Abstract:
A method and apparatus for limiting the application of ablation energy to viable myocardial tissue. An ablation energy generator produces a test signal that is applied to the heart muscle. The response of the heart muscle to the test signal is determined and used to analyze the viability of the heart tissue. If the heart muscle is viable, a higher powered ablation pulse may be delivered. If the tissue is not viable, no ablation energy is delivered. In one embodiment of the invention, the test signal is a low voltage, high frequency signal and the impedance of the tissue in response to the test signal is detected to determine tissue viability.

Description:
FIELD OF THE INVENTION 
     The present invention relates to medical devices in general, and in particular to systems for performing myocardial revascularization. 
     BACKGROUND OF THE INVENTION 
     Myocardial revascularization is a surgical technique whereby small holes or craters are created in the myocardium to allow oxygenated blood within the ventricle to contact the myocardium. One technique, called transmyocardial revascularization (TMR), involves routing an energy delivery catheter through an opening in the chest wall to the exterior of the patient&#39;s heart muscle. Ablation energy is then delivered to the catheter to create a hole that extends from the epicardium, or exterior of the patient&#39;s heart, to the interior of the ventricle such that oxygenated blood flows into and out of the holes. These holes rapidly seal at the outside of the heart but remain open towards the interior of the ventricle. Another technique, called percutaneous myocardial revascularization (PMR), utilizes an energy delivery catheter that is routed through a patient&#39;s vasculature to the interior wall of the left ventricle. Ablation energy is then supplied to the ventricular wall to remove a portion of the endocardium and expose a portion of the myocardium to oxygenated blood flow. 
     Myocardial revascularization is most often used when an area of the myocardium is not receiving adequate blood flow because of clots or diseases that inhibit the ability of the vessels to supply blood to the heart. It is not known whether the procedure induces new blood vessels to form in the myocardium or simply deadens nerve endings in the myocardium to alleviate patient discomfort. 
     Not all cardiac tissue can be helped using myocardial revascularization. For example, if the myocardial tissue has been deprived of oxygenated blood for too long, it may be dead and no benefits to the tissue will be obtained if treated. Applying ablation energy to such myocardial tissue is not only a waste of time, but the tissue may be more susceptible to ventricular perforation. To increase the efficiency and efficacy of a myocardial revascularization procedure, there is a need for a system that can guide a physician to only perform a myocardial revascularization procedure in viable cardiac tissue. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for limiting the delivery of ablation energy to viable areas of the myocardium. An energy delivery catheter is routed to the patient&#39;s myocardium. A determination is made if the myocardium adjacent to the distal end of the catheter is viable and, if so, ablation energy is delivered through the catheter to the myocardial wall. If the tissue adjacent the distal end of the energy delivery catheter is not viable, then no ablation energy is delivered. 
     In one embodiment of the invention, tissue viability is determined by applying a test signal to the tissue and measuring the impedance of the tissue in response to the test signal applied. If the impedance is greater than a predefined amount, the tissue is deemed not to be viable. Therefore, no ablation energy will be delivered. 
     The energy delivery catheter may be routed through the patient&#39;s vasculature or through an opening in the patient&#39;s chest wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates a system for performing myocardial revascularization according to the present invention; 
     FIG. 2 illustrates a pair of catheters that are used to position an energy delivery catheter at a desired location in the patient&#39;s heart; 
     FIG. 3 illustrates in further detail the energy delivery catheter used in one embodiment of the present invention to deliver the ablation energy; and 
     FIG. 4 is a flow chart of the steps performed by one embodiment of the present invention to limit the application of ablation energy to viable myocardial tissue. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the major components of the presently preferred system for performing PMR according to the present invention. A steerable catheter  10  is routed through a patient&#39;s vasculature and into the left ventricle of the heart  12 . In order to route the steerable catheter to the left ventricle, an incision is made into a patient&#39;s femoral artery and an introducer sheath approximately 12″ long (not shown) is introduced into the wound. Next, the steerable catheter  10  is advanced along the vasculature until it nears the patient&#39;s heart valve. A “pig tail” catheter (also not shown) is inserted into the steerable catheter  10  in order to push past a valve at the entrance of the left ventricle. The pig tail catheter is removed and an inner catheter (described below) including an energy delivery catheter  14  is advanced along the steerable catheter  10  into the left ventricle. 
     The energy delivery catheter  14  delivers ablation energy produced by an ablation energy source  16 . In the currently preferred embodiment, the ablation energy source  16  is a radio frequency (RF) voltage generator that is controlled to selectively supply RF electrical energy to the energy delivery catheter  14 . When the distal end of the energy delivery catheter  14  is adjacent to, or in contact with, an ischemic region  18  of the left ventricle, a physician triggers the RF voltage generator to supply a 400 millisecond radio frequency pulse to the energy delivery catheter  14 . The ablation energy is delivered to the interior of the heart muscle to ablate or remove a portion of the endocardial, or inner lining of the heart, thereby creating regions or craters  20  where the myocardium is exposed. The exposed myocardium is then in contact with the oxygenated blood that is flowing within the left ventricle. The delivery of the RF pulses can take place independently of the cardiac cycle. 
