Biophysics of radiofrequency ablation using an irrigated electrode

Abstract

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.

Fig. 1

Schematic of in vitro ablation system.

Fig. 2

Schematic of ablation lesion, depicted as the light area with viable myocardium dark. MW = maximum width; SD = surface diameter; DMW = depth at which lesion width was maximal; MD = maximal depth.

Fig. 3

Typical trial from experimental Group 2: no irrigation, blood flow velocity = 0.26 m/s, RF titrated to achieve T₀ = 80°C.

Fig. 4

Finite element simulation results, separated by experimental group (labeled on top of each graph): FEM 2 (assuming T₀ predicts intramyocardial temperatures) in red; FEM 1 (assuming T_E predicts intramyocardial temperatures) in green. The data points (mean temperature) with error bars (standard deviation) represent in vitro data. The chi-square value, quantifying the agreement between FEM and in vitro data, is shown in the corresponding color. A variance ratio test was performed between the chi-square values of FEM 1 and FEM 2; the resulting P value indicates whether FEM 2 agreed with the in vitro data significantly better than FEM 1.

Fig. 5

Typical trial from experimental Group 3: no irrigation, no blood flow, RF power was fixed at 50 Watts.

Fig. 6

A. Echocardiographic image prior to initiation of RF application demonstrating myocardium (M), superfusing blood (B), and ablation electrode (E). The dotted line demarcates a portion of the electrode–endocardial interface. Scale is 1 centimeter. B. Echocardiographic image during RF application, at the moment of interfacial bubbling. The interfacial bubbles are difficult to appreciate in a static image, except that they are echodense. A “plume” of bubbles (arrows) can be seen escaping to either side of the electrode–endocardial interface. Scale is 1 centimeter. C. Echocardiographic image after RF application, after resolution of all bubbling. A significant widening of the electrode was observed. Scale is 1 centimeter.

Fig. 7

Typical trial from experimental Group 6: irrigation with room temperature normal saline at 20 cc/min, blood flow velocity = 0.26 m/s, RF power was fixed at 50 Watts. Note that T₀ rises rapidly to boiling, at which point it abruptly deflects downward. This deflection was coincident with the sudden elaboration of echocardiographic bubbles at the electrode–endocardial interface and an audible “pop.” Trials stopped at this instant (data not shown) showed displacement of the interface thermocouple to a site adjacent to the interface. After a few seconds, T₀ resumes its rise, but at a much slower rate. Note that at the moment that the downward deflection in T₀ occurs, despite continued (constant) power application there is also a downward deflection in T_E, and an insignificant (<10 ohm) rise in circuit impedance. We hypothesize that the drop in T_E was due to temporary displacement of the electrode from endocardial contact by the elaboration of bubbles, increasing its exposure to blood cooling.

Fig. 8

Figure 10 from Nakagawa et al [5] demonstrating an irrigated RF application using parameters almost identical to those used in Figure 7 (Group 6). Note that, despite continued (constant) RF application, there are sudden downward deflections in both “interface temp” and electrode temp (arrow), followed by a more gradual rise. The electrode temperature pattern is virtually identical to that in Figure 7. The interfacial temperature pattern is similar in pattern to but different in scale from that in Figure 7. No impedance tracing was supplied with this figure. Reproduced with permission.