A single-chamber implantable device for detecting a patient's atrial activity using a monobody lead is disclosed. The monobody lead (10) includes a ventricular coil (16), a supraventricular coil (18), a distal electrode (14) forming three electrodes for detecting depolarization signals. A generator (12) of the implantable device collects a first unipolar signal (20) between the ventricular coil and the generator housing and a second unipolar signal (22) between the supraventricular coil and the generator housing. An independent component analysis is performed to the detected depolarization signals to determine an estimated atrial activity signal from the first and second unipolar signals.