Medical device

Abstract: The invention relates to a device (1) for detecting hemodynamic conditions in a heart (2), in particular conditions corresponding to dangerous arrhythmias. The device contains a sensing unit in the form of an impedance sensor (6) for a physiological variable, which is blood pressure, the unit emitting a signal (10), on the basis of the variable, with a mean value, a signal conditioning unit (7) connected to the sensing unit (6), a calculation unit (8) connected to the signal conditioning unit (7) and a comparator (9) after the calculation unit (8). The calculation unit (8) is devised to calculate a variability measure (14) relative to the signal's mean value, the variability measure (14) being correlated to mean blood pressure, and the variability measure (14) is compared in the comparator (9) to an adjustable threshold value (16, 17) corresponding to a specific hemodynamic condition, whereupon the comparator (9) emits an indication signal (18, 19) when the variability measure (14) falls below the threshold value (16, 17).

Description: The present invention relates to a medical device for detecting hemodynamic conditions in a heart, in particular conditions corresponding to dangerous arrhythmias. An electromedical implant for electrical treatment of the heart in dangerous arrhythmias must be capable of identifying the presence of these conditions. However, detection of e.g. ventricular fibrillation (VF) is associated with difficulties in the prior art. One such difficulty is that the detection of VF from e.g. the ECG signal (the heart's depolarization signal) requires knowledge of the ECG signal's morphology, as illustrated in US-5 058 599. Through this patent document, a method and a device for detecting a dangerous heart condition is prior art. Here, a sequence of abnormal events is detected among a plurality of normal events in an electric signal, especially in a depolarization signal in the heart. According to the method, a plurality of signal parameters, typical of the event to be detected, are identified. One such parameter can be e.g. the signal's derivative for a special tracing segment. After normal and abnormal events are extracted from the signal, a statistical calculation, based on all recorded events for a sequence of the identified signal parameters, is performed. For example, standard deviation is calculated, and this value is compared to a predefined threshold value. If the value for the standard deviation exceeds the threshold value, this indicates the presence of an abnormal condition. In order to come to grips with this difficulty and achieve an ability to select reliable triggering points in an individual signal tracing in order to identify e.g. the tracing slope during a specific interval, use of the ECG signal according to the procedure shown in US-4 453 551 could be a solution. A device and a method for analyzing and distinguishing dangerous fibrillation conditions from the heart's normal sinus rhythm as well as from other conditions in the heart, such as ventricular tachycardia, are also known from US-4 453 551. This is achieved by study of a number of tests performed on a extracorporeally recorded ECG signal. Before these tests can be performed, the signal is sampled, filtered and standardized. In standardization, the amplitude of the signal, which varies greatly from one patient to another, is standardized. According to one of these tests, the ratio between the energy in an ECG signal, above and below a zero level, is calculated in a time window. In ventricular tachycardia or sinus rhythm which, in contrast to fibrillation, displays a regular ECG signal, the calculated energy ratios are largely constant in a series of consecutive time windows, making it possible to distinguish tachycardia from fibrillation. An alternative calculation method for this test is to calculate and study variance in the energy ratios for a series of consecutive time windows. Another device in which the ECG signal is utilized for detecting ventricular fibrillation and ventricular tachycardia is known through US-4 870 974. Here, the heart's hemodynamic state is studied with the aid of pressure signals, in addition to a study of the ECG signal's tracing morphology. For the pressure signal, the time pressure is below a specific level is measured, and different threshold values are represented for this time for e.g. fibrillation and tachycardia. As with the device according to US-5 058 599, certain parameters must also be identified in US-4 870 974 from an individual signal tracing. For the pressure signal, a reference level, for example, must be identified from the signal tracing for detection to work. The object of the present invention is generally to set forth a simplified and improved device for detecting hemodynamic conditions. The invention is as defined in the appended set of claims. More specifically, the aim of the invention is to achieve more reliable detection of dangerous, hemodynamically unstable conditions, such as tachycardia and fibrillation. As will be described below, changes in the mean level of blood pressure in the heart are studied indirectly. If mean blood pressure in the heart drops, e.g. as the result of ventricular tachycardia or fibrillation, this indicates that the heart's pumping capacity has declined and that the heart is no longer capable of supplying vital organs, such as the brain, with oxygenated blood; damage can then occur very rapidly as a result. According to the invention, this object is achieved with the features set forth in the main claim, and a plurality of advantageous embodiments of this are set forth in the dependent claims. Thus, this object is achieved with a device cited in the preamble in which the sensing unit is an impedance