US 20070270795 A1
A system and method for creating lesions and assessing their completeness or transmurality. Assessment of transmurality of a lesion is accomplished by monitoring the impedance of the tissue to be ablated. Rather than attempting to detect a desired drop or a desired increase impedance, completeness of a lesion is detected in response to the measured impedance remaining at a stable level for a desired period of time, referred to as an impedance plateau. The mechanism for determining transmurality of lesions adjacent individual electrodes or pairs may be used to deactivate individual electrodes or electrode pairs, when the lesions in tissue adjacent these individual electrodes or electrode pairs are complete, to create an essentially uniform lesion along the line of electrodes or electrode pairs, regardless of differences in tissue thickness adjacent the individual electrodes or electrode pairs. Complete or partial submersion in a fluid of the ablating portion of the ablation device may be detected prior to, during or following an ablation procedure.
1. A method of ablating tissue, comprising:
applying ablation energy from an ablation device to a first tissue site;
monitoring submersion of at least a portion of the ablation device at the first tissue site; and
responsive to the degree of submersion of the ablation device at the first tissue site, changing the application of ablating energy to the first tissue site.
This application is a continuation of U.S. patent application Ser. No. 10/923,178, filed Aug. 20, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/364,553, filed Feb. 11, 2003, now U.S. Pat. No. 6,989,010, which is a continuation-in-part application of U.S. patent application Ser. No. 10/132,379, filed Apr. 24, 2002, now U.S. Pat. No. 6,648,883, which claims priority to Provisional U.S. Patent Application Ser. No. 60/287,202, filed Apr. 26, 2001, incorporated herein by reference in their respective entireties.
The present invention relates to tissue ablation devices generally and relates more particularly to devices adapted to ablate lines of tissue, for example for use in conjunction with an electrosurgical version of the Maze procedure.
The Maze procedure is a surgical intervention for patients with chronic atrial fibrillation (AF) that is resistant to other medical treatments. The operation employs incisions in the right and left atria which divide the atria into electrically isolated portions which in turn results in an orderly passage of the depolarization wave front from the sino-atrial node (SA Node) to the atrial-ventricular node (AV Node) while preventing reentrant wave front propagation. Although successful in treating AF, the surgical Maze procedure is quite complex and is currently performed by a limited number of highly skilled cardiac surgeons in conjunction with other open-heart procedures. As a result of the complexities of the surgical procedure, there has been an increased level of interest in procedures employing electrosurgical devices or other types of ablation devices, e.g. thermal ablation, micro-wave ablation, cryo-ablation or the like to ablate tissue along pathways approximating the incisions of the Maze procedure. Electrosurgical systems for performing such procedures are described in U.S. Pat. No. 5,916,213, issued to Hiassaguerre, et al. U.S. Pat. No. 5,957,961, issued to Maguire, et al. and U.S. Pat. No. 5,690,661, all incorporated herein by reference in their entireties. Cryo-ablation systems for performing such procedures are described in U.S. Pat. No. 5,733,280 issued to Avitall, also incorporated herein by reference in its entirety.
In conjunction with the use of electrosurgical ablation devices, various control mechanisms have been developed to control delivery of ablation energy to achieve the desired result of ablation, i.e. killing of cells at the ablation site while leaving the basic structure of the organ to be ablated intact. Such control systems include measurement of temperature and impedance at or adjacent to the ablation site, as are disclosed in U.S. Pat. No. 5,540,681, issued to Struhl, et al., incorporated herein by reference in its entirety.
Additionally, there has been substantial work done toward assuring that the ablation procedure is complete, i.e. that the ablation extends through the thickness of the tissue to be ablated, before terminating application of ablation energy. This desired result is some times referred to as a transmural ablation. For example, detection of a desired drop in electrical impedance at the electrode site as an indicator of transmurality is disclosed in U.S. Pat. No. 5,562,721 issued to Marchinski et al, incorporated herein by reference in its entirety. Alternatively, detection of an impedance rise or an impedance rise following an impedance fall are disclosed in U.S. Pat. No. 5,558,671 issued to Yates and U.S. Pat. No. 5,540,684 issued to Hassler, respectively, also incorporated herein by reference in their entireties.
