Ischemia detector sensing repolarisation of the heart

Abstract: An ischemia detector, comprises repolarization sensing means (42) for sensing repolarization of the heart of a patient (44) and delivering to detecting means (46, 48) corresponding repolarization signals, and workload sensing means (40) for sensing the workload of the patient and delivering to said detecting means corresponding workload signals. The detecting means are arranged to detect as a state of ischemia the occurrence of a predetermined relation between sensed repolarization and sensed workload.

Description: Technical Field The present invention relates to an ischemia detector comprising repolarization sensing means for sensing repolarization of the heart of a patient and delivering to detecting means corresponding repolarization signals. Background Art Ischemia results from insufficient blood flow through the heart muscle. The reason therefor is blocking or passage congestion of coronary blood vessels of the heart. An ischemic heart also loses its ability to adapt the heart blood flow to the demand, e.g. the workload. An ischemia is experienced by the patient as a severe chest pain and is one of the most stressing factors known to the organism and the patient is normally forced to sit down or lie down and feels the need for forced breathing, so called hyperventilation. Compensatory effects will be activated to cope with the ischemic situation. Thus the heart rate will increase due to sympathetic nerve stimulation and increased catecholamines stimulation. In case of a limited or local ischemia this activation can compensate for the reduced capacity of a limited ischemic area of the heart. In US-A-5 199 428 a techniques is described for detecting ischemia and effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart's oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptabel limits to avoid bradyarrhythmias and/or unphysiological AV delays induced by the nerve stimulation. The ischemia detection is based on the occurence of changes in the ST-segment variation different from predetermined or programmed threshold levels, or on changes in the pH and/or in the dissolved blood oxygen in venous return blood in the coronary sinus region of the patient's heart. An ischemic state can also be detected by an analysis of recorded IECG:s or surface ECG:s to determine the heart rate variability. An ischemia can be detected by a lead bend sensor located at the distal end portion of an implanted heart stimulator lead. As the heart wall is thickening and stiffening as the result of an ischemia, the accompanying change in the moving pattern of the heart wall can be detected in this way. Also sound absorption is effected by changes in the stiffness of the heart tissue and by measuring the absorption of sound waves, generated e.g. at the heart valve closure, on their way from the upper portion of the ventricle to the apex region, an ischemic situation can be detected. An ischemia is deteriorating the efficiency of the heart's pumping and an ischemic situation can therefore be detected by studying blood pressures and cardiac outputs too. Thus, by measuring the difference between the systolic and diastolic pressures and comparing this difference obtained from one heart beat to the difference obtained from the next heart beat an ischemia can be detected. With the aid of a flow sensor for measuring cardiac output an ischemic state can be detected, too. An ischemia can also be detected from the occurrence of the abnormal combination of a low workload and high breathing activity, which is typical of ischemic patients. The onset of an increased heart rate related to an ischemic situation can also be detected from changes in the repolarization of the heart, like changes in the QT interval, T-wave amplitude etc. From US-A-5 330 511 it is previously known to study variations in the QT-interval for determining an optimized AV-interval for a predetermined pacing rate for the control of a dual chamber pacemaker. The variation of the QT-interval is then studied as a function of the AV-delay for a fixed pacing rate and as optimum AV-interval is determined that interval which corresponds to the maximum QT. In US-A-4 228 803 a physiologically adaptive cardiac pacemaker is disclosed having a circuitry for measuring the time interval between a stimulus pulse and the following T-wave. The escape interval of the pacemaker pulse generator is varied in accordance with the detected stimulus-T-interval to vary the pacing rate in accordance with this variation. Since this interval corresponds to physiological changes the pacemaker is adapted to automatically follow the patient's physiological changes. Disclosure of the Invention The purpose of the present invention is to provide a new type of ischemia detector the function of which is based on the simultaneous detection of the onset of heart stress or increased heart rate and a low workload. This purpose is obtained with the detector defined Thus the ischemia detector according to the invention comprises workload sensing means for sensing the workload of the patient and repolarization sensing means for sensing repolarization changes which are characteristic of the onset of heart stress. According to advantageous embodiments of the detector according to the invention the workload sensing means comprise an activity sensor for sensing an activity parameter of the patient, like body movements, muscular sounds or pressure waves in body fluids due to the work load of the patient, or a sensor for sensing metabolic changes, like changes in nutrition and oxygen consumption of the patient. According to other advantageous embodiments of the ischemia detector according to the invention said repolarization sensing means comprise time measuring means for determining the QT time intervall between the onset of the QRS complex and a predetermined point on the T-wave, detected by said repolarization sensing means, said detecting means being disposed to detect as a state of ischemia the occurrence of a predetermined relation between said time interval and sensed workload. The predetermined point on the T-wave is preferably selected on the rear slope of the T-wave. Repolarization in ischemic tissue is prolonged and the T-wave is deformed, especially on the leading slope. The rear slope of the T-wave remains, however, substantially unchanged and therefore it is appropriate to select as measuring point for the