sensor devised to sense blood pressure, and the calculation unit is arranged to calculate a variability measure relative to the mean value of the signal emitted by the sensing unit, the variability measure being correlated to mean blood pressure. The variability measure is then compared in the comparator to an adjustable threshold value corresponding to a specific hemodynamic condition, whereupon the comparator emits an indication signal if the variability measure falls below the threshold value. As will be apparent, the invention therefore does not require any analysis of the morphology of the signal dependent on the physiological variable. The invention is based on the discovery of a correlation between a variability measure (e.g. the rectified mean value, the standard deviation or the root mean square value (RMS)), related to the mean value of a signal generated according to a sensed physiological variable, and mean blood pressure in the heart. A decline in the variability measure has been found to be correlated to a drop in the level of mean blood pressure. When the variability measure falls below a given level, this indicates the presence of a dangerous condition. For a continuous, consistently varying signal with a constant amplitude, the signal's variability measure (e.g. standard deviation, rectified mean value or RMS), according to general statistical theory, must be constant, independent of signal frequency. For example, standard deviation is the same for sinus signals with the frequencies f and 5f. As regards amplitudes, halving the amplitude, for example, also halves the variability measure. A consequence of these correlations is then that the amplitude of a signal, which normally does not change its morphology but displays a varying frequency can be calculated, by determining the variability measure. If the amplitude of the signal changes, these changes can be determined, since the variability measure displays similar changes. When this argument is applied to a device according to the invention for determining mean blood pressure, this means that the device will work independently of the prevailing heart rate and can, thus, detect, dangerous tachyarrhythmias which can even occur at heart rates under 150 beats/min, since heart rate alone is not a measure of this tachyarrhythmia. The signal generated on the basis of a sensed physiological variable must retain its morphology for the method to work according to the aforementioned argument. When this condition is met, the device according to the invention sets forth a method for detecting dangerous ventricular tachyarrhythmias. The signal can have an unchanged morphology, even in fibrillation in which heart rates can exceed 400 beats/min, so the invention is also applicable to detection of fibrillation. Animal experiments have surprisingly shown that a variability measure, related to the mean value for a signal generated according to a sensed physiological variable and which displays changes in morphology, is correlated to mean blood pressure in the heart. This means that study of the change in a variability measure which is the mean value for a signal generated on the basis of a sensed physiological variable makes it possible to detect dangerous ventricular tachycardia and ventricular fibrillation, both with a stable signal morphology and an unstable signal morphology, with greater certainty and with simpler equipment than has hitherto been possible. According to a preferred embodiment, the variability measure is calculated for a specific period of time in a time window with a predefined duration. This value for the variability measure is compared to a first adjustable threshold value, representing a detection threshold for dangerous tachycardia, and to a second adjustable threshold value, representing a detection threshold for fibrillation, the first value being larger than the second. An indication signal is emitted when a value falls below any of these values so suitable therapy can be instituted. The invention will now be described in greater detail, referring to 2 FIGURES in which: FIG. 1 shows a medical device which contains the detection device according to the invention; FIG. 2 shows the detection device according to the invention. The same or similar components in the FIGURES have the same reference designations. The invention will be used e.g. in a medical device in the form of a tachycardia-terminating pacemaker. As will be obvious to anyone well-versed in the art, no limitation to this application is to be understood. The device according to the invention could also be used in e.g. a defibrillator or with a device for sensing hemodynamic conditions in the heart for diagnostic purposes. The device can be applied to both implanted an extracorporeal medical devices. In FIG. 1 is shown a medical device 1 connected to a heart 2. The medical device 1 comprises a detection device 3 for sensing a physiological variable and processing and evaluating a signal 10 which depends on the variable, a programmable control device 4 for controlling and coordinating the parts in the device 1 and a therapy means 5 for instituting suitable therapy for the heart 2, all according to prior art in the field. As previously noted, the medical device 1 is exemplified with a tachycardia-terminating pacemaker, whereby the therapy means 5 is a pulse generator, controlled by the control device 4, for stimulating the heart 2 with stimulation pulses. The therapy means 5 is of conventional design and will not be further described here, since it does not contribute to an understanding of the detection device according to the invention. FIG. 2 is a block diagram of the detection device 3. According to the invention, it contains a sensing unit 6, a signal conditioning unit 