Abstract:
A risk assessment device, in particular for the risk of a severe heart functional disorder such as sudden death due to heart disease, comprising an input unit which is adapted to record or read in an electrocardiographic input signal and to output a corresponding output signal comprising a plurality of cardiac cycles and corresponding intervals, and a detection unit which is connected to the input unit and which is adapted to detect events of predetermined nature in the output signal, wherein there are provided a summing unit which is connected to the detection unit and the input unit and is adapted to add up the length of predetermined intervals connected to a detected event within the input signal and to output a corresponding sum parameter, and an evaluation unit which is adapted to evaluate the sum parameter under predetermined conditions and to output a risk parameter which is dependent on the evaluation.

Description:
The invention concerns a risk assessment device, in particular for the risk of a severe heart functional disorder such as sudden death due to heart disease, comprising an input unit which is adapted to record or read in an electrocardiogram as an input signal and output a corresponding output signal including a plurality of cardiac cycles and corresponding intervals, and a detection unit which is connected to the input unit and which is adapted to detect events of a predetermined nature in the output signal. 
   The intervals can be R—R intervals but also p—p intervals or other intervals of an electrocardiogram. Usually, an electrocardiogram includes a plurality of intervals of normal length. Particularly in the case of extrasystoles however intervals of reduced or increased length may occur, which do not correspond to a normal interval. 
   BACKGROUND OF THE ART 
   Published international application WO 99/23944 discloses a method and an apparatus which assess electrocardiograms, with the aim of obtaining parameters which can be associated with the individual risk of a person, by assessing electrocardiograms in the area of extrasystoles. 
   In the time domain, for example, an ‘onset’ is ascertained for that purpose. That is the difference in the mean values of the last normal RR-intervals before the extrasystole and the first normal RR-intervals after the extrasystole. 
   Another parameter in the time domain is the ‘slope’, That is the greatest retardation in frequency within a sequence of a plurality of heartbeat intervals after an extrasystole over a predetermined number of successive RR-intervals. The gradient of the regression degrees describing the retardation in frequency is ascertained. In conjunction with the slope its correlation coefficient is determined, that is to say a measurement in respect of the regularity of the slope which is formed by numerical averaging of a plurality of successive slope values. 
   It is known from WO 99/23944 that, with a low onset, a shallow slope or a low correlation coefficient of the slope, the risk of dying as matters progress is significantly increased. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a device which is suitable for risk assessment and which can determine the risk of severe heart functional disorders with a degree of sensitivity which is high as possible and a level of specificity which is as high as possible. High sensitivity means that as many as possible of candidates for sudden death due to heart disease are warned and only a few candidates are overlooked, that is to say remain unwarned, in which respect admittedly there are also persons in respect of whom the risk as matters proceed is not confirmed are also warned. High specificity means that a warning does not happen in relation to most patients who are not endangered, but in actual fact there are also some patients who are not endangered, who are erroneously warned (that is to say disquieted). 
   In accordance with the invention that object is attained by a device of the kind set forth in the opening part of this specification, including a summing unit which is connected to the detection unit and the input unit and is adapted to add up the length of predetermined intervals connected to the detected event within the input signal and to output a corresponding sum parameter, and an evaluation unit which is adapted to evaluate the sum parameter under predetermined conditions and to output a risk parameter dependent on the evaluation. Evaluation of the sum parameter in the evaluation unit to form the risk parameter is typically effected in that the configuration in respect of time of the sum parameter is analysed and a risk parameter corresponding to the respective trend parameter in respect of time is formed. 
   In at least one embodiment, the detection unit detects ventricular extrasystoles as a predetermined event. 
   In many embodiments, the detection unit is connected to a selection unit that selects among the detected events those events which have a predetermined property. 
   In that respect the selection unit will typically select such ventricular extrasystoles which are involved, with a change in the morphology of a QRS-complex associated with the extrasystole, compared to QRS-complexes which are not linked to an extrasystole, in the output signal of the input unit. 