Three basic approaches have been employed to create elongated lesions using electrosurgical devices. The first approach is simply to create a series of short lesions using a contact electrode, moving it along the surface of the organ wall to be ablated to create a linear lesion. This can be accomplished either by making a series of lesions, moving the electrode between lesions or by dragging the electrode along the surface of the organ to be ablated and continuously applying ablation energy, as described in U.S. Pat. No. 5,897,533 issued to Mulier, et al., incorporated herein by reference in its entirety. The second basic approach to creation of elongated lesions is simply to employ an elongated electrode, and to place the elongated electrode along the desired line of lesion along the tissue. This approach is described in U.S. Pat. No. 5,916,213, cited above and. The third basic approach to creation of elongated lesions is to provide a series of electrodes and arrange the series of electrodes along the desired line of lesion. The electrodes may be activated individually or in sequence, as disclosed in U.S. Pat. No. 5,957,961, also cited above. In the case of multi-electrode devices, individual feedback regulation of ablated energy applied via the electrodes may also be employed. The present invention is believed useful in conjunction with all three approaches
The present invention is directed toward an improved system for creating lesions and assessing their completeness or transmurality. In particular, the preferred embodiments of the invention are directed toward an improved system for creating elongated lines of lesion and assessing their completeness or transmurality. In the preferred embodiment as disclosed, the apparatus for producing the lesions is an electrosurgical device, in particular a saline-irrigated bipolar electrosurgical forceps. However, the mechanism for assessing lesion transmurality provided by the present invention is believed useful in other contexts, including unipolar radio-frequency (RF) ablation and RF ablation using catheters or hand-held probes. The mechanism for assessing transmurality may also be of value in the context of other types of ablation systems, particularly those which ablation occurs in conjunction with an induced rise in tissue temperature such as those applying ablation energy in the form of microwave radiation, light (laser ablation) or heat (thermal ablation).
According to the present invention, assessment of transmurality of a lesion is accomplished by monitoring the impedance of the tissue to be ablated. The inventors have determined that, particularly in the case of a saline-irrigated electrosurgical ablation device, tissue impedance first falls and then reaches a stable plateau, during which portion the lesion is completed. Thereafter, the impedance rises. Rather than attempting to detect a desired drop or a desired increase impedance as described in the above cited Yates, Hassler and Marchlinski patents, the present invention detects completeness of a lesion in response to the measured impedance remaining at a stable level for a desired period of time, hereafter referred to as an impedance plateau. In the context of RF ablation, measurement of impedance may be done using the ablation electrodes or may be done using dedicated electrodes adjacent to the ablation electrodes. In the context of the other types of ablation discussed above, impedance measurement would typically be accomplished by means of a dedicated set of impedance measurement electrodes.
In the context of RF ablation, the invention is believed most valuable in the conjunction with an ablation device having multiple, individually activatable electrodes or electrode pairs to be arranged along a desired line of lesion. In this context, the mechanism for determining transmurality of lesions adjacent individual electrodes or pairs may be used to deactivate individual electrodes or electrode pairs, when the lesions in tissue adjacent these individual electrodes or electrode pairs are complete. This allows the creation of an essentially uniform lesion along the line of electrodes or electrode pairs, regardless of differences in tissue thickness adjacent the individual electrodes or electrode pairs. However, the invention is also believed useful in conjunction with assessment of transmurality of lesions produced by devices having only a single electrode or single electrode pair. Similar considerations apply to the use of the present invention in the contexts of other types of ablation as listed above.
In yet another aspect of the invention, RF power delivery can also be controlled in order to assess lesion transmurality. The ablation will commence at a first power level and will be increased to a second power level as certain conditions are satisfied. The power level can also be increased from the second power level to a third power level as certain conditions are satisfied. The increase in power level can continue in the same stepwise manner, increasing the power level from an N power level to an n+1 power level until a set of conditions are met that indicates that transmurality has been achieved and/or that the power should be turned off. Conditions for an increase in power level can include one or more of: the detection of a plateau in impedance or the achievement of a maximum permitted time for that power level. Conditions indicating that transmurality has been achieved can include one of: the lack of change in detected impedance in response to the change in power level or the detection of a rapid rise in impedance. If transmurality is detected by satisfying one of these conditions, the power is turned off and the ablation is complete. If a condition indicating that transmurality has been achieved is not detected, such as a drop in detected impedance in response to the change to the higher power level, ablation is continued at the higher power level until the conditions are met for again increasing the power level or conditions indicating transmurality are met. This process may be continued in a stepwise fashion until the conditions for detection of transmurality are detected, a predetermined number of plateaus in impedance have been detected or until an overall maximum time for the ablation is achieved.
In yet another aspect of the invention, complete or partial submersion in a fluid of the ablating portion of an ablation device may be detected prior to, during or following an ablation procedure. During an ablation procedure one or more fluids, such as blood, CPB fluids and/or surgical field irrigation fluids, can collect or pool in the atria and/or pericardial sac. When the ablation device is positioned and/or clamped onto tissue, a portion of the device, such as one or more ablating elements of the device, may become fully or partially submerged by these fluids. One or more submersion sensors positioned on the device can be used to detect and monitor the submersion of at least a portion of the device. Since total submersion, oscillating submersion, or partial submersion of one or more ablating elements, e.g., ablation electrodes, can cause, for example, impedance changes and affect the ablation cycle and lesion quality, different power or ablating energy and transmurality algorithms may be applied for the various degrees of submersion.
In use, the hemostat is arranged so that the tissue to be ablated is located between the jaws 18 and 19, and pressure is applied in order to compress the tissue slightly between the jaws to ensure good electrical contact. All electrode pairs may be activated individually and may be.individually deactivated when the lesions between the individual electrode pairs are completely transmural. Alternatively, electrode pairs could be activated sequentially, with one pair deactivated upon a detection of a complete lesion between the electrode pair, followed by activation of the next sequential electrode pair. Corresponding use of the invention in conjunction with a series of unipolar electrodes, for example corresponding to electrodes along one of the two jaws in conjunction with a remote ground plate or a similar series of individually activatable electrodes on a catheter or probe in conjunction with a ground plate is also possible.
Display 804 and controls 802 are connected to a digital microprocessor 800, which permits interface between the user and the remainder of the electrical components of the system. Microprocessor 800 operates under control of stored programming defining its operation including programming controlling its operation according to the present invention, as discussed in more detail below. Microprocessor 800 provides control outputs to and receives input signals from the remaining circuitry via address/data bus 806. In particular, the microprocessor 800 provides for monitoring of power, current, voltage, impedance and temperature. As necessary, the microprocessor will provide this information to the display 804. Additionally, the microprocessor 800 permits the user to select the control mode (either temperature or power) and to input the power set point, temperature set point, and a timer set point to the system. The primary source of power for the radio-frequency generator may be a 12 V battery rated at 7.2 ampere-hours. Alternatively, the device may be AC powered. A back-up battery (not shown) such as a lithium cell may also be provided to provide sufficient power to the microprocessor 800 to maintain desired memory functions when the main power is shut off.
The power supply system as illustrated includes a desired number M of individually controllable RF power supplies and receives temperature inputs from a desired number N of temperature sensing devices in the ablation device, illustrated schematically at 836 and receives temperature inputs from a desired number M of impedance monitoring circuits. Each RF power supply includes a transformer (822, 824, 826), a power control circuit (810, 812, 814) and a power measurement circuit (816, 818, 820). A crystal-locked radio-frequency oscillator 840 generates the switching pulses, which drive both the power transformers (822, 824, 826) and the power controllers (810, 812, 814). Power controllers (810, 812, 814) may be analog controllers which operate by pulse-width modulation by comparing a power set point signal from microprocessor 800 with an actual power signal generated by a power measurement circuit (816, 818, 820), which may, for example, include a torroidal transformer coupled to the power output from the associated transformer (822, 824, 826). The power measurement circuits (816, 818, 820) multiply the output current and voltage and provide the resulting actual power signal to both the power controllers (810, 812, 814) and the microprocessor 800.
The RF power output of the transformers (822, 824, 826) is provided to inter face board 808, and thereby is provided to the ablation electrode or electrodes on the ablation device 838. Separate analog comparator circuits (not illustrated) may also be provided for monitoring the output of the power measurement circuits (816, 818, 820), in order to shut-off current to the output transformers (822, 824, 826) if the power exceeds a limit, typically 55 watts. Power transformers (822, 824, 826) may include center taps, which receive the outputs of the power controllers (810, 812, 814). Secondary windings of the transformers (822, 824, 826) may provide for continuous monitoring of the applied voltage in order to permit the power calculations by power measurement circuits (816, 818, 820).
The illustrated power RF generator system 799 employs software controlled temperature processing, accomplished by microprocessor 800, which receives the N temperature input signals from temperature measurement circuits (828, 830, 832), each of which are coupled to a corresponding temperature sensor in ablation device 838 by means of an electrical connector, illustrated schematically at 836 and interface board 808. If programmed to operate in the temperature controlled mode, processor 800 receives the N temperature signals and, based upon the indicated temperatures, defines power set points for each of the power control circuits (810, 812, 814), which in the manner described above control the power levels applied to electrodes on the ablation device through interface 808. Processor 800 may also selectively enable or disable any of the M provided RF power supplies, in response to external control signals from controls 802 or in response to detected anomalous temperature conditions.