7, a calculation unit 8 and a comparator 9. The sensing unit 6 for the physiological variable sends the signal 10, which depends on the variable, to the signal conditioning unit 7 in which the signal can be filtered in a filter section 11. The signal 12 emitted by the signal conditioning unit 7 is sent to the calculation unit 8 in which a variability measure 14 is calculated for the signal 12 by an addition unit 15. The signal 12 can also be rectified in a rectifier 13 in the calculation unit 8 before being sent to the addition unit 15. The variability measure 14 is compared in the comparator 9 to a threshold value 16, 17, and the comparator emits an indication signal 18, 19 when the variability measure 14 falls below the threshold value 16, 17. According to a preferred embodiment, the signal conditioning unit 7 also contains, in addition to the filter section 11, a standardization section 20, consisting of an amplifier, for standardizing the signal. According to this preferred embodiment, a window generator 21 is additionally connected to the addition unit 15 which generates a time window with a predefined duration during which the variability measure 14 is calculated. The sensing unit 6 is an impedance sensor. Both a pressure sensor and an impedance sensor can be used simultaneously. The pressure sensor is placed in the right ventricle and can consist of e.g. a piezoelectric crystal. Pressure measurement with a piezocrystal is a fast, well-tried measurement method which supplies easily interpreted measurement values. An additional advantage is that the absolute level of pressure can also be determined. Disadvantages in measuring pressure are the circumstance that the sensor's output signal is very weak and a special electrode is required to perform the measurements. Impedance measurement is performed between two points in the right ventricle with the aid of a standard electrode in which measurement is between the electrode tip and a ring on the electrode cable at a distance from the electrode tip. Measurement is performed with the alternating current method (AC method) to prevent the polarization effects occurring at the emission of direct current (DC method). The measurement current could be e.g. a continuous square wave with an amplitude of 10 µA and a frequency of 4 kHz. A signal representing impedance is sent from the sensing unit 6 to the signal conditioning unit 7. Measurement of impedance in the heart is a method well-known to anyone well-versed in the art and will not be further described here. The signal 10 from the sensing unit 6 is sent to the signal conditioning unit 7 in which the signal can be filtered in a filter section 11. Whether or not the signal 10 is to be filtered depends on the variability measure selected for calculation. The different variability measures used are the rectified mean value, standard deviation and the RMS value. When the rectified mean value or the RMS value are calculated, the mean value for the signal 10 must be zero, i.e. any direct current level for the signal 1 is filtered out. This is achieved when the filter section 11 filters out signal components less than e.g. 0.2 Hz. The mean value for the signal does not have to be zero for calculation of standard deviation, so filtering out the DC level is not necessary. Filtering out high frequency noise signals in the signal is often desirable. This is achieved when the filter section 11 filters out signals higher than e.g. 15 Hz. Filtration has very little effect on the morphology of pressure or impedance signals. According to a preferred embodiment of the invention, the signal is standardized in the standardization section 20 after filtration. This section consists of an amplifier which amplifies the signal with appropriate gain so variations in signal amplitude are kept within specific limits. The filtered and standardized signal is then sent to the calculation unit 8 in which a variability measure 14 is calculated by the addition unit 15 for the rectified signal. Calculations can be made either on a continuous signal or on a discrete signal. The calculations entail integration of the signal, in the case of a continuous signal, and addition of discrete values, in the case of a discrete signal. Calculations in the addition unit 15 also include division, squaring and root extraction according to the calculation method selected. If the calculations are to be made from the discrete signal, the continuous signal is sampled in the signal conditioning unit 7 before the signal is sent to the addition unit 15. Sampling is performed at a sufficiently high frequency, e.g. 10 Hz to 100 Hz, with a view to the signal's frequency, all according to technics familiar to anyone well-versed in the art and will not be described here. According to the preferred embodiment, the variability measure 14 is calculated in a time window with a predefined duration. An appropriate time has proved to be about 6 seconds. The duration of the time window is selected so calculations are made over a plurality of heart cycles with normal heart rhythm. When a duration of about 6 seconds is chosen, at least one complete respiratory cycle is also covered. The time windows can be arrayed in many different ways. The time window can be isolated, with a long interval to the next window, or the interval between consecutive time windows can be of the same duration as the duration of the time window. A new time window can immediately follow a preceding window, without any interval between windows, and a new calculation made for each window. A new calculation can be made after every half window in a time window, and the time window can also be floating. For a discrete signal this means that a new calculation is made after every sampling. Another possibility