FIELD OF THE INVENTION

The field of the invention relates generally to the treatment of organic tissues using ablation therapy, and more specifically to the prediction and prevention of “steam pop” in catheter-based contact ablation delivery systems.

BACKGROUND

There are many known conditions that affect the electrical impulses that drive the normal operation of the heart. Atrial fibrillation is one common cardiac arrhythmia involving the two upper chambers (atria) of the heart. In atrial fibrillation, disorganized electrical impulses that originate in the atria and pulmonary veins overwhelm the normal electrical impulses generated by the sinoatrial node, leading to conduction of irregular impulses to the ventricles that generate the heartbeat. Atrial fibrillation can result in poor contraction of the atria that can cause blood to recirculate in the atria and form clots. Thus, individuals with atrial fibrillation have a significantly increased risk of stroke. Atrial fibrillation can also lead to congestive heart failure or, in extreme cases, death.

Common treatments for atrial fibrillation include medications or synchronized electrical cardioversion that convert atrial fibrillation to a normal heart rhythm. Surgical-based therapies have also been developed for individuals who are unresponsive to or suffer serious side effects from more conventional treatments. The surgical techniques include making incisions in the right and left atria to block propagation of the abnormal electrical impulse around the atrial-chamber.

U.S. Patent Application No. 2005/0256522 to Francischelli et al. (Francischelli) discloses a surgical-based technique for creating linear lesions along the heart wall by making an incision and inserting a jaw of a dual-jawed ablation head into the heart and clamping a selected portion of the heart wall between the jaws. The jaws are used to measure the thickness of the heart wall tissue. A known clamping force is applied to the jaws, from which a strain on the heart wall tissue can be inferred. Based on the thickness of the heart wall, a combination of jaw force, RF energy and ablation time is selected to fully ablate the clamped tissue. The strain imposed by the jaws is also used to infer the transmurality of the lesion.

Catheter-based contact ablation techniques have evolved as a minimally invasive alternative to surgical-based techniques, and also as an alternative for individuals who are unresponsive to or suffer serious side effects from more conventional treatments (e.g., medications). Contact ablation techniques involve the ablation of groups of cells near the pulmonary veins where atrial fibrillation is believed to originate, or the creation of extensive lesions to break down the electrical pathways from the pulmonary veins located on the posterior wall of the left atrium. Methods of energy delivery include radiofrequency, microwave, cryothermy, laser, and high intensity ultrasound. The contacting probe is placed into the heart via a catheter that enters veins in the groin or neck and is routed to the heart, thus negating the need for an incision in the heart wall from the outside. The probe is then placed in contact with the posterior wall of the left atrium and energized to locally ablate the tissue and electrically isolate the pulmonary veins from the left atrium. Where complete electrical isolation is desired, the process is repeated to form a continuous line of ablated tissue between the left atrium and the pulmonary veins.

The advantages of contact ablation techniques have been recognized; there is no open body and thus risks of infection and recuperation time are reduced. Further, utilizing the aforementioned techniques often reduce or remove the need of pacing hardware or other forms of electronic or mechanical therapy.

Lesion depth is an important parameter in determining the effectiveness of RF ablation therapy in the treatment of atrial fibrillation. Generally, sufficient contact force between the ablation head and the target tissue for a given ablation power is required.

On the other hand, excessive contact force can result in the phenomenon of “steam pops.” Steam pops are a risk associated particularly with irrigated radiofrequency catheter ablation, wherein subsurface heating causes rapid vaporization and expansion that disrupts the proximate tissue and is accompanied by an audible popping sound. If the disruption is of sufficient magnitude (i.e. the volume of the vaporizing expansion large enough), cardiac perforations can lead to “tamponade,” wherein blood accumulates in the space between the myocardium (the muscle of the heart) and the pericardium (the outer covering sac of the heart), causing compression of the heart.

