1. Field of Inventions
The present invention relates generally to medical devices that support one or more therapeutic elements in contact with body tissue.
2. Description of the Related Art
There are many instances where therapeutic elements must be inserted into the body. For example, therapeutic elements may be used to form lesions to treat conditions in the heart, prostate, liver, brain, gall bladder, uterus, breasts, lungs and other solid organs. The application of electromagnetic radio frequency (“RF”) energy to heat and eventually kill (i.e. “ablate”) tissue is one method of forming a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. “Tissue coagulation” is the process of cross-linking proteins in tissue to cause the tissue to jell. In soft tissue, it is the fluid within the tissue cell membranes that jells to kill the cells, thereby killing the tissue. The tissue coagulation energy is typically supplied and controlled by an electrosurgical unit (“ESU”) during the therapeutic procedure. More specifically, after an electrophysiology or electrosurgical device has been connected to the ESU, and one or more electrodes or other energy transmission elements on the device have been positioned adjacent to the target tissue, energy from the ESU is transmitted through the electrodes to the tissue to from a lesion. The amount of power required to coagulate tissue ranges from 5 to 150 W. The energy may be returned by an electrode carried by the therapeutic device, or by an indifferent electrode such as a patch electrode that is secured to the patient's skin.
With respect to the formation of therapeutic lesions in the heart to treat cardiac conditions such as atrial fibrillation, atrial flutter and arrhythmia, some procedures form lesions on the endocardium in order to create a maze for electrical conduction similar to that created by surgical maze procedures. The lesions are carefully placed to interrupt the conduction routes of the most common reentry circuits.
Lesions may be formed by ablating tissue with an electrode that is carried by a probe, such as a catheter or surgical probe. Catheters typically include a relatively long and relatively flexible shaft that carries a distal tip electrode and, in some instances, one or more additional electrodes near the distal end of the catheter. The proximal end of the catheter shaft is connected to a handle which may or may not include steering controls for manipulating the distal portion of the catheter shaft. The length and flexibility of the catheter shaft allow the catheter to be inserted into a main vein or artery (typically the femoral artery), directed into the interior of the heart where the electrodes contact the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter. Exemplary catheters are disclosed in U.S. Pat. Nos. 6,013,052, 6,203,525, 6,214,002 and 6,241,754.
Surgical soft tissue coagulation probes (or “surgical probes”) carry one or more electrodes on relatively short, stiff shafts. These probes may be used in endocardial and epicardial procedures where access to the heart is obtained by way of a thoracostomy, thoracotomy or median sternotomy. Such probes also allow endocardial lesions to be formed as a secondary procedure during a primary open heart surgical procedure such as mitral valve replacement, aortic valve replacement, and coronary artery bypass grafting. Exemplary surgical probes are disclosed in U.S. Pat. Nos. 6,142,994, 6,468,272 and 6,645,200.
The present inventors have determined that proper electrode/tissue contact is important issue, for reasons of efficiency and safety, in both catheter-based and surgical procedures. Poor electrode contact with the target tissue increases the amount of coagulation energy that is transmitted into the surrounding tissue and blood. More specifically, the amount of coagulation energy transmitted to surrounding tissue and blood increases as the proximity to the target tissue decreases. With respect to efficiency, the reduction in the amount of energy that is transmitted to the target tissue reduces the likelihood that a transmural, or otherwise therapeutic, lesion will be formed. Poor electrode/tissue contact can also increase the amount of time that it takes to complete the procedure. Turning to safety, transmission of excessive amounts of energy into the surrounding tissue can result in the formation of lesions in non-target tissue which, in the exemplary context of the treatment of cardiac conditions, can impair heart function. The transmission of excessive amounts of energy into the blood can result in the formation of coagulum and emboli. It also increases the amount of energy that is returned by the patch electrode, which can result in skin burns.