is for calculations to be performed over a plurality of time windows with a new calculation made in each time window. The following table shows how the different variability measures are calculated for a continuous signal and for a discrete, sampled signal: in which T W is the duration of the time window, with a start at time t=0 S(t) is the continuous signal S i is sample no. i in the continuous signal taken at time t i , i = 1 , 2 , ..... N N is the number of samples in the time window. The table assumes the mean value of the signal is S mean =0 . As noted above, the mean value does not have to be zero for calculation of standard deviation. The complete equation for the standard deviation of a discrete signal is: in which S mean is obtained from : for a discrete signal. In an analogous manner, the standard deviation of a continuous signal can be calculated when the mean value is other than zero. In calculation of the rectified mean value, the signal is rectified before calculations are performed in the addition unit 15. No rectification of the signal has to be performed if standard deviation or the RMS value are calculated instead. The variability measure calculated in the addition unit 15 is then compared in the comparator 9 to a threshold value 16, 17 for a hemodynamic state. According to the preferred embodiment, the variability measure 14 is compared to a first adjustable threshold value 16 and a second adjustable threshold value 17, and the comparator emits a first indication signal 18 indicating the presence of ventricular tachycardia when the variability measure 14 falls below the first threshold value 16. The comparator 9 emits a second indication signal 19 indicating 0 the presence of ventricular fibrillation when the variability measure 14 falls below the second threshold value 17, the first value 16 being greater than the second value 17. The indication signals 18, 19 emitted by the comparator 9 are sent to the control device 4 which is arranged to take steps relating to the indication signals. Calculations for a continuous signal can also be performed with a simple averaging circuit, consisting of a resistor and capacitor, with appropriately selected component values, connected to a reference level at which the signal's mean value is determined between the resistor and the capacitor. No window generator is required in this simple averaging procedure. It is not necessary for signal standardization to take place in the signal conditioning unit's standardization section 20. Instead, standardization can be performed by adapting the threshold values 16, 17 in the comparator 9 in some manner appropriate to the magnitude of the variability measure. FIGURE designations 1 Medical device 2 Heart 3 Detection device 4 Control device 5 Therapy means 6 Sensing unit (impedance sensor) 7 Signal conditioning unit 8 Calculation unit 9 Comparator 10 Signal from sensing unit 11 Filter section 12 Signal emitted by signal conditioning unit 13 Rectifier 14 Variability measure 15 Addition unit 16 First threshold value 17 Second threshold value 18 First indication signal 19 Second indication signal 20 Standardization section 21 Window generator

Claims: A device (1) for detecting hemodynamic conditions in a heart (2) and containing a sensing unit (6) for a physiological variable, the unit emitting a signal (10), on the basis of the variable, with a mean value, a signal conditioning unit (7) connected to the sensing unit (6), a calculation unit (8) connected to the signal conditioning unit (7) and a comparator (9) after the calculation unit (8), characterized in that the sensing unit (6) comprises an impedance sensor devised to sense blood pressure, the calculation unit (8) is devised to calculate a variability measure (14) relative to the signal's mean value, whereby the variability measure (14) is correlated to mean blood pressure, and the variability measure (14) is compared in the comparator (9) to an adjustable threshold value (16, 17) corresponding to a specific hemodynamic condition, whereupon the comparator (9) emits an indication signal (18, 19) when the variability measure (14) falls below the threshold value (16, 17) . A device according to claim 1, characterized in that the signal conditioning unit (7) consists of a filter section (11) for filtering the signal (10) and a standardization section (20) for standardizing the signal (10), and a window generator (21), which generates a time window with a predefined duration during which time window the variability measure (14) is calculated, is connected to the calculation unit (8). A device according to claim 2, characterized in that the calculation unit (8) consists of a rectifier (13) for rectifying the signal (12) filtered in the signal conditioning unit, and a following addition unit (15) for calculation of the variability measure (14) for the rectified signal. A device of any of claims 1, 2 or 3, characterized in that the comparator (9) has a first adjustable threshold value (16) and a second adjustable threshold value (17). A first indication signal is emitted, indicating the presence of ventricular tachycardia, when the variability measure (14) falls below the first value, and a second indication signal (19) is emitted, indicating the presence of ventricular fibrillation, when the variability measure (14) falls below the second value (17), the first value (16) being greater than the second value (17) . A device of any of claims 1-4, characterized in that the sensing unit (6) consists of a pressure sensor and an impedance sensor.

Inventors: HAGEL PIA, HOEGNELID KURT, NOREN KJELL, HAGEL, PIA, HOEGNELID, KURT, NOREN, KJELL, Högnelid, Kurt, Norèn, Kjell
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1995
Main classification: A61N1
IPC codes: A61N1