One study concludes that maintaining catheter tip temperatures below 45° C. will prevent steam popping during RF energy delivery. See Watanabe, et al., “Cooled-Tip Ablation Results in Increased Radiofrequency Power Delivery and Lesion Size in the Canine Heart: Importance of Catheter-Tip Temperature Monitoring for Prevention of Popping and Impedance Rise,”Journal of Interventional Cardiac Electrophysiology, vol. 6, no. 2, pp. 9-16 (2002). By contrast, another study determined that steam pops are not related to the temperature of the contacting ablation head, but instead are a strong function of the decrease in target tissue impedance, and recommends monitoring the impedance so that it does not decrease more than a predetermined amount. See Seiler et al., “Steam pops during irrigated radiofrequency ablation: Feasibility of impedance monitoring for prevention,”Heart Rhythm, vol. 5, no. 10, pp. 1411-16 (2008).

A draw back of impedance-based measurement to establish good ablation contact is that the organ wall may not have a uniform behavior. Fat areas have very different impedance than muscle areas. The differences make the impedance reading unreliable indicia of contact integrity. In addition, safety may be compromised because the attending physician may exert a greater contact force to obtain a better impedance indication while not having the benefit of knowing the contact force.

Another study has concluded that steam popping can be avoided by proper selection of power level/lesion diameter combinations. See Topp et al., “Saline-linked surface radiofrequency ablation: Factors affecting steam popping and depth of injury in the pig liver,”Ann. Surg., vol. 239, no. 4, pp. 518-27 (2004). U.S. Patent Application Publication No. 2008/0097220 to Lieber et al. discloses a method of detecting subsurface steam formation by measuring the tissue reflection spectral characteristics during ablation.

Another concern with contact ablation techniques is whether the lesion size is sufficient to accomplish the electrical isolation. At the same time, excessive ablation is also problematic. Excessive ablation can cause damage to the tissues of other organs proximate the heart (e.g. the esophagus), and can also damage the structural integrity of the atrium and lead to “breakthrough,” wherein blood leaks through the atrium wall. Techniques to control lesion size during contact ablation procedures include: an impedance measurement between the contacting probe and ground through the target tissue (WO 2008/063195, US 2008/0275440); monitoring the current output of intervening tissues (serving as an electrolyte) during RF ablation for an inflection that occurs before the onset of harmful tissue charring (U.S. Pat. No. 6,322,558); the use of external auxiliary electrodes to increase lesion depth (U.S. Pat. No. 7,151,964); a microwave probe to heat sub-surface tissue in combination with cryogenic contact cooling of the surface tissue to extend lesion depth without harming surface tissue (U.S. Pat. No. 7,465,300); measuring the temperature of the lesion immediately after energy delivery (US 2008/0275440). Several patents disclose methods for cooling and/or monitoring the temperature of the tip of an RF ablation probe to prevent overheating of the probe tip and the attendant buildup of coagulant that interferes with RF transmission, thereby enhancing lesion depth (US 2005/0177151, US 2007/0093806, US 2008/0161793).

Despite advances in the control of lesion size and steam popping, the effectiveness and risks associated with catheter-based ablation can be highly variable. See Calkins et al., “HRS/EHRA/ECAS expert Consensus Statement on catheter and surgical ablation of atrial fibrillation: recommendations for personnel, policy, procedures and follow-up. A report of the Heart Rhythm Society (HRS) Task Force on catheter and surgical ablation of atrial fibrillation,”Heart Rhythm, v. 4, no. 6, pp. 816-61 (2007). Calkins notes that the results of catheter ablation are widely variable, due in part to differences in technique and to the experience and technical proficiency of the administering physician.

Catheter-based ablation techniques also present challenges relating to visualization of the procedures and providing the operator indications of success, problem areas or potential complications. Early methods for visualization of ablation techniques include mapping the heart cavity utilizing catheter endocardial mapping (U.S. Pat. No. 4,940,064) which relies on the analysis of electric signals. These early methods proved unreliable and more advanced methods were developed to increase the accuracy of the cavity modeling. More recent methods of visualization utilize a Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scan to first model the patient's body cavity at high resolutions followed by a “fusion” with system that establishes a relationship between the 3-dimensional image and physical coordinates. Certain systems utilize a catheter position sensor to create a morphologic map of the organ. Other systems utilize a catheter position sensor that maps the image coordinates of the CT/MRI scan with the sensed physical organ regions in the patient. Some of these systems utilize position sensors based on electrical signals while other systems utilize electromagnetic signals. Still other systems utilize ultrasound arrays to determine location in creating an accurate map of the heart cavity. Commercial mapping implementations are available including the Biosense CARTO® mapping system which utilizes magnetic field sensors and a specialized catheter to detect chamber geometry and EnSite NavX™ Navigation and Visualization Technology which utilizes electrical sensors and a standard catheter to generate 3D models. Still other methods utilize X-Ray machines mated with image fusion technologies such as XMR to generate a 3D visualization of the heart cavity.

