Biophysics of radiofrequency ablation using an irrigated electrode.

BACKGROUND: Previous reports have proposed that prevention of electrode-endocardial interfacial boiling is the key mechanism by which radiofrequency application using an irrigated electrode yields a larger ablation lesion than a non-irrigated electrode. It has been suggested that maximal myocardial temperature is shifted deep into myocardium during irrigated ablation. PURPOSE: To examine the biophysics of irrigated ablation by correlating electrode and myocardial temperatures with ablation circuit impedance and lesion morphology, and to perform a comparison with non-irrigated ablation modes. To assess the influence of irrigant rate, composition, temperature and blood flow velocity. METHODS: I. Ablation with and without electrode irrigation was performed in vitro utilizing a whole blood-superfused system. Electrode, electrode-endocardial interface, and intramyocardial temperatures were assessed, as were ablation circuit impedance, total delivered energy, and lesion and electrode morphology. Irrigants assessed were room temperature normal saline, iced normal saline, and dextrose. Irrigant flow rates assessed were 20 and 100 cc/min. Blood flow velocities assessed were 0 and 0.26 m/s. II. Finite element simulations of myocardial temperature during irrigated ablation were performed to further elucidate irrigation biophysics and provide a more detailed myocardial temperature profile. Two models were constructed, each utilizing a different core assumption regarding the electrode-tissue boundary: 1. electrode temperature measured in vitro; 2. interfacial temperature measured in vitro. Intramyocardial temperatures predicted by each model were correlated with corresponding temperatures measured in vitro. RESULTS: I. Ablation during electrode irrigation with normal saline was associated with greater ablation energy deposition and larger lesion dimensions than non-irrigated ablation. The mechanism underlying the larger lesion was delay or inhibition of impedance rise; this was associated with attenuation or prevention of electrode coagulum. Irrigation did not prevent interfacial boiling, which occurred during uninterrupted radiofrequency energy deposition and lesion growth. Irrigation using saline at 100 cc/min was associated with no impedance rise regardless of blood flow velocity, whereas during irrigation at 20 cc/min impedance rise was blood flow rate-dependent. Iced saline produced results equivalent to room temperature saline. Irrigation with dextrose was associated with curtailed energy application and relatively small lesions. II. The finite element simulation that used electrode-endocardial interfacial temperature as the core assumption predicted a myocardial temperature profile which correlated significantly better with in vitro than did the simulation which used electrode temperature as the core assumption. Regardless of irrigant and blood flow rates, maximal myocardial temperature was always within 1 mm of the endocardial surface. CONCLUSIONS: Radiofrequency energy application via a saline irrigated electrode resulted in a larger lesion due to attenuation or eradication of electrode coagulum, thus preventing an impedance rise. Irrigation did not prevent interfacial boiling, but boiling did not prevent lesion growth. The site of maximal myocardial temperature during irrigated ablation was relatively superficial, always within 1 mm of the endocardial surface. Irrigation with iced saline was no more effective than with room temperature saline; both were far more effective than dextrose. Higher irrigation rates immunized the electrode from the influence of blood flow. The biophysical effects of blood flow and irrigation were similar.

Comparison of irrigated electrode designs for radiofrequency ablation of myocardium.

