Abstract:
An ischemia detector has a patient workload sensor and a patient breathing sensor which emits a signal representing sensed workload, and a patient breathing sensor which emits a signal representing sensed breathing activity of a patient. These signals are supplied to a detector unit which identifies a state of ischemia upon an occurrence of a predetermined relation between the sensed workload and the sensed breathing activity. This predetermined relation is a sensed low workload and a simultaneously sensed high breathing activity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ischemia detector and to an implantable heart stimulator having such an ischemia detector. 
     2. Description of the Prior Art 
     The blood flow and penetration in the circulatory system of a living subject are dependent on the arterial muscular tension, the so called tonus. Thus the blood flow is controlled by the tonus, the driving force of the flow is the blood-pressure in the elastic aorta and the pressure in the aorta is maintained by the pumping action of the heart. For this pumping action the heart needs energy in the form of oxygen and glucose. About 60% of the oxygen in the heart interstitial fluid is consumed within one heart beat. If the energy supplied to the heart is disturbed the heart contractibility and the pumping action of the heart are severely deteriorated and an oxygen shortage or ischemic situation will rapidly develop. Ischemia results from insufficient blood flow through the heart muscle. Due to blocking or passage congestion of coronary blood vessels of the heart. I ischemia is experienced by the patient as a severe chest pain and is one of the most stressing factors known to the organism. Several techniques for detecting ischemia are known. In U.S. Pat. No. 5,156,148 a system for treating a malfunctioning heart, e.g. ischemia, is known using the variation of selected physiologic parameters. In U.S. Pat. No. 4,821,735 a method and an apparatus for detecting myocardial ischemia are described, wherein the systemic vascular resistance (SVR) in a subject is monitored and the presence of myocardial ischemia is detected when the SVR increases by at least 60% over a base line value. 
     In U.S. Pat. No. 5,497,780 an apparatus is described for determining an ischemia by measurements of electric potentials between at least three implanted measuring electrodes, two of these electrodes being implanted with their poles in the heart and the third electrode being implanted with its pole lying outside the heart. 
     In U.S. Pat. No. 5,199,428 a technique is described for detecting ischemia and effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart&#39;s oxygen requirements while providing pacing therapies to maintain the patient&#39;s heart rate within acceptable limits to avoid bradyarrhythmias and/or unphysiological AV delays induced by the nerve stimulation. The ischemia detection is based on the occurrence of changes in the ST-segment variation different from pre-determined 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&#39;s heart. 
     An ischemic state can also be detected by analysis of recorded IECG&#39;s or surface ECG&#39;s to determine the heart rate variability. 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 ischemic state, the accompanying change in the moving pattern of the heart wall can be detected in this way. Also sound absorption is affected 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. Ischemia deteriorates the efficiency of the heart&#39;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 heartbeat to the difference obtained from the next heartbeat an ischemic can be detected. An ischemic state can be detected using a flow sensor for measuring cardiac output, as well. 
     As mentioned above an ischemic state is normally associated with severe pain forcing the patient to sit down or lie down with a reduced heart rate as a consequence. At the same time the patient feels a need for forced breathing, so called hyperventilation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ischemia detector the functioning of which is based on the above mentioned needs of a patient experiencing ischemia. 
     The above object is achieved in accordance with the principles of the present invention in an ischemia detector having a patient workload sensor and a patient breathing sensor, which respectively emit signals which are supplied to a detector unit, the detector unit detecting a state of ischemia dependent on the occurrence of a predetermined relation between the sensed workload and the sensed breathing activity, this predetermined relation being a sensed low workload and a simultaneously sensed high breathing activity. 
     For a healthy person the need of an increased respiration rate normally results from an increased effort or workload of the person. The ischemia detector according to the invention is based on the occurrence of the abnormal combination of low workload and high breathing activity, which is typical of ischemic patients. 
     In an advantageous embodiment of the ischemia detector according to the invention an averaging unit is connected to the workload sensor and breathing sensor which forms average values during time periods of pre-determined lengths of workload and breathing activity respectively, and delivers corresponding average signals to said detector unit. In this way false detections due to accidental variations of transitory nature in the workload and breathing activity are avoided. 
     In another embodiment of the detector according to the invention an alerting unit is activated in response to a detected ischemia. 
     According to another aspect of the invention an implantable heart stimulator has means for varying the stimulation rate and an ischemia detector as defined above. 
     The heart stimulator can have a control unit connected to the ischemia detector for controlling the stimulation rate varying means to lower the stimulation rate in response to the detection of an ischemia. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a generalized block diagram of an ischemia detector according to the invention. 
     FIG. 2 is a block diagram illustrating details of an embodiment of the ischemic detector according to the invention. 
     FIG. 3 is a simplified block diagram of one embodiment of the heart stimulator according to the invention; and 
     FIG. 4 shows a pacemaker as shown in FIG. 3 with its lead implanted in the right ventricle of the heart. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a workload sensor  40  and a breathing activity sensor  42  disposed for sensing the workload and the breathing activity respectively of a patient  44  and delivering corresponding signals to an averaging unit  46 , in which average values during time periods of a predetermined length are formed of the workload and breathing activity signals. These average signal values are supplied to detecting unit  48 . 
     The detecting unit  48  contains a memory  50 , in which one or more relations between workload and breathing activity are stored, and a comparator  52 , in which the relation obtained between the signals from the averaging unit  46  representing workload and breathing activity is compared to the predetermined relations stored in the memory  50 . 
     When a predetermined relation between the signals from the averaging unit  46  is detected alerting means  54  connected to the comparing means is triggered to indicate the occurrence of ischemia. A heart stimulator  56  is connected to the alerting unit  54  for lowering the stimulation rate in response to the detection of an ischemia, as will be described more in detail below. 
     An example of the predetermined relation between workload x and breathing activity y stored in the memory  50  is a linear relationship like 
     
