**3. Experimental studies on HPSD ablation**

In case of conventional, low-power RF applications, most of the thermal injury is a result of heat conduction from the resistively heated thin surface layer. On the other hand, lesion size could be increased by producing direct resistive heating deeper in the tissue by applying higher RF power. The use of a high-power level is allowed by irrigation of the ablation electrode with saline. Saline irrigation maintains a low electrode-tissue interface temperature during radiofrequency application at high power, which prevents a rapid temperature and impedance rise.

Nakagawa et al. found that by applying higher power with an irrigated catheter, a higher temperature can be measured at 3.5 mm tissue depth than at the electrodetissue interface, which indicates that direct resistive heating occurred deeper in the tissue (rather than by conduction of heat from the surface) [11]. Other early studies also found that high-power ablations are effective; however, there was a concern regarding the safety of the procedures based on the animal study results [17]. This seems justified since high-power ablations necessitate reliable real-time feedback on lesion formation to avoid serious complications. Lesion-predicting parameters were not available before the contact force era; therefore, research interest regarding high-power RF ablation decreased transiently. However, after introducing CF-sensing catheters and lesion-predicting parameters, the topic became interesting again. If high power is applied for a long duration, it leads to a big resistive heating zone to which a large conductive heating zone is added, resulting in the creation of extensive lesions. Therefore, high-power ablations should only be applied for a short time to avoid the injury of extracardiac structures.

Nowadays, intensive research is going on examining high-power, short-duration RF ablation technology. Bourier et al. [9] showed that HPSD ablation results in a different lesion geometry (e.g., larger diameters but smaller depth) compared to conventional lower-power ablation. Still, the depth of the high-power applications seems sufficient to reach transmural lesions in the atria (**Figure 2**). Moreover, the larger diameters might improve the chance of creating a contiguous lesion set [9].

Two other preclinical studies evaluated the efficacy and safety of very high-power, short-duration RF ablation (90-W power applied for 4 s) compared to low-power, long-duration ablation in swine models. Both studies showed that HPSD ablation results in improved lesion continuity, lesion transmurality, and shorter ablation time, while the safety profile is comparable to conventional low-power ablation [10, 18].

#### **Figure 2.**

*Properties of lesions created by high-power, short-duration and low-power, long-duration radiofrequency ablation. The high-power, short-duration ablation (panel A) results in a different lesion geometry (e.g., larger diameters but smaller depth) compared to conventional lower-power ablation (panel B). Still, the depth of the high-power application's lesion seems sufficient to reach transmurality in the atria.*
