decrease in systolic blood pressure , this physiologic phenomenon actually represents a decrease in left ventricular stroke volume (“ lvsv ”) during the inspiratory phase of the respiratory cycle and / or an augmentation of lvsv during the expiratory phase of the respiratory cycle . this subtle distinction is important in development of the most accurate means by which to measure pulsus paradoxus . the mechanisms of pulsus paradoxus are incompletely understood and can differ among various diseases . in severe acute asthma , for example , airway obstruction results from bronchospasm , mucous membrane edema and mucus plugging . overcoming this obstruction to maintain airflow and ventilation necessitates the generation of increased negative intrapleural pressure on inspiration and increased positive intrapleural pressure on expiration . these exaggerated swings of intrapleural pressure result in increased left ventricular afterload during inspiration and decreased left ventricular afterload during expiration and concomitant decreased lvsv and increased lvsv during inspiration and expiration , respectively . air - trapping and hyperinflation result in alveolar distension , pulmonary arteriolar vasoconstriction , and increased impedance to right ventricular ejection during inspiration . this results in decreased left ventricular filling and further impaired left ventricular stroke output . diminished left ventricular filling also results from shift of the interventricular septum leftward as a result of augmented filling of the right ventricle . conversely , these events are influenced inversely during expiration and result in increased lvsv during expiration . each of these alterations contribute to paradoxical pulse in asthma and other respiratory and cardiovascular disease states . more generally , pulsus paradoxus is caused by variably altering intrapleural pressure , left ventricular compliance , pulmonary hyperinflation , or systemic or pulmonary venous return . any of these events , individually or collectively , lead to an accentuation of the decrease in lvsv during the inspiratory phase respiratory cycle and / or an augmentation of lvsv during the expiratory phase of the respiratory cycle , and thus the phenomenon of pulsus paradoxus . these changes in lvsv and resulting pulsus paradoxus have been demonstrated to result from the following physiologic events : 1 . increasingly negative intrapleural pressure during inspiration resulting in increased transmural ( absolute intracardiac minus intrapleural ) pressure . 2 . impairment to left ventricular ejection during inspiration resulting from increased afterload ( as the left ventricular must create greater contractile force to overcome the increasingly negative intrapleural pressure ). 3 . mechanical impairment to left ventricular filling ( decreased compliance ) due to shift of the interventricular septum leftward ( resulting from increased venous return to the right heart as a result of increased negative intrapleural pressure during inspiration ), resulting in decreased left ventricular stroke volume during inspiration . 4 . increased impedance to right ventricular ejection during inspiration due to increased alveolar volume and increased resistance to flow through alveolar capillaries , resulting in decreased pulmonary venous return to the left atrium and ventricle and decreased lvsv . typical methods of detecting pulsus paradoxus have numerous , significant disadvantages . for example , using intra - arterial catheters to detect pulsus paradoxus is painful , associated with significant risk and should only be done by highly trained medical personnel using sophisticated monitoring equipment , preferably in a hospital setting . determining pulsus paradoxus through the use of a sphygmomanometer is often difficult and frequently results in unreliable readings . through photoplethysmography , pulsus paradoxus can be determined in non - invasive fashion , but results can not be reliable and provide limited information to a physician or caregiver . previous attempts to quantify pulsus paradoxus from oximeter plethysmographic and other waveforms representing pulsatile arterial flow have done so by measuring changes in waveform height , a one - dimensional parameter , similar to the traditional methods described above , or by measuring variation of the plethysmograph baseline . because pulsus paradoxus represents changes in lvsv and flow , these methods lack the sensitivity and accuracy of a method based upon two or three - dimensional parameters . moreover , the existing techniques only measure changes in the height of the systolic waveform and do not adequately measure diastolic changes occurring during the cardiac cycle that contribute to pulsus paradoxus . the multiple physiologic events contributing to pulsus paradoxus and the corollary , that the diastolic and systolic contributions to pulsus paradoxus vary depending on the disease process involved , make evident the need to consider these diastolic contributions when measuring pulsus paradoxus . in the preferred embodiment , a method and