QT-time interval determination a point on the rear slope of the T-wave. According to yet another advantageous embodiment of the detector according to the invention said detecting means comprise averaging means forming an average value of repolarization signals from a predetermined number of cardiac cycles to form an average repolarization signal representing the repolarization to be used for the detection of an ischemia. In this way influences from momentary variations due to e.g. breathing, position changes of the patient etc., are reduced. According to still another advantageous embodiment of the ischemia detector according to the mentioned alerting means are provided for activation in response to a detected ischemia. This is of particular value to patients having a so called silent ischemia, the occurrence of which the patient otherwise would not be aware of. When being alerted the patient may suitably lower his or her activity. According to another aspect of the invention an implantable heart stimulator is provided having means for varying the stimulation rate and an ischemia detector. According to an advantageous embodiment of the heart stimulator according to the invention control means are connected to the ischemia detector for controlling the stimulation rate varying means in response to the detection of an ischemia. Thus by lowering the stimulation rate at an early stage of an ischemia the further development thereof can be inhibited. Brief Description of the Drawings To explain the invention more in detail embodiments of the invention, chosen as examples, will be described below with reference to the drawings, on which Fig. 1 is a block diagram of an embodiment of an ischemia detector according to the invention, Fig. 2 shows IEGM:s recorded in the ventricle and illustrating changes in the T-wave region associated with an ischemia, Fig. 3 is a block diagram illustrating one specific realization of the operation of the detector according to the invention, Fig. 4 is a simplified block diagram of one embodiment of the heart stimulator according to the invention, and Fig. 5 shows the pacemaker with its lead implanted in the right ventricle of a heart. Description of Preferred Embodiments In Fig. 1 a workload sensor 40 and a repolarization sensor 42 are disposed for sensing the workload and the repolarization respectively of the heart of a patient 44 and delivering corresponding signals to an averaging means 46, in which averaged values for time periods of predetermined lengths are formed of the workload and repolarization signals. These averaged signal values are supplied to detecting means 48. The detecting means 48 comprises a memory 50, in which one or more relations between repolarization and workload are stored, and comparing means 52, in which the relation between the signals from the averaging means 46 representing sensed workload and repolarization is compared to the predetermined relations stored in the memory 50. When a predetermined relation between the signals from the averaging means 46 is detected alerting means (54) connected to the comparing means 52 is triggered to indicate the occurrence of an ischemia. The heart stimulator 56 is connected to the alerting means 54 for changing, normally lowering, the stimulation rate in response to the detection of an ischemia, as will be described more in detail below. The workload sensing means 40 can comprise an activity sensor, e.g. an accelerometer, for sensing body movements of the patient or a sensor for sensing muscular sounds or pressure waves in body fluids of the patient. Alternatively, the workload sensing means can comprise sensors for sensing metabolic changes, like changes in nutrition and oxygen consumption of the patient. The repolarization sensing means can be disposed to determine the magnitude of a characteristic portion of the T-wave, e.g. the T-wave amplitude or width. Repolarization in ischemic tissue is prolonged and the T-wave is deformed, the polarity of the T-wave can even be reversed, and the onset of increased heart rate due to ischemic stress results in an increased T-wave amplitude, cf Fig. 2, which shows IEGM:s recorded in the ventricle before an ischemia a, during an induced ischemia b, and at different times after removal of the occlusion inducing the ischemia c and d. As appeares from this figure the T-wave is deformed when an ischemic situation occurs, but the rear slope of the T-wave remains welldefined and the repolarization sensing means therefore can comprise time measuring means for determining the QT-time interval between the onset of the QRS-complex and a predetermined point on the rear slope of the T-wave, this QT-time interval being shortened when an ischemia is developed. An alternative embodiment of the detecting means is shown in Fig. 3. In this embodiment the detecting means 58 comprises two comparators 60, 62 to which the workload signal and the repolarization signal respectively are supplied for the comparison of the signals with predetermined threshold values Ref 1 and Ref 2. The repolarization signal can then represent e.g. the amplitude of the T-wave or the length of the QT-time interval as described above, the threshold value Ref 2 then being a predetermined amplitude threshold value or a predetermined QT-time interval limit. The outputs of the comparators 60, 62 are connected to the inputs of an AND-gate 64. As described above the occurrence of onset of an increased heart rate, detected from the repolarization of the heart, without a corresponding sensed increased workload, is detected as an ischemia. The comparator 60 is thus arranged to deliver an output signal when the workload signal is below the predetermined workload treshold value Ref 1 and the comparator 62 delivers an output signal if the repolarization signal in the form of the peak amplitude of the T-wave is above the threshold value Ref 2 or QT-time interval is shorter than the QT-time interval limit Ref 2. In this case an output signal is obtain from the AND-gate 64 for e.g. activation of ischemia alerting means and possible control of the stimulation rate of a heart stimulator. Fig. 4 is a simplified block diagram of an implantable heart stimulator 2 according to the invention. The heart stimulator 2 comprises an ischemia detector comprising ischemia detecting means 4, and control means 6, connected to the ischemia detecting means 4. The control means 6 are also connected to a pulse generator 8 