BACKGROUND: Previous reports have demonstrated that radiofrequency energy delivered to myocardium via an irrigated electrode results in a more voluminous ablation lesion than a non-irrigated electrode. Different irrigated electrode designs have been utilized; no direct comparisons have been reported. PURPOSE: To compare different irrigated electrode designs. METHODS: Three irrigation electrode designs were compared to a control (non-irrigated electrode) group: 1. internal; 2. showerhead; 3. sheath. For each electrode, prior to ablation Doppler echocardiographic assessment of the irrigant flow along the electrode outer surface was performed. Ablation was performed in vitro utilizing a whole blood-superfused system. Electrode, electrode-endocardial interface, and intramyocardial temperatures were assessed, as were ablation circuit impedance, total delivered energy, and lesion and electrode morphology. Room temperature normal saline was utilized as the irrigating fluid, delivered at 20 cc/min. Electrode-endocardial interfacial blood flow was assessed at rates of 0 and 0.26 m/s. RESULTS: Irrigant was contained within the internal electrode design and therefore the electrode outer surface manifested no significant flow during irrigation. Irrigant spread primarily radially away from the showerhead electrode design, yielding relatively high electrode outer surface flow at the irrigation holes, but low elsewhere. Irrigant traveled in parallel to and enveloped the electrode outer surface of the sheath electrode design, yielding relatively moderate but uniform flow. Ablation via each of the irrigated electrodes yielded greater ablation energy deposition and larger lesion dimensions than the non-irrigated electrode. Irrigation did not necessarily prevent interfacial boiling, which could occur during uninterrupted radiofrequency energy deposition and lesion growth. The results for the 3 irrigation designs were incongruent. The duration of radiofrequency energy application via the internal electrode design was significantly shorter than the other designs, curtailed by impedance rise. This yielded the smallest total radiofrequency energy deposition and smallest ablation lesion volume. Relative to this, duration using the showerhead design was significantly longer, associated with greater total energy deposition and larger lesion volume. The sheath design permitted the longest duration, associated with the largest total energy deposition and lesion volume. CONCLUSIONS: Although each of the irrigated electrode designs yielded larger lesions than the non-irrigated electrode, they were not comparable. Ablation duration and lesion size were directly correlated with flow along the electrode outer surface.

Optical absorption modeling of thermal infrared detectors by use of the finite-difference time-domain method.

The optical absorption of thin-film thermal infrared detectors was calculated as a function of wavelength, pixel size, and area fill factor by use of the finite-difference time-domain (FDTD) method. The results indicate that smaller pixels absorb a significantly higher percentage of incident energy than larger pixels with the same fill factor. A polynomial approximation to the FDTD results was derived for use in system models.

High spatial resolution measurements of specific absorption rate around ICD leads.

INTRODUCTION: It has been shown that strong electric shocks can cause local refractoriness in the heart. This is of particular concern if the region of refractoriness is the area sensed by an implant to determine cardiac rhythm, as is the case with many Implantable Cardioverter Defibrillator (ICD) leads which use the same electrodes for shocking and sensing. Failure to sense the true cardiac rhythm can cause application of unnecessary shocks and potential induction of arrhythmias. We developed a system to accurately map the areas where local refractoriness is most probable. We measured the Specific Absorption Rate (SAR) around typical ICD leads. Current density (J), a parameter that determines defibrillation effectiveness, is proportional to the square root of SAR. METHODS AND RESULTS: SAR measurements were performed in a homogeneous saline media using a variety of ICD leads. Gated 60 Hz shocks were used to produce heating, which was measured by thermistor probes. The temperature-rate-of-change is directly proportional to the SAR. Measurement techniques were developed that produced accurate SAR results at high spatial resolutions. Multiple polarities and configurations of ICD leads were tested. CONCLUSIONS: We confirmed the spatial distribution of the SAR and corresponding current density possessed sharp peaks and were highly localized around the leads' electrodes. Scans with a resolution of 1 mm or less are required in the area of peak SAR in order to capture the peak's value.

Three-dimensional analysis of subwavelength diffractive optical elements with the finite-difference time-domain method.

We present a three-dimensional (3D) analysis of subwavelength diffractive optical elements (DOE's), using the finite-difference time-domain (FDTD) method. To this end we develop and apply efficient 3D FDTD methods that exploit DOE properties, such as symmetry. An axisymmetric method is validated experimentally and is used to validate the more general 3D method. Analyses of subwavelength gratings and lenses, both with and without rotational symmetry, are presented in addition to a 2 x 2 subwavelength focusing array generator.

Electrical impedance properties of normal and chronically infarcted left ventricular myocardium.

