Long-term electrocardiographic and physiological monitoring over a period lasting up to several years in duration can be provided through a continuously-recording subcutaneous insertable cardiac monitor (ICM). The sensing circuitry and the physical layout of the electrodes are specifically optimized to capture electrical signals from the propagation of low amplitude, relatively low frequency content cardiac action potentials, particularly the P-waves that are generated during atrial activation. The ICM is intended to be implanted centrally and positioned axially and slightly to either the left or right of the sternal midline in the parasternal region of the chest, with at least one of the ECG sensing electrodes of the ICM being disposed for being positioned in a region overlying the sternum or adjacent to the sternum and the other of the electrodes also being disposed for being positioned over the sternum or adjacent to the sternum of on the patient's chest.