A system, comprising: a first sensor circuit, including a sense amplifier circuit to sense a cardiac electrical signal of a patient; a second sensor circuit, including a sense amplifier circuit to sense a heart sound signal of the patient; a memory circuit, a first signal metric generator circuit, coupled to the first sensor circuit and the memory circuit and configured to: detect from the cardiac electrical signal at least one signal component, generate a cardiac electrical signal metric using the at least one signal component, store the cardiac electrical signal metric in the memory circuit, wherein the cardiac electrical signal metric is indicative or correlative of atrial electrical activity, and wherein the first signal metric generator is configured to determine a cycle length variability over measurements of multiple cardiac cycles using the sensed cardiac electrical signal; a second signal metric generator circuit, coupled to the second signal sensor circuit and the memory circuit and configured to: generate a S4 heart sound metric using the sensed HS signal, store the S4 heart sound metric in the memory circuit and, wherein the S4 heart sound metric is indicative or correlative of atrial mechanical contraction; and characterized in further comprising: an atrial tachyarrhythmia detector circuit, communicatively coupled to the first and second signal metric generator circuits, configured to detect an AT event using the determined CLV and the generated S4 heart sound metric. a first sensor circuit, including a sense amplifier circuit to sense a cardiac electrical signal of a patient; a second sensor circuit, including a sense amplifier circuit to sense a heart sound signal of the patient; a memory circuit, a first signal metric generator circuit, coupled to the first sensor circuit and the memory circuit and configured to: detect from the cardiac electrical signal at least one signal component, generate a cardiac electrical signal metric using the at least one signal component, store the cardiac electrical signal metric in the memory circuit, wherein the cardiac electrical signal metric is indicative or correlative of atrial electrical activity, and wherein the first signal metric generator is configured to determine a cycle length variability over measurements of multiple cardiac cycles using the sensed cardiac electrical signal; a second signal metric generator circuit, coupled to the second signal sensor circuit and the memory circuit and configured to: generate a S4 heart sound metric using the sensed HS signal, store the S4 heart sound metric in the memory circuit and, wherein the S4 heart sound metric is indicative or correlative of atrial mechanical contraction; and characterized in further comprising: an atrial tachyarrhythmia detector circuit, communicatively coupled to the first and second signal metric generator circuits, configured to detect an AT event using the determined CLV and the generated S4 heart sound metric. detect from the cardiac electrical signal at least one signal component, generate a cardiac electrical signal metric using the at least one signal component, store the cardiac electrical signal metric in the memory circuit, wherein the cardiac electrical signal metric is indicative or correlative of atrial electrical activity, and wherein the first signal metric generator is configured to determine a cycle length variability over measurements of multiple cardiac cycles using the sensed cardiac electrical signal; a second signal metric generator circuit, coupled to the second signal sensor circuit and the memory circuit and configured to: generate a S4 heart sound metric using the sensed HS signal, store the S4 heart sound metric in the memory circuit and, wherein the S4 heart sound metric is indicative or correlative of atrial mechanical contraction; and characterized in further comprising: an atrial tachyarrhythmia detector circuit, communicatively coupled to the first and second signal metric generator circuits, configured to detect an AT event using the determined CLV and the generated S4 heart sound metric. generate a S4 heart sound metric using the sensed HS signal, store the S4 heart sound metric in the memory circuit and, wherein the S4 heart sound metric is indicative or correlative of atrial mechanical contraction; and characterized in further comprising: an atrial tachyarrhythmia detector circuit, communicatively coupled to the first and second signal metric generator circuits, configured to detect an AT event using the determined CLV and the generated S4 heart sound metric.