system are provided which utilizes changes in area under a measured waveform curve (“ auc ”), a two - dimensional parameter , to determine pulsus paradoxus . preferably , the provided method and system also accurately incorporate diastolic changes in order to make a more sensitive and accurate assessment of the physiologic changes contributing to pulsus paradoxus . fig1 depicts a pulse oximeter system , generally designated as 100 , connected to a mammalian patient 102 . preferably , pulse oximeter system 100 includes an optical transducer or probe 104 , a sensor interface 106 , processing hardware 108 , a graphical display 110 and an audible alarm 112 . in the preferred embodiment , the probe 104 is attached to the patient 102 on a digit , earlobe or other site overlying an arteriolar vascular bed . as illustrated in fig1 , the probe 104 can be attached to the fingers of the patient 102 , but other locations are also possible . the probe 104 typically comprises an exterior housing that applies active elements of the probe 104 to tissue of the patient 102 containing an arterial or arteriolar vascular bed that is to be monitored . in the preferred embodiment , the probe 104 also contains one or more light - emitting diodes (“ leds ”) and one or more light detectors to monitor the level of light transmitted through or reflected from the patient &# 39 ; s 102 vascular tissue . the light detectors typically measure the changing optical absorption of light transmitted from leds at approximately 660 nm and 940 nm which results from volume expansion of the arteriolar and capillary beds during pulsatile cardiac blood flow . the probe transmits signals describing the detected light to at least one sensor interface 106 . in addition , although a single probe 104 and a single sensor interface 106 are illustrated , one skilled in the art will recognize that alternative embodiments employing multiple probes 104 and / or multiple sensor interfaces 106 are also possible . one exemplary design of a typical pulse oximeter system is disclosed in u . s . pat . no . 6 , 385 , 471 by mortz , the entire disclosure of which is hereby incorporated by reference . the signal received by the sensor interface 106 from the probe 104 is traditionally an analog signal and is preferably processed by additional analog circuitry and converted by an analog - to - digital converter circuit into a set of digital measurements before being transmitted to the processing hardware 108 . the processing hardware 108 can comprise a specific device , such as a digital signal processor specifically programmed or hardwired to implement the features of embodiments of the present invention . the processing hardware 108 can also comprise a general purpose computer , incorporating elements such as an intel pentium ™ processor , physical memory , input and output devices , etc ., programmed with software implementing features embodied by the present invention . the primary function of the processing hardware 108 is to measure the maximal change in area under a continuous optical plethysmographic waveform curve corresponding to arterial pulsation , to display data related to this measurement on the graphic display 110 , and to trigger an audible alarm 112 to alert physicians or other caregivers to the presence and severity of pulsus paradoxus . the operation of the processing hardware 108 is described in additional detail below and , for the purpose of this disclosure , it is assumed that the other elements disclosed in fig1 are the conventional components found in existing pulse oximeter systems . fig2 depicts an arterial pressure waveform 202 , as detected by the pulse oximeter system 100 . as is evident from the diagram , the waveform 202 represents the product of two distinct pressure waves flowing throughout the patient &# 39 ; s 102 vascular system . specifically , waveform 202 is comprised of an incident pressure wave component 204 and a reflected pressure wave component 206 . the incident pressure wave component 204 represents the pressure wave created by the ventricular ejection of the heart of the patient 102 , which travels through the circulatory system and causes blood vessels to expand . the reflected pressure wave component 206 represents the reflected wave traveling back through the blood vessels as they contract to their original size . of note in the waveform 202 is its systole 208 , its dicrotic notch 211 and its diastole 210 . end systole 208 represents the highest point and end diastole represents the lowest point of arterial pressure . the dicrotic notch 211 represents closure of the aortic valve and the backsplash of blood against the closed valve . because the waveform 202 represents both systolic and diastolic pulsatile cardiac events , it is recognized to represent these individual events during different phases of the waveform 202 . an ascending limb phase 212 ( designated “ iauc ”) of the waveform 202 , from the end ( the lowest portion ) of diastole 210 to the peak of systole 208 represents primarily lvsv . during the iauc phase 212 , the patient &# 39 ; s 102 stroke volume