Temperature response in radiofrequency catheter ablation.

T he ablative effect of radiofrequency (RF) energy is based on resistive heating of tissue adjacent to the delivery electrode. Generally, the amount of heat generated relates to the amount of power delivered. Thus, under controlled experimental conditions, delivered power can be used as a parameter to control the amount of myocardial damage.1,2 However, in the clinical situation, a large proportion of power delivered is dissipated and lost in circulating cavitary blood,3 and the amount of power that is effectively used for heating myocardial tissue will vary depending on the quality of electrode-tissue coupling. Several studies2'4 including the article by Langberg et a15 in this issue of Circulation have now demonstrated that temperature measurement yields better control over the creation of RF lesions. Some basic undeistanding of the mechanism of tissue heating by high-frequency power dissipation is necessary to appreciate the value and limitations of these techniques.26 The amount of power, expressed in watts, dissipated in a patient during RF ablation equals the square of total current (I) multiplied by total resistance (R): watts-42xR. Similarly, in a three-dimensional medium, the amount of power dissipated per unit volume (w) equals the square of current density (i) multiplied by the specific resistance (p) of the medium: w=i2xp. This implies that power dissipation and, thus, heat generation are not evenly distributed but rather concentrated in area(s) with the highest current density, thus near the ablation electrode(s). With RF delivery in the unipolar mode, current density decreases approximately with the square of the distance from the ablation electrode. This, in combination with the equation above, thus results in a power dissipation per unit volume that decreases with the fourth power of the distance from the ablation electrode.