BACKGROUND: Previous reports have disclosed that a significant difference exists between the electrical impedance properties of healthy and chronically infarcted ventricular myocardium. PURPOSE: To assess the potential utility of electrical impedance as the basis for mapping in chronically infarcted left ventricular myocardium. Specifically: (1) to delineate electrical impedance properties of healthy and chronically infarcted ventricular myocardium, with special emphasis on the infarction border zone; (2) to correlate impedance properties with tissue histology; (3) to correlate impedance properties with electrogram amplitude and duration; (4) To demonstrate that endocardial impedance can be measured effectively in vivo using an electrode mounted on a catheter inserted percutaneously. METHODS: An ovine model of chronic left ventricular infarction was utilized. Sites of healthy myocardium, densely infarcted myocardium and the infarction border zone were investigated. Bulk impedance was measured in vitro using capacitor cell, four-electrode and unipolar techniques. Epicardial and endocardial impedances were measured in vivo using four-electrode and unipolar techniques. Impedance was measured at multiple frequencies. Electrographic amplitude, duration and amplitude/duration ratio were measured using bipolar electrograms during sinus rhythm. Quantitation of tissue content of myocytes, collagen, elastin and neurovascular elements was performed. RESULTS: Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the border zone transitioning between healthy myocardium and dense infarction. Decreasing impedance correlated with a decrease in tissue myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as the measurement frequency decreased. Healthy myocardium exhibited a marked frequency dependence in its impedance properties; this phenomenon was not observed in densely infarcted myocardium. There was a direct association between impedance and both electrogram amplitude and amplitude/duration ratio. There was an inverse association between impedance and electrogram duration. Endocardial impedance, measured in vivo using a electrode catheter inserted percutaneously, was demonstrated to distinguish between healthy and infarcted myocardium. CONCLUSIONS: The electrical impedance properties of healthy and infarcted left ventricular myocardium differ markedly. The properties of the infarction border zone are intermediate between healthy and infarcted myocardium. Impedance may be a useful assay of cardiac tissue content and adaptable for cardiac mapping in vivo. Condensed Abstract. To delineate the electrical impedance properties of healthy and chronically infarcted left ventricular myocardium emphasizing the infarction border zone, impedance was measured in chronically infarcted ovine hearts. Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the infarction border zone in transition between healthy myocardium and dense infarction. This correlated with a decreasing myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as measurement frequency decreased. There was a direct association between impedance and electrogram characteristics. Endocardial impedance, measured in vivo using an electrode catheter inserted percutaneously, distinguished between healthy and infarcted myocardium

Radiofrequency multielectrode catheter ablation in the atrium.

We developed a temperature-controlled radiofrequency (RF) system which can ablate by delivering energy to up to six 12.5 mm long coil electrodes simultaneously. Temperature feedback was obtained from temperature sensors placed at each end of coil electrodes, in diametrically opposite positions. The coil electrodes were connected in parallel, via a set of electronic switches, to a 150 W 500 kHz temperature-controlled RF generator. Temperatures measured at all user-selected coil electrodes were processed by a microcontroller which sent the maximum value to the temperature input of the generator. The generator adjusted the delivered power to regulate the temperature at its input within a 5 degrees C interval about a user-defined set point. The microcontroller also activated the corresponding electronic switches so that temperatures at all selected electrodes were controlled within a 5 degrees C interval with respect to each other. Physical aspects of tissue heating were first analysed using finite element models and current density measurements. Results from these analyses also constituted design input. The performance of this system was studied in vitro and in vivo. In vitro, at set temperatures of 70 degrees C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90 degrees C were contiguous. The lesion length increased almost linearly with the number of electrodes. Power requirements to reach a set temperature were larger as more electrodes were driven by the generator. The system impedance decreased as more electrodes were connected in the ablation circuit and reached a low of 45.5 ohms with five coil electrodes in the circuit. In vivo, right atrial lesions were created in eight mongrel canines. The power needed to reach 70 degrees C set temperature varied between 15 and 114 W. The system impedance was 105+/-16 ohms, with one coil electrode in the circuit, and dropped to 75+/-12 ohms when two coil electrodes were simultaneously powered. The length and the width of the lesion set varied between 17.6+/-6.1 and 59.2+/-11.7 mm and 5.9+/-0.7 and 7.1+/-1.2 mm respectively. No sudden impedance rises occurred and 75% of the lesions were contiguous. From the set of contiguous lesions, 90% were potentially therapeutic as they were transmural and extended over the entire target region. The average total procedure and fluoroscopy times were 83.4 and 5.9 min respectively. We concluded that the system can safely perform long and contiguous lesions in canine right atria.