   Commonly, the selection unit will also select such ventricular extrasystoles in which a QRS-complex is at least 20% premature compared to the QRS-complexes of preceding intervals. The typical selection unit is one which selects such ventricular extrasystoles which are followed by an interval which is extended by at least 10% in relation to preceding intervals. For selection which goes still further, the selection unit is adapted to select a predetermined number of intervals which do not contain any extrasystole prior to and after a ventricular extrasystole, for forming the sum parameter. Those measures each in themselves and in particular in combination contribute to substantially avoiding misclassifications due to disturbances in the signal such as noise, hum or other signals which are superimposed on the electrocardiogram. The number of the intervals respectively considered, preceding or following an extrasystole, can be for example between 2 and 20 and can be used to predetermine the most appropriate values for sensitivity and specificity, in accordance with further conditions. In that respect the sequence of the stated features can be adapted to the given aspects of the signal in regard to signal quality and processing time and the arrangement of the measures can be interchanged. 
   In a further alternative configuration the detection unit includes an interval selection unit which is adapted to select for the formation of the sum parameter such intervals with which there are associated at least two ventricular extrasystoles which occur within a predetermined period of time. That period of time is commonly of the order of magnitude of an hour. 
   Supplementally or alternatively the object of the invention is attained in a risk assessment unit having an input unit which is adapted to record or read in an electrocardiographic input signal and to output a corresponding output signal including a plurality of cardiac cycles and corresponding intervals, and a first evaluation unit which is at least indirectly connected to the input unit and which is adapted to evaluate the output signal under predetermined conditions and to output a risk parameter dependent on the evaluation, characterised by a second evaluation unit which is connected to the first evaluation unit to take over the risk parameter as a first parameter and has at least one input for at least one further parameter and is adapted to calculate a risk signal corresponding to a probability of a severe heart functional disorder by linear combination of the parameters as it is usually determined by logistical regression, or by non-linear combination of the parameters. 
   It has been found that not only the risk parameter as described hereinbefore and formed from the sum parameter is a suitable input value for logistical regression analysis, but for example also the above-described onset or slope as well as the statistical fluctuation range of those values. However particularly specific and significant results are attained if the sum parameter or the configuration in respect of time thereof, besides age and blood pressure of a patient, are involved in the linear combination of the parameters. 
   In summary the essential aspects of the invention lie on the one hand in the formation of the described sum parameter and on the other hand in the linking of various risk parameters by a linear combination. Both aspects alone and independently of each other contribute to an improvement in sensitivity and specificity. The object of the invention is particularly well attained by a risk assessment device in which the sum parameter is one of the risk parameters which are involved in the parameter combination. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail by means of an embodiment with reference to the single Figure. 
     The Figure is a diagrammatic block circuit diagram of a risk assessment device according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The risk assessment device  10  includes as its main component an ECG risk parameter branch  12  and a risk analyser  14  which in the illustrated embodiment are advantageously connected together but which can also be implemented separately from each other. 
   On the input side the ECG risk parameter branch  12  has a connection for an ECG measurement value pick-up  16 . Alternatively, an ECG which is stored on a data carrier  18  can also be read in, by way of a second connection. On the output side the ECG risk parameter branch  12  is connected to an input of the risk analyser  14 . In the illustrated embodiment the risk analyser  14  has two further parameter inputs  20  and  22 . 
   On the output side the risk analyser  14  is connected to an output unit  24 . The output unit can have an interface for further units which are connected downstream thereof, it can include a display in order to display a calculated value for example for the risk of a sudden death due to heart disease or however it may also include a threshold value unit which outputs an alarm signal whenever the calculated risk exceeds a limit value. Such a threshold value unit can moreover be connected to a therapy unit which is triggered when the threshold value is exceeded and which for example causes medicational therapy or electrotherapy for the heart. 
   In an alternative configuration the illustrated risk assessment device  10  is so designed that it is part of an implanted device which by means of the ECG input  16  and an input unit  26  connected thereto, is capable of recording as its input signal an ECG which is derived directly from the heart or obtained from another location in the interior of the body. A measurement value pick-up, for example a sense electrode of an electrode line, can be connected to the ECG unit  16 , as is known to be used in relation to cardiac pacemakers. The input unit  26  includes the necessary input amplifiers and filters to process the input signal and to make a processed ECG signal available at its output. 