expands the arterial tree and peripheral arteriolar and capillary tissue beds . the subsequent downstroke phase 214 ( designated “ dauc ”) of the waveform 202 represents primarily outflow from the elastance vessels of the tissue bed , is influenced significantly by local vasomotor tone , and is much more variable than iauc . fig3 depicts both a normal arterial pressure waveform 302 and a pulsus paradoxus waveform 304 . as is evident from the diagram , in a healthy patient 102 ( fig1 ), arterial pressure is typically cyclic within a bounded range during both expiration 306 and inspiration 308 . here , the blood pressure illustrated in the normal waveform 302 is about 120 mm hg / 80 mm hg . systolic pressure varies little and , correspondingly , pulsus paradoxus is less than 5 mm hg and is normal . however , in a patient with a condition generating pulsus paradoxus , peak arterial blood pressure can increase during expiration 306 and decrease during inspiration 308 , as is evidenced by the pulsus paradoxus waveform 304 . as indicated in fig3 , there is a change in peak height 310 of the waveform 304 between 110 mm hg and 150 mm hg and correspondingly , a pulsus paradoxus of 40 mm hg . fig4 illustrates measurements made upon a waveform 400 through one embodiment of the present invention . this waveform 400 depicts arterial pressure in a typical patient 102 , and it comprises a number of smaller periods , or components . in the preferred embodiment of the present invention , a first component 402 of the waveform 400 is selected by the processing hardware 108 ( fig1 ). as previously described , the first component 402 preferably includes an ascending limb phase 404 and a descending limb phase 406 , which are bounded by the lowest portions of diastole and divided at the uppermost peak of systole . preferably , a baseline 408 is then determined by the processing hardware 108 , and the area under the curve component 402 and bounded by the baseline 408 is subsequently computed and stored . in alternate embodiments , the area under the curve component 402 during the ascending limb phase 404 and the descending limb phase 406 can also be computed . these computations will be readily apparent to one skilled in the art , and can be completed through integration , numerical approximation or summation , fourier analysis , etc . after calculating the area under the curve component 402 , a second wave component 410 is selected . preferably , the second component 410 also includes an ascending limb phase 412 and a descending limb phase 414 , which are bounded by the lowest portions of diastole and divided at the uppermost peak of systole . preferably , a baseline 416 is then determined by the processing hardware 108 , and the area under the curve component 410 and bounded by the baseline 416 is subsequently computed and stored . in alternate embodiments , the area under the curve component 410 during the ascending limb phase 412 and the descending limb phase 414 can also be computed . in the preferred embodiment , the auc of components 402 and 410 are then compared to determine the presence and / or the magnitude of pulsus paradoxus . preferably , the difference in the areas under the curve components 402 , 410 (“ δauc ”) is calculated as a percentage . as calculated by embodiments of the present invention , moderate airway obstruction and corresponding increases in pulsus paradoxus typically result in δauc of greater than about 20 – 40 %. similarly , levels of δauc greater than about 60 % can result from and can be indicative of more severe airway obstruction and pulsus paradoxus . however , percent values of δauc vary amongst individuals and may be valuable when determinations of δauc are monitored in an individual longitudinally over time . in alternate embodiments , changes in iauc and dauc ( designated “ δiauc ” and “ δdauc ”, respectively ) can also be measured and compared in a fashion similar to that provided herein for δauc , in order to provide additional information about the state of the patient 102 . while a single calculation of δauc for two given curve components 402 , 410 , can often diagnose pulsus paradoxus and a variety of related ailments , the accuracy can be greatly improved by iteratively repeating the measurements and corresponding calculations , and analyzing the aggregate data from these measurements as it is collected . in this fashion , it is contemplated that a variety of alternate embodiments can be employed which make additional processing calculations , including : an average δauc over a plurality of comparisons , a maximum and minimum δauc , a weighted or floating average for δauc , standard deviation and variance of auc , artifact suppression , time interval measurement and adjustment , etc . in the preferred embodiment , weighted signal averaging would be employed to maximize the accuracy of the calculated pulsus paradoxus while maintaining minimal response times to changing pulsus paradoxus . specifically , weighted signal averaging comprises calculating a rolling average of δauc over a time interval . advantageously , weighted