An embodiment of the invention provides an apparatus for the detection and treatment of atrial arrhythmia comprising an electrical lead having proximal and distal portions. The distal portion is positionable in an atrial chamber and the end of the proximal portion is configured to be coupled to a pacemaker. The lead comprises a plurality of conductive wires clad with an insulative coating and has sufficient flexibility to be positioned in the atria from a percutaneous introductory site. The conductive wires are coupled to a plurality of pairs of bipolar electrodes positioned on a membrane attachable to an endocardial wall. The electrode pairs are distributed in a pattern defining an area for detecting a location of a foci of aberrant electrical activity located within or adjacent the area and sending a pacing signal to that location to prevent or stop an occurrence of atrial fibrillation caused by that foci.