   The processed ECG signal is fed to a detection unit  28  which is adapted to detect the QRS-complexes contained in the input signal and in particular various kinds of extrasystoles in the processed ECG signal. Such extrasystoles are, inter alia, supraventricular extrasystoles, ventricular extrasystoles without a change in the morphology of the corresponding QRS-complex in the ECG (VES-1) and ventricular extrasystoles which are involved with a change in the morphology of the corresponding QRS-complex (VES-2). Hereinafter, in connection with extrasystoles, more detailed information will be set forth, in accordance with the classification system described herein, only when that is required to explain the function and the requirement involved. 
   From the detection unit  28  the processed ECG signal together with the information about detected extrasystoles passes to a selection unit  30  which, besides normal memory elements, includes a morphology comparison unit  32  which is adapted to detect variations in the morphology of a QRS-complex. For that purpose the morphology comparison unit  32  is connected to a signal portion memory  34  for storing the respective form or parameters extracted therefrom of that QRS-complex which was ascertained for the last-detected reference interval. In that respect, the reference interval is an interval which corresponds to a predetermined duration or a predetermined number of normal events and does not contain any extrasystole. 
   On the basis of the comparison of the signal portion stored in the signal portion memory  34  or the extracted parameters and the corresponding signal portion associated with a ventricular extrasystole or its extracted parameters, the morphology comparison unit  32  is capable of detecting variations in the morphology of a QRS-complex in connection with a ventricular extrasystole and outputting a corresponding marker signal for the ventricular extrasystole with a QRS-complex morphology variation (type VES-2) and the R—R interval preceding the ventricular extrasystole with the QRS morphology variation (type VES-2) and the following R-R interval. Following the morphology comparison unit  32  the processed ECG signal with the information about VES-2 extrasystoles which have occurred is fed to a reference interval comparison unit  36  of the selection unit  30 . The reference interval comparison unit  30  serves for further selection of the intervals containing a ventricular extrasystole. For that purpose, the reference interval comparison unit  36  is connected to a reference interval memory  38  for storing a reference interval derived from the average of the last normal intervals. This usually involves a relatively small number of normal intervals, for example 5 intervals, in order to permit sufficiently rapid adaptation to physiological changes as constantly occur without any connection to extrasystoles. The reference interval comparison unit  36  is so adapted that it only selects such intervals containing a VES-2 extrasystole, in which the extrasystole is premature in relation to the reference interval at least by a predetermined limit value (typically 20%) and which have a post-extrasystole pause which is longer at least by a limit value (typically 10%) than the pause after the systole in the reference interval. In addition, the reference interval comparison unit  36  is adapted only to select such intervals containing VES-2 extrasystoles, which are preceded by a predetermined number of normal intervals and which are followed by an also predetermined number of normal intervals. 
   In that respect, depending on the given aspects of the signal, in terms of signal quality and processing time, the morphology comparison unit  32  in conjunction with the signal portion memory  34  can be interchanged with the reference interval comparison unit  36  in conjunction with the reference interval memory  38 . 
   The information about the processed ECG signal and the selected intervals is available at the output of the reference interval comparison unit  36  or the morphology comparison unit  32 , in the case of the above-described interchange of the functional units, and is applied to the input of an interval selection unit  40 . The interval selection unit  40  executes a further selection in such a way that only those intervals are selected, which relate to at least two VES-2 extrasystoles which occur in succession within a predetermined time, in specific terms for example within an hour. If therefore only one VES-2 extrasystole occurs within that time, the corresponding interval is not selected. If more than one VES-2 extrasystole occurs within an hour, the corresponding intervals are selected. Once again, the processed ECG signal and the information about the selected intervals are available at the output of the interval selection unit  40  and thus at the input of the summing unit  42 . The summing unit  42  is adapted to form the sum of the durations of the selected intervals. In addition the summing unit  42  is adapted to determine the sum of the selected intervals for a respective predetermined time, for example 24 hours, optionally in each case beginning at zero or floating over the predetermined period of time preceding the last VES-2 extrasystole. That therefore affords a kind of density function over the occurrence of intervals which are detected by the interval selection unit  40 . That is applied to the input of a first evaluation unit  44 . The first evaluation unit  44  is adapted to determine the trend pattern of the incoming sum parameters. 