for controlling the rate of generated stimulation pulses. The pulse generator 8 in its turn is connected to a lead 10 provided with electrodes 12 at the distal end portion for delivery of stimulation pulses and for possible electrical measurements, which lead 10 is intended to be implanted into the heart of the patient, preferably with the electrodes 12 in the right ventricle, cf Fig. 5. Sensing means 14 are also provided at the distal end portion of the lead 10 and sensed signals are supplied to the ischemia detecting means 4 through the lead 10. The sensing means 14 can be used for recording IECG:s and comprise electrodes as will be described in connection with Fig. 5. The signals are supplied by the lead 10 to an IECG recording unit 5, in which e.g. the magnitude of a characteristic portion of the T-wave, like the T-wave peak amplitude, or the QT-time interval, is determined and a corresponding output signal is delivered to the ischemia detecting means 4. Workload sensing means 11 of any suitable kind mentioned above are also provided in the heart stimulator 2. The heart stimulator 2 is also provided with alerting means 13, e.g. of wrist watches "beeper-type". These alerting means 13 are connected to the ischemia detecting means 4 to be activated when the predetermined relation occurs between the repolarization signal received from the unit 5 and the workload signal received from the workload sensing means 11, indicating the presence of an ischemic situation. Alternatively the alerting means 13 can be connected to the control means 6 to be activated when the stimulation rate is lowered as a result of a detected ischemia. These alerting means 13 is of particular value for patients having a "silent" ischemia, the occurence of which the patient otherwise would not be aware of. Fig. 5 shows an implanted heart stimulator in the form of a pacemaker 16, connected to the right ventricle 18 of the heart of a patient by the pacemaker lead 20, which is of a bipolar type with an electrode ring 22 and a tip electrode 24. The lead 20 is also provided with sensing means 26, 28, e.g. electrodes for recordal of IECG:s, as mentioned above. As an alternative the repolarization sensing means 26, 28 can be provided to determine e.g. both the T-wave amplitude and the QT-time interval and deliver a repolarization signal only when both the T-wave amplitude and the QT-time interval fulfil their predetermined ischemia conditions. An important advantage of the present invention is that the normal electrode system can be used for detecting the changes in the repolarization or, if separate sensing means are used, these can be mounted on the lead intended to be implanted into the patient's heart for stimulatory purposes. The stimulation rate of the heart stimulator 16 is reduced in response to an ischemia, detected as described above. There are different possibilites of reducing the stimulation rate. The control means can inhibit the delivery of a particular stimulation pulse, thus temporarily producing a longer interval between two consecutive pulses. The control means can also be arranged to more regularly inhibit a stimulation pulse out of a specified number of stimulation pulses in response to a detected ischemia. As a further alternative the control means can be arranged to control the pulse generator such that the stimulation rate is uniformly reduced on the detection of an ischemia, or the stimulation rate can be shifted to selected lower rates.

Claims: An ischemia detector comprising ischemia detecting means (4; 46, 48; 58), repolarization sensing means (5,14;26,28;42) for sensing repolarization of the heart of a patient (44) and delivering to said detecting means (4;46,48;58) corresponding repolarization signals, and workload sensing means (11,40) for sensing the workload of the patient (44) and delivering to said detecting means (4;46,48;58) corresponding workload signals,, characterized by said repolarization sensing means (5,14;26,28;42) comprising time measuring means for determining the QT time interval between the onset of the QRS complex and a predetermined point on the T-wave, detected by said repolarization sensing means, said detecting means being arranged to detect as a state of ischemia the occurrence of a predetermined relation between said time interval and sensed workload. The ischemia detector according to claim 1, characterized in that said workload sensing means comprise an activity sensor (11,40) for sensing an activity parameter of the patient, like body movements, muscular sounds or pressure waves in body fluids due to the workload of the patient (44). The ischemia detector according to claim 1, characterized in that said workload sensing means comprise a sensor (11,40) for sensing metabolic changes, like changes in nutrition and oxygen consumption, of the patient (44). The ischemia detector according to any one of the preceding claims, characterized in that said predetermined point on the T-wave is selected on the rear slope of the T-wave. The ischemia detector according to any one of the preceding claims, characterized in that said detecting means (58) are disposed to detect as a state of ischemia a sensed workload which is below a predetermined workload threshold value (Ref 1) and a simultaneously determined QT time interval which is shorter than a predetermined QT time interval limit (Ref 2). The ischemia detector according to any of the claims 1 through 5, characterized in that said detecting means comprise averaging means (46) forming an average value of repolarization signals from a predetermined number of cardiac cycles to form an average repolarization signal, representing the repolarization, to be used for the detection of an ischemia. The ischemia detector according to any of the claims 1 through 6, characterized by alerting means (13) provided for activation in response to a detected ischemia. Implantable heart stimulator, having means (8) for varying the stimulation rate, characterized by an ischemia detector according to any of the claims 1 through 7. The heart stimulator according to claim 8, characterized in that control means (6) are connected to the ischemia detector for controlling the stimulation rate varying means (8) in response to the detection of an ischemia.

Inventors: EKWALL CHRISTER, NOREN KJELL, EKWALL, CHRISTER, NOREN, KJELL, Norén, Kjell
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1998
Main classification: A61B5
IPC codes: A61N1