Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2001-08-31
2004-02-24
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S663000, C324S071100, C606S042000, C600S547000
Reexamination Certificate
active
06696844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to apparatus and methodology for electronically determining the qualitative and quantitative physical state, in real time, of either inorganic and organic materials by monitoring their electrical properties for data acquisition, manipulation, analysis and system control.
2. Background of the Invention
Radio frequency (RF) generators used in tissue ablation medical procedures provide RF energy between one or more electrodes supported on an ablation catheter and a ground electrode applied to the patient; alternatively, the catheter may include one or more rings or other electrode(s) that serve in the manner of a ground electrode. The temperature of the catheter obtained from a thermocouple or thermistor embedded in the electrode tip of the catheter is usually used to control the delivery of ablation energy. While such a thermocouple/thermistor measurement may be sufficiently accurate to reflect the temperature of the catheter tip electrode, it is inherently inaccurate and imprecise in determining the temperature of the tissue during ablation (Hindricks, et. al., “Radiofrequency coagulation of ventricular myocardium: Improved prediction of lesion size by monitoring catheter tip temperature”, Eur Heart Journal 1989; 10:972-984; Langberg et. al., “Temperature monitoring during radiofrequency catheter ablation of accessory pathways”, Circulation 1992;86:1469-1474; Haines et. al., “Observation on electrode-tissue interface temperature and effect on electrical impedance during radiofrequency ablation of ventricular myocardium”, Circulation 1990;82:1034-1038; Blouin, et. al, “Assessment of effects of radiofrequency energy field and thermistor location in an electrode catheter on the accuracy of temperature measurement”, PACE 1991; Part I 14:807-813.). Because of the thermocouple's (thermistor's) inability to accurately reflect tissue temperature, there is a propensity to overheat the ablation site, which could lead to four potentially injurious conditions (He, et. al., “Temperature monitoring during RF energy application without the use of the thermistors or thermocouples”, (abstract) PACE 1996; 19:626; He, et. al., “In vivo experiments of radiofrequency (RF) energy application using bio-battery-induced temperature monitoring”, (abstract) J. Am Coll Cardiol 1997; 29:32A). First, the delivery of RF energy from the catheter tip may become ineffective due to blood coagulation, and furthermore, the coagulum can be dislodged into the blood stream potentially causing a stroke due to occlusion of downstream blood vessels in critical organs. Second, overheated tissue at the ablation site may “stick to” the catheter tip and result in tearing of the tissue upon removal of the catheter. Third, inadequate tissue temperature control can result in unnecessary injury to the heart, including perforation. Fourth, extreme heating of the tissue can cause one or more micro-explosions, which micro-explosions are undesirable since they may displace a piece of tissue into the blood stream and possibly cause a stroke. Because of these potentially dangerous conditions, an apparatus for accurately determining, on a real time basis, the state of tissue during ablation would be of great advantage in performing medical electrophysiological (EP) procedures. Furthermore, if prevention of micro-explosions is not possible for any reason, perhaps due to tissue abnormalities, it would also be advantageous if the apparatus would identify the occurrence of a micro-explosion(s) so that the patient can be closely monitored for adverse reaction during and after the procedure.
Another factor that complicates a cardiac EP procedure in a dynamic beating heart with blood flow is the quality of the electrode-tissue contact. The main aim of an EP procedure is to damage a selected site on cardiac tissue to interrupt errant electrical pulses that cause arrhythmias. To ensure a successful EP procedure, consistent and reliable lesion creation is needed. However, in order to create lesions in a consistent and reliable manner, it is necessary to have good physical contact between the active catheter electrode and the tissue surface. Prior art generators (such as ultrasound, RF, and cryogenic generators) include no device or mechanism that provides adequate information on the quality of electrode-tissue contact. Methods used by cardiologists to determine contact include monitoring the ECG injury current signal before and during energy delivery, monitoring tissue impedance and electrode temperature during energy delivery. (Strickberger, et. al., “Relation between impedance and endocardial contact during radiofrequency catheter ablation”, American Heart Journal 1994; 128:226-229; Avitall, et. al., “The effects of electrode-tissue contact on radiofrequency lesion generation”, PACE 1997; 20:2899-2910). The prior art technology for determining the quality of the electrode-tissue contact is inefficient and ineffective for various reasons. First, because the contact impedance can only be faintly determined during the delivery of energy, this approach unnecessarily lengthens the duration of the procedure. To help illustrate this point, consider the following presently practised methodology. A physician first has to maneuver the catheter with the aid of a fluoroscope to the site on the tissue to be ablated. The physician would then have to deliver a small dose of RF energy so that the change in temperature or impedance signals can be monitored for approximately 30 to 40 seconds. If the monitored signals do not indicate sufficient contact, the whole process would have to be repeated until sufficient contact was achieved. This repetitive process also subjects the patient to unnecessary exposure to radiation from the fluoroscope. Second, the difference between poor contact (i.e. catheter electrode lightly touching the tissue) and good contact (i.e. catheter electrode firmly in contact with tissue) is of insufficient resolution due to inadequate sensitivity of the impedance measurements in prior art RF generators. One prior art RF generator typically provides an approximate initial impedance of 113±16&OHgr; (mean ±SD) for poor contact and 139±24&OHgr; for good contact (Strickberger, et. al., “Relation between impedance and endocardial contact during radiofrequency catheter ablation”, American Heart Journal 1994; 128:226-229). The approximate changes in impedance during a short ablation with another prior art generator (40 second application of 20 W of RF energy) are 14±10&OHgr; for poor contact and 20±2&OHgr; for good contact (Avitall, et. al., “The effects of electrode-tissue contact on radio frequency lesion generation”, PACE 1997; 20:2899-2910). These data show that the difference in the initial impedance (measured during short duration low energy delivery) that is used to estimate good contact is only 26&OHgr; and the large standard error makes this measurement uncertain since there is substantial overlap in the values measured. In the second example the difference in the change of impedance (during ablation) is only 6&OHgr;. Third, the technique using the change in temperature for estimating contact pressure can not be used effectively with an irrigated or chilled catheter since the electrode of the catheter is intentionally not responsive to increases in tissue temperature, unlike a non-cooled catheter. Due to the above reasons, cardiologists find these existing approaches of determining contact quality cumbersome and inefficient, and usually skip the process. Thus, an apparatus capable of effectively monitoring and providing accurate and sensitive tissue impedance measurement before, during, and after an EP procedure as well as quantifying with sufficient resolution the contact quality for discriminating between no, poor, fair, and good electrode-tissue contact would be of great utility and advantage in performing an EP procedure.
Catheter ablation treatment of atrial fibrillation often requires the formation of lin
Hansen Kristi A.
Taylor Junius E.
Wong Yee Chin
Yool Andrea J.
Cahill von Hellens & Glazer, P.L.C.
Deb Anjan K.
Engineering & Research Associates, Inc.
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