There are various devices known in the art for monitoring heart function. Many of these devices typically function by analyzing signals such as an electrocardiogram (ECG) signal, which can be representative of heart function. Many methods have been developed to analyze a patient's ECG signal to diagnose whether the patient may be susceptible for sudden and life-threatening ventricular arrhythmias. Two general techniques developed to diagnose a patient for this susceptibility are the signal-averaged ECG method and the T-wave alternans method.
The Kroll and Kroll patents (U.S. Pat. Nos. 5,117,834 and 5,351,687) disclosed that a transthoracic application of subthreshold current across a patient's heart changed myocardial depolarization, that these changes were measurable in the patient's surface ECG, and that this method and apparatus would discern these changes by computing differences between vector magnitudes constructed from signal-averaged ECG signals comprising a plurality of current-injected cardiac cycles and undisturbed cardiac cycles. These methods and apparatus have been further developed in the Brewer-Taghizadeh patents (U.S. Pat. Nos. 5,555,888 and 5,694,943) and in the Hoium-Ryan-Malik patents (U.S. Pat. Nos. 5,951,484, 6,129,678, 6,445,947, 6,512,947, 6,625,483, and 6,850,795). These methods and apparatus have been shown to significantly improve patient diagnosis compared to conventional signal-averaged ECG methods.
The Cohen and Smith patent (U.S. Pat. No. 4,802,491) disclosed a passive method to detect subtle alternations in the morphologic features of the ECG, known as microvolt T-wave alternans (MTWA), to determine a patient's increased risk for life-threatening arrhythmias. Their method and apparatus was determined to provide a capability equivalent to general signal-averaged ECG methods. To generate a significant level of diagnostic alternans information, an important element to using the MTWA technology requires the measurements for MTWA to be drawn from a patient's recorded ECG signals derived during the course of an exercise stress procedure for the patient, and as such the MTWA technology may be limited in its application and utility because of the inability of many cardiac patients to participate in exercise induced stress or the risk that they will be harmed by it.
A recently published review (Haghjoo M, et al, Microvolt T-wave alternans: A review of techniques, interpretation, utility, clinical studies, and future perspectives, Internat Jour Cardiol 2006; 109; 293-306) carefully delineates the technical limitations to MTWA testing: (1) MTWA cannot be measured in patients with a trial fibrillation, a common arrhythmia in patients with structural heart disease, (2) the presence of frequent atrial and ventricular ectopy, excessive motion artifacts, and inability to reach a target heart rate render the results indeterminate, (3) the accuracy of MTWA testing is reduced in patients with prolonged QRS complex, (4) the spectral methods for MTWA do not perform well soon after an acute myocardial infarction, and (5) exercise MTWA testing may be impossible in subgroups of patients who are not able to perform bicycle or treadmill testing.
Further, as a clinical example regarding this appraisal, recent research has been published showing that there were a significant higher number of patients with sustained MTWA and a significantly lower number of patients with an indeterminate test when using simultaneous ventricular and atrial pacing in patients following myocardial infarction when compared to the same patients when using bicycle based exercise stress testing (Raatikainen M J P, et al, Microvolt T-wave alternans during exercise and pacing in patients with acute myocardial infarction, Pacing Clin Electrophysiol 2005; 28: S193-S197). Such clinical results continue to illustrate the truly demanding procedures required to support the application and accuracy of the MTWA as it is presently practiced.
Despite the need in the art for an ECG apparatus or methods which overcome the shortcomings and limitations of the prior art, none insofar as is known has been developed or proposed.