Methods and apparatus for discriminating polymorphic tachyarrhythmias from monomorphic tachyarrhythmias facilitating detection of fibrillation

In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a system of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising:

• provisional declaring means for provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria;
• signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events;
• window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets;
• zero crossing point determining means for determining baseline zero crossing points from the sampled signal amplitudes in the baseline window and event zero crossing points from the sampled signal amplitudes in the sensed event window;
• slope determining means for determining the absolute signal slope at each zero crossing point in the baseline and sensed event windows;
• weighting means for weighting each zero crossing point by the determined absolute signal slope;
• summing means for summing the weighted zero crossing points in the baseline window to provide a baseline weighted zero crossing sum and the weighted zero crossing points in the sensed event window to provide a sensed event weighted zero crossing sum;
• means for subtracting the baseline weighted zero crossing sum from the sensed event weighted zero crossing sum to provide a weighted zero crossing sum metric related to the morphology of the cardiac signal;
• means for comparing the weighted zero crossing sum metric to a weighted zero crossing sum threshold;
• counting means for accumulating a count x of weighted zero crossing sum metrics that meet the weighted zero crossing threshold among y data sets as long as a polymorphic tachyarrhythmia is provisionally declared;
• withholding means for withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia;
• and
• final declaring means for making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared.

A method of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of:

• a) successively sampling, processing, and temporarily storing the cardiac signal to provide sampled signal amplitudes;
• b) determining a baseline window between sensed events and a sensed event window encompassing the sensed event;
• c) determining baseline zero crossing points from the sampled signal amplitudes in the baseline window and event zero crossing points from the sampled signal amplitudes in the sensed event window;
• d) determining the absolute signal slope at each zero crossing point;
• e) weighting each zero crossing point by the determined absolute signal slope;
• f) summing the weighted zero crossing points in the baseline window to provide a baseline weighted zero crossing sum;
• g) summing the weighted zero crossing points in the sensed event window to provide a sensed event weighted zero crossing sum;
• h) subtracting the baseline weighted zero crossing sum from the sensed event weighted zero crossing sum to provide a weighted zero crossing sum metric related to the morphology of the cardiac signal;
• i) comparing the weighted zero crossing sum metric to a weighted zero crossing sum threshold;
• j) accumulating a count x of weighted zero crossing sum metrics that meet the weighted zero crossing threshold over y repetitions of steps a) through i);
• and
• k) declaring a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia.

A system of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising:

• signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events;
• window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets;
• zero crossing point determining means for determining baseline zero crossing points from the sampled signal amplitudes in the baseline window and event zero crossing points from the sampled signal amplitudes in the sensed event window;
• slope determining means for determining the absolute signal slope at each zero crossing point in the baseline and sensed event windows;
• weighting means for weighting each zero crossing point by the determined absolute signal slope;
• summing means for summing the weighted zero crossing points in the baseline window to provide a baseline weighted zero crossing sum and the weighted zero crossing points in the sensed event window to provide a sensed event weighted zero crossing sum;
• means for subtracting the baseline weighted zero crossing sum from the sensed event weighted zero crossing sum to provide a weighted zero crossing sum metric related to the morphology of the cardiac signal;
• means for comparing the weighted zero crossing sum metric to a weighted zero crossing sum threshold;
• counting means for accumulating a count x of weighted zero crossing sum metrics that meet the weighted zero crossing threshold among y data sets;
• and
• means for declaring a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia.