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In vitro measurement of myocardial impedivity anisotropy with a miniature rectangular tube
(2003-04-01)
Tsai, Jang Zern
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Will, James A.
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Vorperian, Vicken R.
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Hubbard-Van Stelle, Scott
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Cao, Hong
Due to rapid change of fiber orientation, it is difficult to measure myocardial impedivity separately in a longitudinal or transverse fiber direction without mutual influence in the two directions. Previously published values of the longitudinal and the transverse myocardial impedivity were derived indirectly from measurements that mixed the impedivity in all directions. Those values are questionable because the derivations were based on a simplified uniform myocardial fiber model. In this paper, a miniature rectangular tube was devised to facilitate direct measurement of myocardial impedivity in a uniform fiber direction. The average transverse-to-longitudinal ratio of the measured in vitro swine myocardial impedivity was about 1.66 from 1 Hz to 1 kHz and dropped to 1.25 at 1 MHz. The result is important for accurate modeling of the electrical property of myocardium in biomedical research of radio-frequency cardiac catheter ablation.
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Mechanical compliance of the endocardium
(2002-12-01)
Bin Choy, Young
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Cao, Hong
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Tungjitkusolmun, Supan
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Tsai, Jang Zern
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Haemmerich, Dieter
Radio-frequency (RF) ablation is an accepted treatment for cardiac arrhythmias related to abnormal focal cardiac substrate. The penetration depth of the electrode into the endocardium affects lesion size, a critical determinant of success of RF ablation. We measured the relation between the mechanical compliance and the penetration depth of RF ablation catheter electrode at frequently ablated areas of the endocardium and examined the influence of time after death on mechanical properties of the tissue. We measured force versus time for eight insertion depths of the catheter electrode into full-thickness endocardial samples derived from the mitral valve annulus, the left ventricular free wall and the tricuspid valve annulus. We varied the time after death at 15, 40min, 3, 8, and 18h and repeated our measurements. At 15min after death, the first 0.5mm penetration depth caused the fastest relaxation at 55s. Force decay decreased dramatically at 15min after death as the penetration depth increased from 0.5 to 4mm. We used the force data sampled at 60s after insertion to approximate the elasticity. We observed the relations between the force versus the insertion depth. The force increased by a factor of 5 for the mitral valve annulus and 8 for the left free wall from 15min to 18h. We derived coefficients of a second-order polynomial equation relating the force data to insertion depth with R<sup>2</sup>>0.99. © 2002 Elsevier Science Ltd. All rights reserved.
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In-vivo measurement of swine myocardial resistivity
(2002-04-30)
Tsai, Jang Zern
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Will, James A.
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Stelle, Scott Hubbard Van
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Cao, Hong
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Tungjitkusolmun, Supan
We used a four-terminal plunge probe to measure myocardial resistivity in two directions at three sites from the epicardial surface of eight open-chest pigs in-vivo at eight frequencies ranging from 1Hz to 1 MHz. We calibrated the plunge probe to minimize the error due to stray capacitance between the measured subject and ground. We calibrated the probe in saline solutions contained in a metal cup situated near the heart that had an electrical connection to the pig's heart. The mean of the measured myocardial resistivity was 319 Ω cm at 1 Hz down to 166 Ωcm at 1 MHz. Statistical analysis showed the measured myocardial resistivity of two out of eight pigs was significantly different from that of other pigs. The myocardial resistivity measured with the resistivity probe oriented along and across the epicardial fiber direction was significantly different at only one out of the eight frequencies. There was no significant difference in the myocardial resistivity measured at different sites.
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Error analysis of tissue resistivity measurement
(2002-04-30)
Tsai, Jang Zern
;
Will, James A.
