Abstract:
For automatic recognition of ventricular tachycardia and ventricular fibrillation, pulse sequences are compared which are obtained when sensing at at least one position on each ventricular epicardial surface of a heart. Changes in the sequence of activations and in the timing from pulsing at one sensor position to a next pulsing at that same position will indicate both ventricular tachycardia and ventricular fibrillation so as to enable a response to be made to restore a pulse sequence representing normal ventricular activity of the heart.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to apparatus for recognition of ventricular tachycardia and ventricular fibrillation from epicardial electrogram timings, and for termination thereof. 
     Ventricular fibrillation is defined as a condition characterized by fibrillary electrical activity of the ventricular muscle, the electrical impulses traversing the ventricles so rapidly that coordinated contractions cannot occur. This must be distinguished from ventricular tachycardia which may be defined as a rapid (greater than 100 beats per min.) cardiac rhythm originating in the ventricles. If sustained, it is usually synchronized in terms of overall ventricular contraction. Both should be differentiated from the normal situation of sinus rhythm where the heart&#39;s rhythm is controlled by depolarization originating from the sinus node and which spread sequentially through the atria, the AV node, the His-Purkinje system, and ventricular myocardium. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an apparatus for recognizing both ventricular tachycardia and ventricular fibrillation, and means for responding to both these conditions to restore normal heart rhythm. 
     According to the present invention, an apparatus is provided for the automatic recognition of ventricular tachycardia and ventricular fibrillation. At least two sensors are provided, one sensor being attached at least to each ventricular epicardial surface of a heart. Signal paths connect the sensors to programmed means for detecting a pulse sequence representing the ventricular electrical activity of the heart and for comparing the pulse sequence detected with that representing the electrical activity of the heart during normal ventricular rhythm of the heart. Means are provided for converting the detected pulse sequence into a form which will be useful for providing a corrective response to a pulse sequence representing the electrical activity of the heart during abnormal ventricular rhythm of the heart. The apparatus is generally used in association with means for supplying to the heart stimuli to restore normal rhythm to the heart following detection of abnormal ventricular rhythm. In such a case there may be no need for converting the detected pulse sequence into a readable form or other form, such as audible form, which permits identification of a pulse sequence representing electrical activity of the heart during abnormal ventricular rhythm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically an arrangement of four ventricular activation sites; 
     FIG. 2 shows the electrograms obtained from the four sites during normal sinus rhythm; 
     FIGS. 3A, B, and C show electrograms obtained under simulated ventricular tachycardia conditions; 
     FIG. 4 shows the electrograms obtained at the four sites under conditions of ventricular fibrillation; 
     FIG. 5 is a block diagram of a cardiac implant embodying this invention; and 
     FIG. 6 is a flow chart indicating the characteristic operation of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The feasibility of automatic recognition of ventricular tachycardia and ventricular fibrillation has been examined in a number of patients undergoing coronary artery surgery. Bipolar epicardial electrograms from four discrete points on the surface of the heart have been recorded during operation. The points are indicated on the ventricles of the heart. It has been observed that during normal rhythm, the points which are recorded are activated in a certain sequence which is at least consistent, although not always specific to that rhythm. Thus, referring to FIG. 1 of the accompanying drawings, the locations of four discrete points numbered A, B, C, and D on the left ventricle (LV) and right ventricle (RV) are shown, two of the points (A and D) being on the left ventricular and right ventricular apices, and points B and C being at left ventricular and right ventricular paraseptal positions. A pacing site is located on the right ventricle adjacent the third point. During normal rhythm, activation took place in the sequence C, D, B, A in this particular case (see FIG. 2). Furthermore, the timing from the first detected deflection to the last of the four was always the same during normal rhythm, and in this example, because of normal rhythm, the timing is short, and is of the order of 25 msec. 
     With abnormal rhythm, this timing will generally be increased and the sequence of activations changed. 
     This latter observation was established by simulation of an abnormal rhythm by pacing from the site on the right ventricle. It was observed that 8 out of a group of 10 patients paced at this particular site showed a change of sequence of activation compared with that seen during normal sinus rhythm. Recording of the sequences obtained showed that activations change from C, D, B, A to C, B, D, A. Another abnormality which was induced (because a normal conducting system was not used) was that the spread of activity took longer across the heart. Thus, the timing from the onset of depolarization detected first at site C and finally at site A took 85 msecs, as opposed to 25 msecs. FIGS. 3A, 3B, and 3C indicate that this duration and sequence of activation is not affected by the rate of the abnormal rhythm, provided that its site of origin remains constant. 
