Importance of anatomy in cavotricuspid isthmus

We were delighted to read the article by Iwasawa et al. ‘Cavotricuspid isthmus ablation using a catheter equipped with mini electrodes on the 8 mm tip; a comparison with an 8 mm dumbbell shaped tip catheter and 8 mm cryothermal catheter’,1 which in many aspects echoed our own experience with this novel ablation catheter (IntellaTip MiFi, Boston Scientific, Natrick, MA, USA) for atrial flutter ablation. Whilst we agree with the authors' view on the lack of power achieved in temperature-limited applications (a fact we have also found), we would like to draw the authors' attention to the potential interaction of the cavotricuspid isthmus (CTI) anatomy and the use of signals from the mini electrodes for selection of ablation points. In our respective institutions, we routinely utilize the voltage-directed CTI ablation technique.2,3 The concept of this technique is to ablate the conducting bundles of the CTI whilst avoiding ablating the intervening non-conducting fibrous tissue (Figure 1). Thus, the operator sequentially targets the highest amplitude signals in the CTI after repeatedly mapping the voltage across the CTI on pullback from the tricuspid valve annulus to the inferior vena cava at the 6 o’clock position. This is repeated until bidirectional block is achieved. With conventional catheters, block can often be obtained after 3–6 applications; however, lesion volume is related to the surface area of contact between electrode tip and endocardium.4 This in turn is dependent upon electrode placement within the 8 mm tip and if the catheter is employed in a horizontal or vertical fashion. Using the mini-electrode signals instead of the conventional bipole, we experienced great difficulty to achieve block and on a couple of occasions had to change to irrigated tip catheter. Investigating this further with electroanatomical mapping, we discovered that mapping with the mini electrodes placed the ablation surface more proximal than when mapping with conventional bipoles. Thus, our hypothesis is that with conventional bipole guidance more of the bundle is covered by the 8 mm ablation surface in a horizontal configuration, whereas with mini-electrode guidance, only the tip of the catheter is at the point of maximum diameter of the bundle reducing the effective surface area of contact (Figure 2). Such positioning will also have a negative effect with catheter stability as cardiac and respiratory movements will tend to move the ablation surface off the bundle. Smaller ablation tip (irrigation) and vertical orientation would mitigate this effect. Diagrammatic representation of bundles within CTI. (IVC, inferior vena cava; RV, right ventricle; TV, tricuspid valve).5 (A) Diagram of hypothetical placement of catheter over bundle with conventional bipole voltage guidance and (B) the more proximal catheter placement hypothesized with mini-electrode voltage guidance.