     In the presently preferred embodiment PMR device, the energy delivery catheter  14  is a unipolar device including a single electrode. A return electrode  21  is positioned on the exterior of the patient to provide a current path back to the RF voltage generator. 
     FIG. 2 illustrates the presently preferred steerable catheter  10  that routes the energy delivery catheter  14  into the patient&#39;s heart muscle. The steerable catheter  10  comprises an outer catheter  30  and inner catheter  32  that are relatively flexible at their distal ends. The outer catheter  30  has a predefined “J-shaped” bend  34  at its distal end, and the inner catheter  32  has a predefined “J-shaped” bend  36  at its distal end. The radius of the bend  36  is smaller than that of the bend  34 . Each of the catheters  30 ,  32  has a connector at its proximal end with a pair of opposed “wings” or tabs on it. The diameter of the inner catheter  32  is selected such that it can be threaded into a lumen that extends along the length of the outer catheter  30 . When the inner catheter  32  is inserted into the outer catheter  30 , the tabs on the proximal ends of the catheters allow the inner catheter  32  to be rotated with respect to the outer catheter  30 . The predefined bends  34  and  36  cooperate to vary the orientation of a distal tip  38  of the inner catheter  32 . The bends  34  and  36  may be aligned so they both bend in the same direction, in opposite directions, or at any position in between. 
     As illustrated in FIG. 3 above, the energy delivery catheter  14  houses a flexible electrode  35 . The electrode  35  is threaded through two holes of a ceramic cap  37  at the distal end of the catheter  14 . Specifically, the electrode  35  exits a first hole  37   a  in the distal direction and then is routed proximally through a second hole  37   b  in the ceramic cap  37  such that a portion of the electrode  35  is exposed at the distal end of the catheter. The distal end of the electrode  35  does not extend all the way back along the length of the catheter  14  but terminates at a point generally near the distal end of the energy delivery catheter  14 . The ceramic cap  37  may include a pair of additional holes  37   c  and  37   d , that allow fluids such as dyes or drugs to be supplied through a lumen in the energy delivery catheter  14  and delivered to the ablation site. Finally, the energy delivery catheter  14  may include a radiopaque marker band  39  that surrounds the ceramic cap  37  in order to enhance the visibility of the energy delivery catheter  14  under fluoroscopy or other imaging techniques, as the PMR procedure is being performed. 
     As indicated above, to prohibit the application of ablation energy to myocardial tissue that would not benefit from the procedure, the present invention determines the viability of such tissue prior to the delivery of ablation energy. FIG. 4 illustrates a series of steps performed by the present invention to ensure that ablation energy is not applied to non-viable cardiac tissue. Beginning with a step  50 , a physician positions the energy delivery catheter at the desired location on or inside the heart. As indicated above, the catheter may be placed either against the endocardial or epicardial layer of the heart muscle. At a step  52 , a test signal is delivered to the heart muscle and the heart&#39;s response to the test signal is measured at a step  54 . 
     From the results of the test signal, a decision is made at step  56  to determine whether the myocardium in the area adjacent the energy delivery catheter is viable. If the tissue is viable, the ablation energy is delivered at a step  58 . If the tissue is not viable, the physician is prevented from delivering ablation energy to that spot on the ventricle. The physician then moves the probe and processing returns to step  50  as described above. 
     In the presently preferred embodiment of the invention, the test signal delivered at step  52  is a low energy, high frequency RF energy pulse. Preferably, the signal has a frequency greater than 50 kHz in order to avoid fibrillating the heart. The impedance of the heart muscle in response to the test signal delivered is the presently preferred criteria by which viability of the heart is determined at step  54 . 
     If the impedance is greater than a predefined level, such as 1700 ohms, it is assumed that the heart muscle is dead or would otherwise not respond to the myocardial revascularization treatment, and no ablation energy is delivered at that point. In order to measure impedance, the ablation energy source  16  shown in FIG. 1 includes a circuit that determines the magnitude of the current received in response to the low power, RF test signal applied. Based on the magnitude of the current sensed, a switch or other control within the ablation energy source is inhibited from delivering an RF pulse to the cardiac tissue at that position. In the presently preferred embodiment of the invention, the test signal is delivered when the physician activates a foot pedal or other control to initiate the delivery of ablation energy. The test signal is delivered first and if the impedance indicates the tissue is viable, the ablation energy pulse follows immediately or very shortly thereafter. 
     Although the present invention uses a low energy, high frequency test signal in order to measure the impedance of the cardiac tissue, it will be appreciated that other criteria could be used to determine tissue viability. For example, the catheter may include an electrode or other sensor to determine if the cardiac tissue in the area of the electrode is responding to the heart&#39;s own pacing signals. If no tissue response is observed, no ablation energy will be applied to that portion of the heart muscle. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the invention. It is therefore intended that the scope of the invention be determined from the following claims and equivalents thereto.