Surgery – Instruments – Electrical application
Reexamination Certificate
1999-01-20
2001-04-17
Cohen, Lee (Department: 3739)
Surgery
Instruments
Electrical application
C606S034000, C607S099000, C607S105000, C607S113000
Reexamination Certificate
active
06217574
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electrophysiology catheter system having a split tip electrode catheter and a signal processing system for providing a safe and effective RF ablation of the heart tissue.
BACKGROUND OF THE INVENTION
The heart has a natural pacemaker and conduction system which causes the heart muscle to contract, or beat, in an orderly rhythmical manner. The normal pacing rate for an adult at rest is about 60 to 70 beats per minute. There are many physiologic abnormalities which cause one or more chambers of the heart to beat more rapidly (tachycardia or flutter) or chaotically (fibrillation). A patient cannot live with ventricular fibrillation because there would be no blood pumped through the arteries, but may live with atrial fibrillation so long as the chaotic impulses are filtered out at the AV node and do not reach the ventricals. A patient may also live with atrial flutter and various forms of tachycardia but quality of life may be considerably compromised.
Many of these arrhythmias can be treated effectively by ablation using radio-frequency (RF) energy. Other arrhythmias are less effectively treated, requiring more RF lesions for a successful outcome or resulting in no successful outcome. RF ablation is performed with a catheter having one or more electrodes which deliver the RF energy to the cardiac tissue. In operation the catheter is guided through a vein or artery into the heart chamber and positioned at one or more sites, determined by an electrophysiologist, to correct the arrhythmia. The catheter delivers energy from an external source (generator) to the tissue, generating sufficient heat to kill the tissue, which is thereafter replaced by scar tissue. In a successful ablation procedure, the lesions formed interrupt the electrical pathways that cause the arrhythmia so that heart rhythm is improved or returns to normal.
During ablation, it is important to control the temperature of the tissue, both at the tissue surface and below. If the surface temperature becomes excessive, dehydration and charring. results. Charred tissue presents a dangerous situation as it may flake off resulting in blockage of a blood vessel. Excessive heating below the surface is also dangerous as it may result in a “steam pop.” A “steam pop” occurs when deep tissue is heated to a temperature sufficient to boil the water of the tissue which creates a steam bubble within the tissue. Such a steam bubble erupts through the surface of the myocardium with substantial force. This eruption can typically be heard as a “pop” by the electrocardiologist.
SUMMARY OF THE INVENTION
The present invention provides an improved system and method for ablating myocardial tissue. The system comprises an electrophysiology catheter having a split tip electrode at its distal end. The split tip electrode preferably comprises a tip electrode assembly having two or more orthogonally arranged electrode members. In a preferred embodiment, there are five electrode members which comprise the split tip electrode. The electrode members are arranged so that four of the members form sectors of a hemisphere and the fifth forms a ring behind the four member hemisphere. The split tip electrode catheter preferably comprises means for passing a fluid, e.g., saline, through each of the electrode members for cooling the electrode members during ablation. A particularly preferred irrigated split tip electrode is disclosed in patent application entitled IRRIGATED SPLIT TIP ELECTRODE CATHETER to Webster, Jr. (U.S. application No. 09/205,116) which is filed concurrently herewith and incorporated herein by reference.
The system further comprises means, electrically connected to each of the electrode members of the split tip electrode, for receiving electrical signals from each of the electrode members and for generating a record and/or display indicative of those signals, preferably an electrogram.
Also electrically connected to each of the electrode members of the split tip electrode is a means for measuring the electrical impedance between each electrode member and a reference electrode to determine which of the electrode members are in contact with the myocardium. The impedance measuring means comprises an RF generator for generating a low level RF electrical signal, preferably about 2 microamperes at a frequency of about 50 KHz, means for delivering the low level RF current to each of the electrode members, at least one reference or indifferent electrode, e.g., a skin patch electrode, and preferably means for generating a record and/or display of the impedance between each electrode member and the reference electrode(s).
Means are also provided for delivering an ablating RF current to one or more of the electrode members of the split tip electrode for ablating myocardial tissue in contact with those electrode members. Such means comprises an RF generator for generating RF current sufficiently strong to ablate heart tissue. Preferred ablating currents are from about 0.25 to about 1.0 amperes at about 400 KHz to about 700 KHz, more preferably about 0.5 to 0.75 amperes at 500 KHz. A signal processor is provided for activating the RF generator to transmit the low level RF current to each of the electrode members of the split tip electrode and for comparing the impedances associated with each electrode member of the split tip electrode to determine which electrode member(s) is (are) associated with the highest impedance. This allows the identification of the electrode members in contact with the myocardium as those electrode members will be associated with a higher impedance than those electrode members in contact only with the blood pool. The signal processor also activates the RF generator to selectively transmit RF ablation current to only those electrode members in contact with the myocardium.
It is preferred that the system comprise an irrigated split tip electrode catheter and a metering pump for pumping a cooling fluid through the catheter to cool the ablating electrode(s) during ablation. In such an embodiment, it is also preferred that the signal processor be capable of activating the pump and the RF generator intermittently so that there are periods of no irrigation and RF ablation energy between periods wherein RF ablation energy and cooling fluid are delivered to the ablation electrodes.
In a preferred embodiment of the invention, the ablation system further comprises an irrigated split tip electrode catheter and means for monitoring the surface and/or subsurface and temperatures of the tissue being ablated. Preferred surface temperature monitoring means comprises a thermocouple or thermistor coupled to each electrode member of the split tip electrode for generating an electrical signal indicative of the temperature of the electrode members in contact with the myocardium and signal processor intermittently activating and deactivating the RF ablation current generator and the irrigation pump. During the deactivated periods, the electrode temperature reaches the approximate surface temperature of the tissue being ablated and is therefore an estimate of the tissue temperature at the tissue-electrode interface. Preferred sub-surface temperature monitoring means comprises impedance monitoring means for monitoring the impedance associated with the electrode members of the split tip electrode, and estimating the maximum deep tissue temperature from the impedance measurements. Preferably, means are also provided for automatically reducing the amount of RF current delivered to the selected electrode members when the temperature and/or impedance reaches pre-determined levels to prevent excessive surface or deep tissue temperature rise.
REFERENCES:
patent: 3920021 (1975-11-01), Hiltebrandt
patent: 4848352 (1989-07-01), Pohndorf et al.
patent: 4960134 (1990-10-01), WEbster, Jr.
patent: 5334193 (1994-08-01), Nardella
patent: 5341807 (1994-08-01), Nardella
patent: 5348554 (1994-09-01), Imran et al.
patent: 5383852 (1995-01-01), Stevens-Wright
patent: 5398683 (1995-03-01), Edwards et al.
patent: 54625
Christie Parker & Hale LLP
Cohen Lee
Webster Cordis
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