A method with rapidly increasing interest is the 3D angiography which utilizes a contrast medium that is injected into the heart cavity. After injection, fluoroscopy equipment rotates around the patient capturing information. Based on the captured information, a computer system is able to construct a 3D rendering of the heart cavity. Recent advances in MRI technologies including Delayed-enhancement Magnetic Resonance Imaging (DE-MRI) techniques have been developed that are providing increased resolution images of the heart cavity without spatial distortion. Other recently developed 3D mapping techniques have been published by Pappone et al., “Non-fluoroscopic mapping as a guide for atrial ablation: current status and expectations for the future”, European Heart Journal Supplements, vol. 9, Supplement I, pp. 1136-1147 (2007), which is hereby incorporated by reference in its entirety except for express definitions defined therein.

However, while 3D visualization techniques have advanced, they are only one component in the analysis of ablation procedures. Traditionally, ablation procedures have been characterized by measuring the power, temperature and/or time of the RF energy being applied. These initial characterizations proved unsuccessful in predicting overall lesion size and effectiveness. Thus, existing technology merely provides the operator with a limited amount of visual information related to their ablation procedure. Existing visualizations may provide the operator with an estimate of power, temperature and time by color coding fixed-size 3D objects overlaid onto a 3D virtualization of the heart cavity. However, there is no technology available that is able to provide an operator with a comprehensive visualization and characterization of the ablation procedure outcome.

Alternative apparatuses and methods for predicting the size of lesions and/or reducing the incidence of injurious steam pops during catheter-based contact ablation procedures, as well as for visualizing the predicted lesion sizes, tissue damage (i.e. perforations and resistive tissues) and isolation gaps during contact ablation procedures would be welcome. A method for predicting the probability of steam pop to better predict ablation therapy outcome would also be welcome.

SUMMARY OF THE INVENTION

Various embodiments of the invention reliably predict the probability that steam pop will occur during RF ablation therapy and control the energy delivered to the ablation probe based on the contact force between the ablation probe and the target tissue to prevent steam pop. In one embodiment, the prediction is based on two parameters: (1) the contact force between the RF ablation head and the target tissue, and (2) the power delivered to the RF ablation head. Prediction of the volume, area and/or depth of lesions created through the use of a force-time integration technique is also presented.

In another aspect, various embodiments of the invention reliably visualize the predicted volume, area and/or depth of lesions created during ablation procedures. One embodiment visualizes the predicted lesions created utilizing a force contact density mapping procedure. Another embodiment visualizes the predicted lesions through the use of a force-time integration technique. Yet another embodiment visualizes the predicted lesions through the use of a force-time and power (and/or current) integration technique. Other embodiments predict the occurrence and locations of tissue damage such as perforation that occurred during the ablation process. Still other embodiments predict the occurrence and location of isolation gaps that may occur during or after the procedure.

Recent advances in catheter-based contact ablation systems have included the ability to measure a reactive force on a catheter that results from contact with the target tissue. A number of patent applications have recently disclosed apparatuses and methods for determining the contact force or stress measurement at the distal tip when in contact with the target tissue. See, e.g., EP 2047797, WO 2008/045958, WO 2007/050960. These disclosures introduce contact force and/or contact pressure as another real-time metric that is available to the practitioner during contact ablation procedures, in addition to time, temperature, power and/or current). U.S. Patent Application Publications 2006/0200049, 2007/0060847, 2008/0294144 to Leo et al. and 2008/0009750 to Aeby et al., assigned to the assignee of the present application and the disclosures of which are hereby incorporated by reference in their entirety except for express definitions therein, disclose devices and methods for resolving a force vector in three-dimensional space for a reactive force on the end effector of a catheter, including an RF ablation head.