signal averaging would assign a comparatively low weight to δauc values obtained from individual intervals which fall significantly outside the current average δauc , thereby minimizing artifacts inherent in the pulsatile cardiovascular data . these , and other calculations will increase the efficacy of embodiments of the present invention by reducing the number of false positive reports ( e . g ., those due to arrhythmia ) and increasing the system &# 39 ; s overall accuracy . advantageously , embodiments of the system and methods provided herein reliably reflect corresponding increases or decreases in airway obstruction and pulsus paradoxus more accurately and less invasively than previous methods . whether by pulse oximeter plethysmography , finger plethysmograph or other waveform , the preferred embodiment involves placing a non - invasive and painless transducer on a finger , ear lobe or other body area . the necessary calculations are conducted in real time , allowing for nearly instantaneous measurement of pulsus paradoxus , rapid notification for medical personnel , and quicker diagnosis and treatment . furthermore , states of low perfusion , such as hypovolemia , will have little or no influence on the accuracy of pulsus paradoxus determined by embodiments of the present invention because the measurement of pulsus paradoxus is based upon relative changes δauc during the respiratory cycle and not upon absolute values of auc or other waveform indices . in addition , the two dimensional parameters measured ( δauc , δiauc and δdauc ) are far more accurate for determining the presence and / or magnitude of pulsus paradoxus than manual measurement or methods utilizing only change in waveform height . the decrease in lvsv that results in pulsus paradoxus is a change in volume , a three - dimensional entity . the two - dimensional mathematical waveform indices , including changes in area under the waveform curve which we utilize to measure pulsus paradoxus , accurately represent the true changes occurring in lvsv and , thus , pulsus paradoxus . previous developers have not recognized the importance of utilizing this two - dimensional parameter , area under the curve , to measure pulsus paradoxus . furthermore , numerous advantages not realized under previous techniques are also achieved . for example : 1 . the preferred embodiment does not require simultaneous determination of the phases of the respiratory cycle . 2 . the preferred embodiment is based upon the percent change in auc and , thus requires less intensive use of processing hardware and / or software , allowing less sophisticated and / or less expensive components to be used . 3 . the preferred embodiment is not significantly influenced by changes in waveform baseline occurring as a result of movement of the patient 102 . 4 . although the timing of events causing pulsus paradoxus during the respiratory cycle is altered during positive pressure ventilation , the preferred embodiment nonetheless provides accurate measurement of pulsus paradoxus during positive pressure ventilation . in addition to the diagnosis of pulsus paradoxus , embodiments of the present invention allow for accurate and timely recognition of airway obstruction ( e . g ., from mucus plugs ), mechanical failure ( equipment failure ), loss of the airway ( tracheostomy tube displacement ) or other conditions associated with pulsus paradoxus such as cardiac tamponade . consequently , the methods and systems provided herein will aid in the rapid diagnosis and treatment of patients currently or potentially suffering from sudden infant death syndrome or obstructive sleep apnea . technology dependent patients dependent on home assisted ventilation and artificial airways ( tracheostomy ) will also benefit from this monitoring . given the embodiments of the invention described herein , it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting , having been presented by way of example only . numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as defined by the appended claims and equivalents thereof .
a method and apparatus are disclosed of utilizing a source of arterial and / or arteriolar pulse waveform data from a patient for the purpose of measuring pulsus paradoxus . the arterial pulse waveform data source described is a pulse oximeter plethysmograph but can be any similar waveform data source , including intra - arterial transducer , blood pressure transducer , or plethysmograph . through incorporation of the measurements of values , such as the area under the pulse waveform curve , that are time - domain functions of a change in height of the pulse waveform over at least a partial duration of the waveform , embodiments of the present invention represent a significant improvement upon previously described methods of measuring pulsus paradoxus and , further , produce improved accuracy in measurement of the multiple contributing variables generating pulsus paradoxus , in particular the contribution of diastolic events , to the extent that the physical signs so measured can be regarded as “ revised pulsus paradoxus .”