An introduction assembly and method for the insertion of medical instruments through a thoracic passage into a selected one of either the left or right atrium of the heart. Catheters or other instruments dedicated to performing required cardiac maneuvers are passed through an introductory sheath having a distal end disposed within the targeted atrium. Upon completion of the required cardiac maneuvers the instruments are removed from the atrium and a closure assembly is passed through the introductory sheath into a closing relation to an entry site, formed in the pericardium and corresponding atrium wall, to facilitate healing thereof. The introductory assembly and method facilitates the concurrent, operative disposition of a plurality of catheters or other instruments into the interior of the selected atrium through different thoracic passages and entry sites thereby allowing synergistic interaction between the multiple catheters in the performance of the required cardiac maneuvers.