It has been found that integrating the reactive force over the time of contact at a known energization level can provide reliable estimates of the size of the resulting lesion. Alternatively, the product of the force and energization level, which can both vary with time, can be integrated over the time of contact. Reliable approximations to the force-time or force-energization-time integrals may also be produced by knowing the time of contact and multiplying by an average or other representative value of the force and/or energization over the time interval. Herein, a “force-time integral” is broadly defined as a measured quantity that involves the measurement of force over time. Accordingly, a “force-time integral” as used herein includes force-time products (e.g. a representative force multiplied by the time interval of application), force-energization-time integrals, force-time-energization products, and combinations thereof.

Studies regarding the relationship between the contact force and lesion size, as well as contact force and incidence of steam pop and thrombus, have been published by Yokoyama et al., “Novel Contact Force Sensor Incorporated in Irrigated Radiofrequency Ablation Catheter Predicts Lesion Size and Incidence of Steam Pop and Thrombus,”Circulation: Arrhythmia and Electrophysiology, vol. 1, no. 5, pp. 354-362 (December 2008), which is hereby incorporated by reference in its entirety except for express definitions defined therein. Yokoyama also discloses the optimization of RF power and application time to maximize lesion formulation and reduce steam pop and thrombosis. Yokoyama, however, does not disclose a nexus between force-time integrals and lesion size prediction.

Herein, an apparatus and method is disclosed for further reducing the incidence of steam pop. Structurally, the apparatus and method utilize a contact force or contact pressure measurement as a feedback element to the power source controlling the delivery of energy to the ablation head. The power level is controlled for higher output when there are low or intermittent contact forces/pressures, and for lower output when there is higher contact forces/pressure. By this technique, steam popping can be reduced.

In some embodiments, an apparatus and method is provided for comprehensively characterizing the results of ablation operation utilizing visualization technology. The visualization aspect can provide the operator with reliable indications of lesion characteristics (e.g. area, volume and/or depth) of the lesions created during ablation procedure, in real time and/or for post-operative analysis. In certain embodiments, contact density is mapped to a three-dimensional (3D) visualization of the heart cavity during the ablation procedure. In other embodiments, force, time and an ablation energization parameter (e.g. power or current) are mapped to a 3D visualization of the heart cavity during the ablation procedure.

The visualization aspects of the invention may enable the professional to increase the efficiency of the ablation procedure by making reliable approximations of lesion size and assessing the completeness of isolation lines. In still other embodiments, contact force and power density is mapped to a 3D visualization of the heart cavity. The visualization aspects may enable the operator to make reliable approximations of potential tissue damage such as perforation prone areas which may produce complications in patient recovery. The visualization aspects may further enable the operator to make reliable approximations of potential isolation gaps that are present during the procedure or may occur in a certain period after the procedure. The visualization aspects can further provide the operator with the ability to differentiate between lesions created by each surgical procedure, thus providing information as to overall coverage and patient history. Thus, information relating to ablation procedures may be stored for further analysis or to become part of the patient's medical history. Finally, some embodiments of the invention may provide information as to the creation of edemas or other cell structures that negatively affect the penetration of RF power during the ablation process. These edemas or structures may be then monitored or targeted for future ablation procedures. By utilizing the described methods and apparatus, the characteristics of lesions created during the ablation process can be predicted reliably, ensuring procedure success and potentially reducing procedure complications and recovery time.

The methods and apparatuses disclosed herein are also adaptable to robotic control of contact and/or lesion size. Calculation of the force-time integral can be performed by a microprocessor that also controls (or provides information to another microprocessor that controls) the manipulation of the catheter. The calculation can be performed in real time or pseudo-real time so that either the power output (or voltage or current output) or the contact force or contact pressure is actively controlled by the system to produce the predicted lesion size, without human intervention in the control of the force-time integral.

