An implantable medical device comprising: a plurality of electrodes comprising: a tissue-piercing electrode implantable from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to deliver cardiac therapy to or sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium of a patient's heart, and a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; a therapy delivery circuit operably coupled to the plurality of electrodes to deliver cardiac therapy to the patient's heart; a sensing circuit operably coupled to the plurality of electrodes to sense electrical activity of the patient's heart; and a controller comprising processing circuitry operably coupled to the therapy delivery circuit and the sensing circuit, the controller configured to: monitor electrical activity of the right atrium using the right atrial electrode; monitor electrical activity of the left ventricle using the tissue-piercing electrode implanted in the basal and/or septal region of the left ventricular myocardium of the patient's heart from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium; and deliver tachycardia therapy based on the monitored electrical activities of the right atrium and the left ventricle. a plurality of electrodes comprising: a tissue-piercing electrode implantable from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to deliver cardiac therapy to or sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium of a patient's heart, and a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; a tissue-piercing electrode implantable from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to deliver cardiac therapy to or sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium of a patient's heart, and a right atrial electrode positionable within the right atrium to deliver cardiac therapy to or sense electrical activity of the right atrium of the patient's heart; a therapy delivery circuit operably coupled to the plurality of electrodes to deliver cardiac therapy to the patient's heart; a sensing circuit operably coupled to the plurality of electrodes to sense electrical activity of the patient's heart; and a controller comprising processing circuitry operably coupled to the therapy delivery circuit and the sensing circuit, the controller configured to: monitor electrical activity of the right atrium using the right atrial electrode; monitor electrical activity of the left ventricle using the tissue-piercing electrode implanted in the basal and/or septal region of the left ventricular myocardium of the patient's heart from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium; and deliver tachycardia therapy based on the monitored electrical activities of the right atrium and the left ventricle. monitor electrical activity of the right atrium using the right atrial electrode; monitor electrical activity of the left ventricle using the tissue-piercing electrode implanted in the basal and/or septal region of the left ventricular myocardium of the patient's heart from the triangle of Koch region of the right atrium through the right atrial endocardium and central fibrous body to sense electrical activity of the left ventricle in the basal and/or septal region of the left ventricular myocardium; and deliver tachycardia therapy based on the monitored electrical activities of the right atrium and the left ventricle.