       
         
           ax+by=c 
         
       
     
     where a, b, and c are constants. However, different kinds of non-linear relations are common too. 
     An alternative embodiment of the detecting unit is shown in FIG.  2 . In this embodiment the detecting unit  58  contains two comparators  60 ,  62  to which the workload signal and the breathing activity signal respectively are supplied for comparing the signals with predetermined threshold values Ref  1  and Ref  2 . The outputs of the comparators  60 ,  62  are connected to the inputs of an AND-gate  64 . 
     The comparator  60  is arranged to deliver an output signal when the workload signal is below the predetermined workload threshold value Ref  1  and comparator  62  delivers an output signal if the breathing activity signal is above the predetermined breathing activity threshold value Ref  2  and in this case an output signal is obtained from the AND-gate  64  for e.g. activation of ischemia alerting means. 
     FIG. 3 is a simplified block diagram of an implantable heart stimulator  2  according to the invention. The heart stimulator  2  contains an ischemia detector having an ischemia detecting device  4 , and a control unit  6 , connected to the ischemia detecting device  4 . The control unit  6  is connected to a pulse generator  8  for controlling the rate of generated stimulation pulses. The pulse generator 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 a patient, preferably with the electrodes in the right ventricle, cf. FIG. 4. A sensor  14  is also provided at the distal end portion of the lead  10  and sensed signals are supplied to the ischemia detecting device  4  through the lead  10 . 
     A workload sensor  11  in the form of e.g. an accelerometer for sensing body movements of the patient or a sensor for sensing muscular sounds of the patient is also provided in the heart stimulator  2 . For detecting muscular sounds the stimulator case can be used as a microphone and the associated electronics for recording the oscillations of the wall of the case can be glued onto the inner side of the case wall. 
     The sensor  14  can be used for recording IECGs and has electrodes as described in connection with FIG.  4 . The signals are supplied by the lead  10  to an IECG recording unit  5 . The IECG recording unit  5  comprises a sensor for sensing the IECG baseline offset and determining the breathing activity from this baseline offset. The baseline offset is preferably measured on a small DC bias voltage. An output signal from the IECG recording unit  5  is delivered to the ischemia detecting device  4 . 
     The DC bias voltage is supplied during a fraction of the breathing cycle and at certain defined positions of the cardiac cycle. 
     The heart stimulator  2  is also provided with alerting unit  13 , e.g. of a wrist watches “beeper-type”. 
     These alerting unit  13  is connected to the ischemia detecting means  4  to be activated by a detected ischemia. Alternatively the alerting means can be connected to the control unit  6  to be activated when the stimulation rate is lowered. This is of value for patients having a “silent” ischemia, the occurrence of which the patient would otherwise not be aware of. 
     FIG. 4 shows an implanted heart stimulator in the form of a pacemaker  16 , connected to the right ventricle  18  on the heart of a patient by its lead  20 , which is of a bipolar type with an electrode ring  22  and with a tip electrode  24  and sensors  26 ,  28 . 
     With the heart stimulator according to the invention the stimulation rate is reduced in response to the detection of an ischemia. There are different possibilities of reducing the stimulation rate. The control unit  6  can inhibit the delivery of a particular stimulation pulse thus temporarily producing a longer interval between two consecutive pulses. The control unit  6  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. The control unit  6  can also 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. 
     The breathing activity can be determined by measuring the AC impedance between the two electrodes  22 ,  24  of the electrode lead  20  or between one of the electrodes  22 ,  24  and the case of the pacemaker  16 . 
     The electrodes  22 ,  24  or the sensors  26 ,  28  can be used for measuring amplitude modulation of sensed cardiac activity for determining the breathing activity from this measured modulation. 
     Other possibilities of determining the breathing activity of the patient is by using a sensor for sensing breathing sounds in the thorax of the patient or a sensor for sensing lung volume changes. 
     Also other kinds of workload sensor can be used in the present invention. Thus the workload sensor can be a sensor for sensing pressure waves in body fluids generated by the workload or activity of the patient. The workload sensor alternatively can be a sensor for sensing metabolic changes, like changes in nutrition and oxygen consumption of the patient. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intension of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.