   The sum parameter itself is already an indicator in respect of the risk of a sudden death due to heart disease. If the value of the sum parameter is high the risk of dying from sudden death due to heart disease is greater than when the value of the sum parameter is low. In addition a rising trend pattern, having regard to the scatter of the sum parameter, points to an increase in the risk of suffering sudden death due to heart disease. 
   The sum parameter at the output of the ECG risk parameter branch  12  or the trend pattern thereof can be added for example to a threshold value unit which, whenever the sum parameter or the trend pattern thereof exceeds a given limit value, gives an alarm which indicates a high risk of sudden death due to heart disease. It is also possible to connect to the threshold value unit a unit for delivering a medicament or for carrying out another preventive-therapeutic measure. 
   In the illustrated embodiment the sum parameter or the trend pattern thereof is applied to one of three inputs of the risk analyser  14 . It is firstly fed to an interval scaling unit  50  of the risk analyser  14 , in which the continuous sum parameter or the trend pattern thereof is subjected to an interval scaling procedure. The value describing the respective interval in which a current sum parameter or the trend pattern thereof falls is multiplied in a multiplication unit  52  by a previously determined coefficient. In the illustrated embodiment this is the coefficient b 1 . For that purpose the multiplication unit  52  is connected to the interval scaling unit  50 . 
   The signals at the parameter inputs  20  and  22  are also subjected to interval scaling in accordance with their respective magnitude in corresponding interval scaling units  54  and  56  and the scale value describing the respective interval is multiplied in multiplication units  58  and  60  by the previously determined coefficients b 2  and b 3 . Therefore, three different scale values multiplied by the respective coefficients are available at the outputs of the multiplication units  52 ,  58  and  60 . Those three values, together with a constant  61 , are added together in a summing unit  62 . A parameter sum is available at the output of the summing unit  62 , which parameter sum in a subsequent exponential unit  64  is accounted for in such a way that a value which corresponds to the Euler&#39;s number e parameter sum  occurs at the output of the exponential unit  64 . That value is divided in the subsequent computing unit  66  by the sum of that value plus 1. A risk factor is available at the output of the computing unit  66 , which describes the probability of dying from a sudden death due to heart disease and the determination of which involves not just the sum parameter from the ECG risk parameter branch  12  but by way of the parameter inputs  20  and  22  also such risk factors as age or blood pressure. Those risk parameters are balanced with each other in the risk analyser  14  on the basis of the approach of logistical regression. In this respect the described structure of the risk analyser  14  leads to a calculation, in which the output value p thereof depends on the input values X 1 , X 2  and X 3 , as follows: 
       p   =       ⅇ     (         b   1     ·   X1     +       b   2     ·   X2     +       b   3     ·   X3     +   a     )         1   +     ⅇ     (         b   1     ·   X1     +       b   2     ·   X2     +       b   3     ·   X3     +   a     )               
 
   In that respect the coefficients b 1 , b 2  and b 3  and a are matched in the manner which is usual for logistical regression procedures. 
   It has been found that a corresponding risk assessment device can determine the risk of sudden death due to heart disease both with a high level of sensitivity and also with a high degree of specificity. The aim of values which are as high as possible both in respect of sensitivity and also in respect of specificity is achieved in the device according to the invention by one of the following measures alone or a suitable combination thereof:
         selection of VES-2 extrasystoles in accordance with predetermined criteria and limit values;   formation of sum parameters and trend parameters derived therefrom;   consideration of risk parameters which have been validated on the basis of a logistical regression model; and   formation of risk factors which are calculated on the basis of weighting coefficients which are established in accordance with the aim involved, and interval-scaled risk parameters.