An implantable dual chamber ICD and method senses atrial (A-A) and ventricular (V-V) intervals, calculates the ratio (A/V interval ratio) of A-A intervals to V-V intervals, classifies heart rhythms with V-V intervals (i) above a revert rate threshold as not tachycardia, (ii) below a high rate threshold as treatable tachycardia, (iii) between the two thresholds and below an A-dominant A/V interval ratio threshold as a non-treatable tachycardia, (iv) between the revert rate threshold and the high rate threshold and above a V-dominant threshold as treatable tachycardia and, when the heart rhythm is intermediate all four thresholds, (v) utilizes one or a combination of advanced discriminators (e.g., ventricular windowing, ventricular interval variability, sudden onset, AV-delay creep, ventricular electrogram morphology, active sensing, minute ventilation, or right ventricular pressure) to classify whether the heart rhythm is a non-treatable or a treatable tachycardia.
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