Cooled Tip Catheter Ablation

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The Chilli® Catheter (EP Technologies/Boston Scientific).
Tissue heating with radiofrequency current. An ablation catheter tip is shown delivering current to tissue. Resistive heating is occurring within the millimeter beyond the catheter tip. The radiant heat which takes place during this process helps to give
Three models for cooled ablation. The three major approaches to cooled ablation are illustrated. (A) the closed system in which internally irrigated saline remains confined to a catheter-pump apparatus; (B) the open irrigation model illustrating the inter
CircuCool Pump And Model 1000TC Generator (EP Technologies/Boston Scientific).
Author(s): 

Linda Moulton

ust over a decade ago, the advent of tip temperature monitoring during radiofrequency (RF) energy delivery marked the beginning of a second generation of ablation catheter technology and furthered the success of catheter ablation as a therapeutic tool. RF generators were designed with a closed-loop feedback system to automatically adjust power output to avoid temperatures that would cause coagulation formation on the ablation catheter tip.

Initially, it was recognized that as long as the tip was located at sites of good blood flow, the tip temperature rarely reached a level resulting in coagulum formation. On the other hand, if the tip was wedged in an area of inadequate blood flow, the system s impedance rapidly rose and power output would abruptly fall to the extent that the lesion was inadequate. This problem prompted interest in whether constant cooling of the ablation catheter tip might result in more effective lesion production and led to the development of the next generation of ablation catheter technology. This article will review the physiology of RF ablation, the use of cooled radiofrequency ablation, clinical trials, and current available systems.

Radiofrequency current is alternating electrical current at a range between 350-750 kHz between an electrode catheter tip resting on the endocardium and a grounding patch placed on the body surface. The grounding patch or plate, also known as a dispersive electrode, has a much greater surface area than the catheter tip. This dispersive effect minimizes any heat delivered to the skin as current flows through the patch. In contrast, the small surface area of the catheter tip is associated with very high current density which produces the desired effect of intensifying heat production in the area around the electrode.

Heating of the tissue around the catheter tip occurs in two phases (Figure 1). First, there is resistive heating, in which a mere millimeter of tissue beyond the catheter tip is heated. With resistive heating, an impedance change occurs between the metallic electrode (low impedance) and the tissue (high impedance). The region of tissue undergoing resistive heating becomes the source of radiant heat which results in tissue damage. It also gives rise to the second phase of heating, known as conductive heating, in which heat transfer to adjacent areas promotes tissue destruction. This conductive heat transfer is also sensed by a thermistor probe located inside the catheter tip and provides the basis for tip temperature monitoring. The resulting lesion size is determined by the balance of tissue conductive heat generation and convective heat loss carried away by tissue blood flow. Thus, lesion volume can be influenced by tissue blood flow within an area deep to the catheter tip.

References: 

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