     While maintaining the same set of sites, further experimentation to induce ventricular fibrillation yielded further results of interest. Ventricular fibrillation was induced by putting AC current onto a heart under cardiopulmonary by-pass (this is a means of obtaining cardiac arrest and is often used during surgery). It was observed that during ventricular fibrillation, the electrical activity at all four sites was extremely rapid, and certainly more rapid than normally seen. However, there was no apparent fixed sequence of activation. The activity can therefore be described as asynchronous. Because of the asynchronous nature of activity, there can be no fixed duration of activity. Thus, this provides a means of using multi site testing to distinguish between ventricular tachycardia (where there is likely to be an altered sequence of depolarization compared with normal rhythm and an increased duration of activation over that occurring during normal sinus rhythm) and ventricular fibrillation when all this synchrony is lost and the electrical activity from different points in the heart becomes asynchronous. 
     The apparatus of the present invention is programmed to respond to ventricular tachycardia or ventricular fibrillation when they are observed based upon an altered sequence and duration of ventricular activation as detected by impulses sensed from the epicardial sensing sites. 
     The present invention is of particular value since ventricular fibrillation has so far been a very difficult rhythm to detect reliably in automatic fashion. Moreover, the energy required by an implantable device to treat ventricular fibrillation is likely to be higher than that required to treat ventricular tachycardia. Therefore, by the use of this technique, lower energies can be selected for termination of ventricular tachycardia, thus prolonging battery life. There is thus provided a reliable method for the first time of detecting ventricular fibrillation. The micro-computer utilized in the circuit for comparing the activation sequence with that during sinus rhythm can then control a defibrillator which can be discharged when a rhythm characteristic of ventricular fibrillation or ventricular tachycardia is detected. Appropriate software is provided for controlling the micro-computer. 
     FIGS. 5 and 6 show practical embodiments of the invention and should be viewed in conjunction with each other. Thus, an implant 1 (sensors are not shown in FIG. 5 but are preferably at positions such as shown in FIG. 1) will monitor heart beat rate at all times using a normal heart beat detector 2 having a time base and backup pacing control 4 whose operation is directed by a microprocessor 5 having a memory 6. Should a high heart rate be detected, then a detector 3 which is normally operating in backup mode is switched on and simultaneous multichannel sensing is carried out (FIG. 1 shows sensing at four sites being carried out). This number is adequate in general, and there is no reason why more or less than four sites may be used for testing, although the use of four sites has been found to be an optimum compromise between cost and sensitivity. The microcomputer 5 which is utilized with the detector and receives signals therefrom will check by means of memory 6 whether activation sequence and duration are compatible with sinus rhythm. If this is the case, then no action will be required. However, if the activation sequence and duration are not compatible with sinus rhythm, then provided that an activation sequence is synchronized indicating ventricular tachycardia, a response appropriate to treatment of ventricular tachycardia will be initiated, i.e. stimuli will be delivered by pulse generator 7. In certain cases which depend on the type of tachycardia, however, ventricular tachycardia may be located by a relatively low energy shock for an associated defibrillator 8. If the activation sequence is not synchronized, indicating that ventricular fibrillation is taking place, then operation of the defibrillator 8 will take place. 
     In the preferred embodiment of the invention and again referring to FIG. 5, the normal heartbeat detector employs a band pass filter which receives the heartbeat signal and connects it to a first input of a threshold comparator whose other input is connected to a reference. The detector 3 is a Tachy detector which is morphology sensitive. The time base and back-up pacer control 4 is the known control logic of the pace function and can be over-ridden by the microcomputer or microprocessor 5. The microprocessor is preferably formed of the Intel 8085 microprocessor chip and the memory 6 is a ROM memory, Intel 27C64. The defibrillator stage 8 generates stimulation pulses in known prior art fashion and such a typical stage is shown in U.S. Pat. No. 4,321,928. 
     Although various minor changes and modifications may be proposed by those skilled in the art, it will be understood that I wish to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within my contribution to the art.