In addition to the circuitry as described above and disclosed in and disclosed in the Maguire, et al. '961 patent, the apparatus of
As an alternative to dedicated impedance monitoring circuits, the microprocessor may employ voltage and current measurements of impedance measurement signals generated by the power transformers (822, 824, 826) to derive impedance and may use such derived impedance values in conjunction with the present invention. For example, measurement of impedance in this fashion is disclosed in U.S. Pat. No. 5,540,681, issued to Struhl, et al, cited above or U.S. Pat. No. 5, 573,533, issued to Struhl, also incorporated herein by reference in its entirety.
In cases in which an alternative ablation energy generation apparatus is employed, particularly those in which a rise in tissue temperature is induced, e.g. laser, microwave or thermal ablation, the RF generation circuitry of
The flow chart of
After initialization at 200, the microprocessor 800 (
At 212, the microprocessor 800 employs the stored impedance measurements to calculate dZ/dt, which may, for example, be calculated as equal to (1(2Δt1)) (Zn−Zn−2). The absolute value of dZ/dt, i.e., |dZ/dt|n is employed to assess whether or not an impedance plateau has been reached at 214. The microprocessor checks at 214 to determine whether |dZ/dt|n is less than a defined constant b, indicative of a minimal rate of change of impedance. In the case of an elongated, fluid irrigated ablation electrode similar to that illustrated in
In the event that |dZ/dt|n is sufficiently small in value at 214, the count m is incremented at 216 and m is compared to a third constant c which sets forth the defined number of low value |dZ/dt|n measurements required to detect an impedance plateau. For example, in a system as described herein, c may be 6-12. Alternatively, rather than requiring an entire series of measured |dz/dt|n values to be less than b, a requirement that a defined proportion of the |dZ/dt|n values must be less than b may be substituted, e.g. 8 of 12, or the like.
If the number of low values of |dZ/dt|n is less than c at 218, the microprocessor waits the impedance sampling interval Δt1 at 220 and continues to acquire successive impedance measurements until sufficient number of sequential low values of |dZ/dt|n have occurred at 218. At that point, the microprocessor then compares the current impedance value Zn with the initial impedance value Zi to determine whether a sufficient impedance drop d has occurred. If not, the microprocessor waits for the next impedance sampling interval at 220 and continues to collect impedance measurements and make calculations of |dz/dt|n until such time as an impedance plateau is recognized at 218 and a sufficient impedance drop is recognized at 220. When both of these criteria have been met at 220, the microprocessor than waits for an additional time interval Δt2 to assure completeness of the lesion at 222 and thereafter terminates the provision of ablation energy to the specific electrode pair being regulated at 224 and the ablation process with regard to that electrode or electrode pair is terminated at 226.
The microprocessor than measures the current impedance Zn at 306, increments the value of n at 310 and checks at 314 to determine whether an adequate number of impedance measurements have been accumulated to make a calculation of dZ/dt at 314, in the same fashion as discussed above in conjunction with
For example, at 316, the microprocessor may calculate the value of dz/dt according to the following method. The microprocessor may employ a 5 point moving average filter to produce an average impedance value Za, which is equal to (Zn+Zn−1+Zn−2+Zn−3+Zn−4)/5. The value of dZ/dtn and |dZ/dt|n may be calculated in the same fashion as in conjunction with the flow charts of
At 318, microprocessor 800 may attempt to determine whether an impedance plateau has been reached employing the following method. The microprocessor reviews the preceding 15 measurements of dZ/dtn, and |dZ/dt|n and applies three criteria to those values, all three of which must be met in order for an impedance plateau to be detected. For a fluid irrigated hemostat as described, the rules may be as follows: for n=1 to 15; |dZ/dt|n must be less than or equal to 1 for all 15; and for n=1 to 15 the value of dZ/dtn must be less than or equal to 0.75, for 13 of the 15; and for n=1 to 15, |dZ/dt|n must be less than or equal to 0.5, for 10 of the 15. If all of these criteria are met at 318, the microprocessor checks at 319 to determine whether an adequate period of time has elapsed since ablation was initiated, if not, the microprocessor waits for the impedance sampling interval At at 312 and continues to measure impedances and calculate values of dZ/dt according to the above described method until both a plateau is present and the defined time period b has elapsed at 319. After the required time period at b at 319 has elapsed, delivery of ablation energy to the associated electrode or electrode pair is terminated at 322 and the ablation process ceases at 324 as to that electrode pair.