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Stelle, Scott Hubbard Van
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Cao, Hong
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Tungjitkusolmun, Supan
We identified the error sources in a system for measuring tissue resistivity at eight frequencies from 1 Hz to 1 MHz using the four-terminal method. We expressed the measured resistivity with an analytical formula containing all error terms. We conducted practical error measurements with in-vivo and bench-top experiments. We averaged errors at all frequencies for all measurements. The standard deviations of error of the quantization error of the 8-bit digital oscilloscope with voltage averaging, the nonideality of the circuit, the in-vivo motion artifact and electrical interference combined to yield an error of ±1.19%. The dimension error in measuring the syringe tube for measuring the reference saline resistivity added ±1.32% error. The estimation of the working probe constant by interpolating a set of probe constants measured in reference saline solutions added ±0.48% error. The difference in the current magnitudes used during the probe calibration and that during the tissue resistivity measurement caused ±0.14% error. Variation of the electrode spacing, alignment, and electrode surface property due to the insertion of electrodes into the tissue caused ±0.61% error. We combined the above errors to yield an overall standard deviation error of the measured tissue resistivity of ±1.96%.
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Using electrical impedance to predict catheter-endocardial contact during RF cardiac ablation
(2002-02-26)
Cao, Hong
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Tungjitkusolmun, Supan
;
Choy, Young Bin
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Tsai, Jang Zern
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Vorperian, Vicken R.
During radio-frequency (RF) cardiac catheter ablation, there is little information to estimate the contact between the catheter tip electrode and endocardium because only the metal electrode shows up under fluoroscopy. We present a method that utilizes the electrical impedance between the catheter electrode and the dispersive electrode to predict the catheter tip electrode insertion depth into the endocardium. Since the resistivity of blood differs from the resistivity of the endocardium, the impedance increases as the catheter tip lodges deeper in the endocardium. In vitro measurements yielded the impedance-depth relations at 1, 10, 100, and 500 kHz. We predict the depth by spline curve interpolation using the obtained calibration curve. This impedance method gives reasonably accurate predicted depth. We also evaluated alternative methods, such as impedance difference and impedance ratio.
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Three-dimensional finite-element analyses for radio-frequency hepatic tumor ablation
(2002-01-05)
Tungjitkusolmun, Supan
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Staelin, S. Tyler
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Haemmerich, Dieter
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Tsai, Jang Zern
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Cao, Hong
Radio-frequency (RF) hepatic ablation, offers an alternative method for the treatment of hepatic malignancies. We employed finite-element method (FEM) analysis to determine tissue temperature distribution during RF hepatic ablation. We constructed three-dimensional (3-D) thermal-electrical FEM models consisting of a four-tine RF probe, hepatic tissue, and a large blood vessel (10-mm diameter) located at different locations. We simulated our FEM analyses under temperature-controlled (90 °C) 8-min ablation. We also present a preliminary result from a simplified two-dimensional (2-D) FEM model that includes a bifurcated blood vessel. Lesion shapes created by the four-tine RF probe were mushroom-like, and were limited by the blood vessel. When the distance of the blood vessel was 5 mm from the nearest distal electrode 1) in the 3-D model, the maximum tissue temperature (hot spot) appeared next to electrods A. The location of the hot spot was adjacent to another electrode 2) on the opposite side when the blood vessel was 1 mm from electrode A. The temperature distribution in the 2-D model was highly nonuniform due to the presence of the bifurcated blood vessel. Underdosed areas might be present next to the blood vessel from which the tumor can regenerate.
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Modeling bipolar phase-shifted multielectrode catheter ablation
(2002-01-05)
Tungjitkusolmun, Supan
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Haemmerich, Dieter
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Cao, Hong
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Tsai, Jang Zern
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Choy, Young Bin
Atrial fibrillation (AFIB) is a common clinical problem affecting approximately 0.5-1% of the United States population. Radio-frequency (RF) multielectrode catheter (MEC) ablation has successes in curing AFIB. We utilized finite-element method analysis to determine the myocardial temperature distribution after 30 s, 80 °C temperature-controlled unipolar ablation using three 7F 12.5-mm electrodes with 2-mm interelectrode spacing MEC. Numerical results demonstrated that cold spots occurred at the edges of the middle electrode and hot spots at the side electrodes. We introduced the bipolar phase-shifted technique for RF energy delivery of MEC ablation. We determined the optimal phase-shift (φ) between the two sinusoidal voltage sources of a simplified two-dimensional finite-element model. At the optimal φ, we can achieve a temperature distribution that minimizes the difference between temperatures at electrode edges. We also studied the effects of myocardial electric conductivity (σ), thermal conductivity (κ), and the electrode spacing on the optimal φ. When we varied σ and κ from 50% to 150%, optimal φ ranged from 29.5° to 23.5°, and in the vicinity of 26.5°, respectively. The optimal φ for 3-mm spacing MEC was 30.5°. We show the design of a simplified bipolar phase-shifted MEC ablation system.
