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.

The kinetics of competitive antagonism by cisatracurium of embryonic and adult nicotinic acetylcholine receptors.

Competitive antagonists to nicotinic acetylcholine receptors are clinically used as muscle relaxants. Previously, we reported the kinetics of inhibition (in the absence of acetylcholine) by (+)-tubocurarine and pancuronium on embryonic receptors. Here, we examine cisatracurium, a commonly used muscle relaxant. Outside-out patches were equilibrated with cisatracurium before application of 300 microM acetylcholine. cisatracurium inhibited the initial peak current, but the decay of these currents displayed a pronounced biphasic behavior. The IC(50) value was 54 +/- 2 nM and 115 +/- 4 nM for adult and embryonic receptors, respectively. We designed a rapid perfusion system to apply or remove cisatracurium for various times before application of acetylcholine. We determined the association (embryonic, 3.4 +/- 0.4 x 10(8) M(-1) s(-1); adult, 1.8 +/- 0.3 x 10(8) M(-1) s(-1)) and dissociation (embryonic, 34 +/- 6/s; adult: 13 +/- 5/s) rates for cisatracurium. Association was 2.9- and 1.3-fold greater than that of (+)-tubocurarine and pancuronium, respectively. Dissociation was 6- and 16-fold higher than (+)-tubocurarine and pancuronium, respectively. These measurements correspond to dissociation of cisatracurium from receptors in the absence of acetylcholine. Physiologically, acetylcholine interacts with receptors equilibrated with antagonist. We developed a mathematical technique that removes the effect of desensitization and determined dissociation (embryonic, 52 +/- 9/s; adult, 33 +/- 5/s) in the presence of acetylcholine. These data suggest that presence of acetylcholine on one binding site of the receptor increases the dissociation rate of antagonist from the other binding site. We incorporated all of these rates into a computer simulation of a comprehensive 11-state Markov model. There was excellent agreement (without curve fitting) between simulated and experimental currents.