1.  Field of the Invention
The invention relates to medical devices for performing mapping and ablation procedures.  More particularly, the invention relates to methods and apparatus for mapping and ablating at or near, for example, the ostia of the pulmonary veins or coronary sinus. 
2.  Discussion of the Related Art
The human heart is a very complex organ, which relies on both muscle contraction and electrical impulses to function properly.  The electrical impulses travel through the heart walls, first through the atria and then the ventricles, causing the corresponding muscle tissue in the atria and ventricles to contract.  Thus, the atria contract first, followed by the ventricles.  This order is essential for proper functioning of the heart. 
Over time, the electrical impulses traveling through the heart can begin to travel in improper directions, thereby causing the heart chambers to contract at improper times.  Such a condition is generally termed a cardiac arrhythmia, and can take many different forms.  When the chambers contract at improper times, the amount of blood pumped by the heart decreases, which can result in premature death of the person. 
Techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and also to disable the short-circuit function of these areas.  According to these techniques, electrical energy is applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways or terminate the focal initiation.  The regions to be ablated are usually first determined by endocardial mapping techniques.  Mapping typically involves percutaneously introducing a catheter having one or more electrodes into the patient, passing the catheter through a blood vessel (e.g. the femoral vein or artery) and into an endocardial site (e.g., the atrium or ventricle of the heart), and deliberately inducing an arrhythmia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions.  When an arrythormogenic focus or inappropriate circuit is located, as indicated in the electrocardiogram recording, it is marked by various imaging or localization means so that cardiac arrhythmias emanating from that region can be blocked by ablating tissue.  An ablation catheter with one or more electrodes can then transmit electrical energy to the tissue adjacent the electrode to create a lesion in the tissue.  One or more suitably positioned lesions will typically create a region of necrotic tissue which serves to disable the propagation of the errant impulse caused by the arrythromogenic focus.  Ablation is carried out by applying energy to the catheter electrodes.  The ablation energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. 
Atrial fibrillation together with atrial flutter are the most common sustained arrhythmias found in clinical practice. 
Another source of arrhythmias may be from reentrant circuits in the myocardium itself.  Such circuits may not necessarily be associated with vessel ostia, but may be interrupted by means of ablating tissue either within the circuit or circumscribing the region of the circuit.  It should be noted that a complete ‘fence’ around a circuit or tissue region is not always required in order to block the propagation of the arrhythmia; in many cases simply increasing the propagation path length for a signal may be sufficient.  Conventional means for establishing such lesion ‘fences’ include a multiplicity of point-by-point lesions, dragging a single electrode across tissue while delivering energy, or creating an enormous lesion intended to inactivate a substantive volume of myocardial tissue. 
Commonly-owned U.S. patent application Ser.  No.  09/396,502, entitled Apparatus For Creating A Continuous Annular Lesion, which is hereby incorporated by reference, discloses a medical device which is capable of ablating a continuous ring of tissue around the ostia of either veins or arteries leading to or from the atria.