Structurally, various embodiments of a system for ablating a target tissue during a medical procedure comprise an elongate flexible catheter adapted to be introduced into a patient during the medical procedure, the catheter including a distal portion. An ablation head is disposed at the distal portion of the catheter, the ablation head adapted to contact the target tissue during the medical procedure. A force sensor is operatively coupled with the ablation head and adapted to detect a contact force exerted on the ablation head from contact with the target tissue, the force sensor outputting a signal in response to the contact force. In one embodiment, the force sensor includes a fiber optic strain gauge. A power source is operatively coupled with the ablation head for energization of the ablation head. A current sensor may be configured to detect the electrical current to the ablation head.

In one embodiment, a control system is adapted to receive the signal from the force sensor to produce a sequence of contact force values. The data acquisition system can also be adapted to determine a time period of energization of the ablation head and for integration of the sequence of contact force values acquired over the time period of energization to produce a force-time integral.

In one embodiment, the control system is adapted to determine an energization parameter to be delivered to the ablation head and to predict a size parameter of a lesion on the target tissue created by the energization parameter. The prediction in this embodiment is based on the force-time integral and the energization parameter. In one embodiment, the energization parameter is power level and/or electrical current. The control system can be adapted to determine the magnitude of the electrical current delivered to the ablation head, and be further adapted to predict a size parameter of a lesion on the target tissue created by the magnitude of current, the prediction being based on the force-time integral and the magnitude of current. The size parameter can be one or more of lesion volume, lesion depth or lesion area.

The control system can also be adapted to control the time period of energization of the ablation head, as well as the magnitude of an energization parameter delivered to the ablation head with the power source. In one embodiment, the control system is adapted to control the magnitude of an energization parameter delivered to the ablation head with the power source, the magnitude of the energization parameter being based on the magnitude of the contact force. The control system can be adapted to substantially disable energization of the ablation head with the power source when the force-time integral reaches a predetermined value. In still other embodiments, the control system can be adapted to increase irrigation, in addition or in place of decreasing or disabling energization. The control system in certain embodiments can be configured to calculate the force-time interval in real time.

The overall system can be configured for manual operation by a human operator, or coupled to a robotic manipulator for movement of the distal portion of the catheter. The robotic manipulator can be controlled by the control system.

In certain embodiments, the control system includes a central processor operatively coupled to the force sensor and the power source. A storage medium can be provided that contains programming instructions to be accessed and carried out by the central processor. In one embodiment, the programming instructions include measuring a sequence of contact forces with the force sensor while the ablation head is in contact with the target tissue, the sequence of contact forces being in reaction to the contact; energizing the ablation head for a period of time while the sequence of contact forces is being measured; and integrating the sequence of contact forces that were measured with the force sensor over the period of time of energizing the ablation head to determine a force-time integral. The programming instructions can further include determining an energization parameter delivered to the ablation head during the energizing of the ablation head, controlling the magnitude of the energization parameter, selecting the energization parameter based on the contact forces of the sequence of contact forces to prevent or reduce the incidence of steam pop, determining a size parameter of a lesion based on the force-time integral and the energization parameter, and/or instructions to terminate energization of the ablation head when the force-time integral reaches a predetermined value.

A force signal conditioning system can also be adapted to digitize the signal received from the force sensor and to provide the digitized signal to the central processor. For configurations utilizing a fiber optic force sensor, the force signal conditioning system can include a fiber optic interrogator operatively coupled with the fiber optic strain gauge and the central processor. The force signal conditioning system can also be adapted for the production of the sequence of contact force values (for example, to digitize the signal received from the force sensor and to provide the digitized signal to the central processor).

Methodologically, various embodiments of the invention include exerting the ablation head of the catheter against the target tissue, measuring a sequence of contact forces with the force sensor while the ablation head is exerted against the target tissue, the contact forces being in reaction to the exerting of the ablation head against the target tissue. The ablation head is then energized for a period of time while the sequence of contact forces is being measured. The sequence of contact forces measured with the force sensor over the period of time of energizing are then integrated to determine a force-time integral. In one embodiment, the method further involves determining an energization parameter delivered to the ablation head during the energizing of the ablation head, and determining the size parameter of the lesion based on the force-time integral and the energization parameter. The magnitude of the energization parameter can be selected based on the contact forces of the sequence of contact forces to prevent or reduce the incidence of steam pop.