In this embodiment, in the event that an impedance plateau fails to manifest itself, the microprocessor may nonetheless safely terminate provision of ablation energy to the electrode pair under consideration in response to a detected rapid rise in impedance, which normally follows the impedance plateau. In the event that a plateau is not detected at 318, the microprocessor checks the preceding stored values of dZ/dtn to look for a rapid rise in impedance. For example, a rapid rise in impedance may be detected using the following rule: for n=1 to 10, dZ/dtn must be greater than or equal to 1.0 for all 10. If this rapid rise criteria is met at 320, the processor checks to see whether the required minimal time period has elapsed at 319, and if so, terminates ablation at 322 as discussed above. If the minimal time interval b has not elapsed, the microprocessor continues to acquire measurements of impedance and continues to calculate values of dZ/dt n and |dZ/dt|n until either an impedance plateau is detected at 318 or a rapid rise in impedance is detected at 320 and the minimum time interval has expired at 319.
After initialization at 400, all electrodes 1−x are activated at 402, meaning that ablation energy is provided to all electrodes and electrode pairs. The microprocessor then checks the value of dZ/dt and/or |dZ/dt| at the first of the electrode pairs at 404, using any of the mechanisms discussed above in conjunction with
The overall operational methodology of
In each of the
As shown in
One embodiment of the present invention includes an ablation system comprising a RF generator, for example generator 799, having a sensor interrogation program and an ablation device, for example as shown in
The signal transmitted from the submersion sensors needs to be physiologically compatible. It has been demonstrated that human and animal cells are affected or stimulated by frequencies below 10,000 Hz. For this reason, it is particularly important for cardiac ablation procedures on a beating heart to maintain frequencies above the 10,000 Hz, for example RF frequencies of about 100 KHz to about 100 MHz may be deployed. Signals transmitted from submersion sensors need to be discemable and/or isolated from the RF ablation current. Using different frequencies, modulation methods, or vector analysis of the signals can allow for simultaneous ablation and sensor monitoring. For example, a first reading of the sensor circuit could be accomplished when the ablation cycle is initiated. A built in delay in RF generator operation could allow for sampling of the sensor circuit prior to delivery of RF power to the ablation electrodes. Alternatively, pulsed RF may be used to perform the ablation procedure. Pulsed RF has been thought to provide benefits in creating ablation lesions. One benefit appears to be the avoidance of over ablating the surfaces closest to the ablation electrode. Pulsed RF has a pulse train of RF energy, wherein the RF energy is oscillated between being off and being on. During the off periods the sensor circuit could be sampled.
During an ablation process, submersion sensors may be in contact with various fluids and tissues having various differences in conductance, impedance, and capacitance. The sensor interrogation program would take into account these various differences. The sensor interrogation program may analyze the various vectors and resulting phase angle between the sensors. For example, it is known that current leads voltage in a capacitive circuit and lags voltage in a reactive circuit; (P=E*I*CosQ; where P=power, E=voltage, I=amperage, and CosQ=phase angle).
In one embodiment, the submersion sensors may be thermocouples thereby allowing for simultaneous temperature and submersion monitoring. Thermocouples generally would require filtering of RF noise. Thermocouples may sense a temperature gradient along the length of an ablation jaw. For example, a thermocouple in a fluid may be at a different temperature than a thermocouple in air or contacting tissue.
In one embodiment, the submersion sensors may be fiber optic sensors that would sample the light refraction between the various sensor locations. For example, air and water have different refraction indices, i.e., air=1.0003 vs. water=1.33. Depending on the light refraction each sensor senses or measures, it can be determined if the sensor is in air, fluid or placed against a tissue surface. Knowing the position of the sensors relative to the device determines what portion of the device is in air or a fluid, for example. A light refraction method has the advantage of being electrically isolated from the ablation current, thereby allowing simultaneous monitoring of submersion conditions while ablating tissue with RF energy. In one embodiment, the submersion sensors may employ ultrasound technology. As with the use of light refraction for monitoring changes in submersion, mechanical wave propagation in various materials could be monitored.
In one embodiment, segmented electrodes that are electrically isolated from each other may also serve as submersion sensors. For example, multiple electrodes of an ablation device, for example, as shown in
It will be appreciated by those skilled in the art that while the invention has been described above in connection with particular embodiments and examples, the invention is not necessarily so limited, and that numerous other embodiments, examples, uses, modifications and departures from the embodiments, examples and uses are intended to be encompassed by the claims attached hereto. The entire disclosure of each patent and publication cited herein is incorporated by reference, as if each such patent or publication were individually incorporated by reference herein.