Prior art cardiac monitoring devices such as cardioverter defibrillators, holter monitors, or ICU monitors often use the same criteria to detect and subclassify cardiac arrhythmias. Prior art devices commonly utilize a single criterion such as cycle length to detect tachyarrhythmias. These devices generally measure cardiac cycle length by measuring the time between the large electrical deflections produced when the ventricles depolarize. The electrical deflections are sensed when the signal amplitude exceeds the amplitude of a programmed threshold. Prior art devices detect tachyarrhythmias by determining when the cardiac cycle length, or time between consecutive ventricular contractions, falls below a programmed level. The programmed levels are typically as follows: a cycle length greater than 500 milliseconds (ms) is identified as normal, a cycle length between 500 and 333 ms is classified as monomorphic tachycardia, and a cycle length less than 333 ms is identified as polymorphic tachycardia.
Thus the effectiveness of prior art devices of this type depend on the accuracy of cycle length detection. There are two major disadvantages with these devices. First, these detectors sometimes miss low amplitude electrical activity, such as during ventricular fibrillation, which can cause the detector to miss dangerous polymorphic ventricular tachycardias. Second, cycle length is not an effective discriminator between monomorphic and polymorphic arrhythmias even when the threshold detectors sense the electrical events appropriately.
Other prior art devices have been developed to overcome these disadvantages by utilizing additional parameters which can be programmed by a physician. Examples of some detection parameters include cycle length cutoff for monomorphic ventricular tachycardias, cycle length cutoff for polymorphic ventricular tachycardias, cycle length regularity, and QRS width. The disadvantage with these programmable devices is that a large number of detection parameters must be programmed by a physician. Further, this programming process can be complex, time consuming and prone to physician error.
It is highly desirable to have a non-programmable device and method for detecting tachyarrhythmias, i.e., a device which does not require programming by a physician. Further, a device is needed for use with cardioverter defibrillators, or monitors which can more accurately classify and discriminate between normal, monomorphic and polymorphic arrhythmias. Still further yet, it is desirable to have a device which can accurately sub-classify and discriminate between different types of arrhythmias within the monomorphic arrhythmia class or the polymorphic arrhythmia class. Thus, it is highly desirable to have an improved device and method which eliminates the prior art problems of missing dangerous polymorphic ventricular tachycardias when there is low electrical activity, and discriminating between monomorphic and polymorphic arrhythmias.