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Flow effect on lesion formation in RF cardiac catheter ablation
(2001-04-23)
Cao, Hong
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Vorperian, Vicken R.
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Tungjitkusolmun, Supan
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Tsai, Jang Zern
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Haemmerich, Dieter
This study investigated the flow effect on the lesion formation during radio-frequency cardiac catheter ablation in temperature-controlled mode. The blood flow in heart chambers carries heat away from the endocardium by convection. This cooling effect requires more power from the ablation generator and causes a larger lesion. We set up a flow system to simulate the flow inside the heart chamber. We performed in vitro ablation on bovine myocardium with three different flow rates (0 L/min, 1 L/rain and 3 L/min) and two target temperatures (60 °C and 80 °C). During ablation, we also recorded the temperatures inside the myocardium with a three-thermocouple temperature probe. The results show that lesion dimensions (maximum depth, maximum width and lesion volume) are larger in high flow rates (p < 0.01). Also, the temperature recordings show that the tissue temperature rises faster and reaches a higher temperature under higher flow rate.
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Guidelines for predicting lesion size at common endocardial locations during radio-frequency ablation
(2001-01-01)
Tungjitkusolmun, Supan
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Vorperian, Vicken R.
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Bhavaraju, Naresh
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Cao, Hong
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Tsai, Jang Zern
We used the finite element method to study the effect of radio-frequency (RF) catheter ablation on tissue heating and lesion formation at different intracardiac sites exposed to different regional blood velocities. We examined the effect of application of RF current in temperature- and power-controlled mode above and beneath the mitral valve annulus where the regional blood velocities are high and low respectively. We found that for temperature -controlled ablation, more power was delivered to maintain the preset tip temperature at sites of high local blood velocity than at sites of low local blood velocity. This induced more tissue heating and larger lesion volumes than ablations at low velocity regions. In contrast, for power-controlled ablation, tissue heating was less at sites of high compared with low local blood velocity for the same RF power setting. This resulted in smaller lesion volumes at sites of low local velocity. Our numerical analyzes showed that during temperature-controlled ablation at 60 °C, the lesion volumes at sites above and underneath the mitral valve were comparable when the duration of RF current application was 10 s. When the duration of RF application was extended to 60 s and 120 s, lesion volumes were 33.3% and 49.4% larger above the mitral valve than underneath the mitral valve. Also, with temperature-controlled ablation, tip temperature settings of 70 °C or greater were associated with a risk of tissue overheating during long ablations at high local blood velocity sites. In power-controlled ablation (20 W), the lesion volume formed underneath the mitral valve was 165.7% larger than the lesion volume above the mitral valve after 10 s of ablation. We summarized the guidelines for energy application at low and high flow regions.
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Temperature measurement within myocardium during in vitro RF catheter ablation
(2000-01-01)
Cao, Hong
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Vorperian, Vicken R.
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Tsai, Jang Zern
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Tungjitkusolmun, Supan
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Je Woo, Eung
While most commercial ablation units and research systems can provide catheter tip temperature during ablation, they do not provide information about the temperature change inside the myocardium, which determines the lesion size. We present the details of a flow simulation and temperature measurement system, which allows the monitoring of the temperature change inside the myocardium during in vitro radio frequency (RF) cardiac catheter ablation at different blood flow rates to which the catheter site may be exposed. We set up a circulation system that simulated different blood flow rates of 0 to 5 L/min at 37°C. We continuously measured the temperature at the catheter tip using the built-in thermistor and inside the myocardium using a three-thermocouple probe. The system provides a means for further study of the temperature inside myocardium during RF catheter ablation under different flow conditions and at different penetration depths.

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