Binary subwavelength diffractive-lens design.

We present a procedure for the design of binary diffractive lenses with pulse-width-modulated subwavelength features. The procedure is based on the combination of two approximate theories, effective medium theory and scalar diffraction theory, and accounts for limitations on feature size and etch depth imposed by fabrication. A design example is presented.

Nonuniform heating patterns of commercial electrodes for radiofrequency catheter ablation.

INTRODUCTION: Previous investigations of the biophysics of myocardial ablation using radiofrequency energy have focused mainly on the electrode-myocardial interface. Relatively little has been written about the overall function of commercial electrodes used for ablation. The purpose of this study was to evaluate the heating patterns of several commercial electrodes used for myocardial ablation. METHODS AND RESULTS: The specific absorption rate, a unit of measure proportional to the initial rate of rise of the temperature at the electrode-tissue interface and reflecting the energy deposition from the electrode, was measured during radiofrequency energy application at multiple points around each electrode. These measurements were combined to form a specific absorption rate pattern, which reflected the spatial pattern of energy deposition from the electrode. There was significant nonuniformity of the specific absorption rate pattern of each electrode evaluated. The proximal edge of the electrode and the curvature of the distal portion of the electrode were two sites of accentuated heating relative to the remaining portions of the electrode. The magnitude of this nonuniformity was similar between the 4-mm length electrodes of different manufacturers, but increased progressively in larger (5- and 8-mm length) electrodes. CONCLUSIONS: The heating patterns of commercial electrodes used for myocardial ablation are nonuniform. These data have implications for electrode design and utilization.

Three-dimensional finite element analysis of current density and temperature distributions during radio-frequency ablation.

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. We used validated three-dimensional finite element models to perform these analyses. We found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density decreased more slowly with distance from the electrode surface. We analyzed the effects of three tissue-electrode angles: 0, 45, and 90 degrees. More power was needed to reach a maximal tissue temperature of 95 degrees C after 120 s when the electrode-tissue angle was 45 degrees. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45 degrees than for 0 and 90 degrees. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.

Myocardial electrical impedance mapping of ischemic sheep hearts and healing aneurysms.

BACKGROUND: This study was designed to examine the bulk electrical properties of myocardium and their variation with the evolution of infarction after coronary occlusion. These properties may be useful in distinguishing between normal, ischemic, and infarcted tissue on the basis of electrophysiological parameters. METHODS AND RESULTS: The electrical impedance of myocardial tissue was studied in a sheep model of infarction. The animal model involved a one-stage ligation of the left anterior descending and second diagonal arteries at a point 40% of the distance from the apex to the base. By use of a four-electrode probe, an epicardial mapping system was developed that allowed for cardiac cycle gated and signal-averaged measurements. Subthreshold current (15 microA) was injected through two of the electrodes at frequencies of 1, 5, and 15 kHz and the induced potential measured with the other two electrodes. Epicardial maps of the left ventricle were obtained during acute infarction and at 1-, 2-, and 6-week intervals after occlusion. Results showed the average specific impedance of the myocardium before infarction to be 158 +/- 26 omega-cm independent of location on the epicardium. By 60 minutes after coronary occlusion, the specific impedance had increased by 199% (p < 0.005, n = 9); it remained elevated for up to 4 hours. One week after infarction, the specific impedance decreased to 59% of the control value (p < 0.025, n = 8). Six weeks after occlusion, the specific impedance remained low at 57% of that of the noninfarcted tissue (p < 0.005, n = 9). The phase angle of the complex impedance was also measured and revealed similar changes. The hydroxyproline content of the tissue was assayed to assess infarct healing. CONCLUSIONS: In this animal model, impedance is a bulk electrical property of tissue that varies with the evolution of myocardial infarction. Impedance mapping revealed significantly different values for normal, ischemic, and infarcted tissue and may prove useful in better defining the electrophysiological characteristics of such tissue.