**2. Theoretical background of radiofrequency lesion formation**

Radiofrequency catheter ablation is the first-line treatment choice for most symptomatic arrhythmias. The tissue injury caused by this energy source is thermally mediated, resulting in discrete and homogeneous lesions.

#### **2.1 Radiofrequency energy delivery and tissue heating**

The standard RF generator used for catheter ablation produces a sine wave alternating current at 350–500 kHz. The RF energy delivery is usually unipolar between the ablation catheter's tip electrode and a large surface indifferent electrode applied to the patient's skin. During RF energy delivery, the alternating electrical current traverses from the ablation catheter's tip electrode through the intervening tissue to the indifferent electrode. The passage of the electric current through the tissue results in electromagnetic heating, termed resistive heating. Resistive heating is proportional to the square of the current density; current density is inversely proportional to the square of the distance from the ablation electrode. Therefore, power dissipation per unit volume decreases dramatically with the distance, and resistive heating decreases with the distance from the ablation electrode to the fourth power. Since the region of the highest current density is at the tissue below the ablation electrode, resistive heating of the tissue only occurs in a thin layer within a very close vicinity to the ablation electrode. Deeper tissue heating occurs as a result of passive heat conduction from this narrow resistive heating zone, termed conductive heating (**Figure 1**). Temperatures above 50°C are required for irreversible myocardial injury. Of note, a non-negligible part of the delivered energy will be lost as a consequence of convective heat loss to the blood pool surrounding the ablation electrode [5].

#### **2.2 Factors influencing radiofrequency lesion creation**

Lesion formation is dependent on optimal electrode-tissue contact force (CF), RF power, size of ablation catheter tip electrode, and the duration of RF delivery.

The role of the ablation catheter tip size will not be discussed in this chapter, as the vast majority of the electrophysiology laboratories only use 3.5–4-mm tip electrodes in everyday practice, and large-tip electrodes are utilized less commonly.

Lesion size is directly proportional to the electrode-tissue contact temperature. Therefore, those factors that increase the temperature at the electrode-tissue interface (e.g., RF power and contact force) will also increase the lesion size.

It is known that the lesion size is proportional to RF power, as a higher RF power results in a larger current density at the ablation electrode leading to greater tissue heating. However, the deliverable power (and time) might be limited by an impedance rise that occurs when the temperature at the electrode-tissue interface reaches 100°C. This impedance rise can be prevented by maintaining the electrode-tissue

*High-Power, Short-Duration Ablation in the Treatment of Atrial Fibrillation Patients DOI: http://dx.doi.org/10.5772/intechopen.100218*

#### **Figure 1.**

*Phases of lesion formation during radiofrequency ablation. The first phase is the resistive heating occurring in a thin layer of the tissue contacting the ablation electrode. The second phase is the heat conduction from the resistively heated zone to the more distant tissue layers (conductive heating).*

interface temperature below 100°C by cooling the tip of the ablation catheter. A landmark *in vivo* study was presented by Nakagawa et al. [11]. They evaluated the role of presence or absence of catheter tip irrigation in eleven anesthetized dogs' tight muscles. They executed temperature measurements on the catheter-tissue surface and tissue temperatures in 3.5 mm and 7 mm depth. The main findings were that in the case of applications with irrigation of the catheter tip, electrode and electrode-tissue interface temperatures were consistently lower than the tissue temperature at 3.5 mm depth. Moreover, lesion sizes were larger in the case of irrigated ablations, most likely because they could deliver higher-power applications in this group [11, 12].

Later, studies concluded that irrigation minimally affects lesion size by cooling the tissue surface (when the applied power is the same). Larger lesions may only be created with the use of irrigation by making the delivery of higher-power levels possible. This is especially important in case of low blood flow areas where high temperatures are reached at relatively low-power levels, resulting in insufficient lesion formation. In such areas, irrigation decreases temperature during ablation and therefore makes the delivery of a higher power possible [13].

When tissue contact is poor, a larger surface area of the ablation electrode is exposed to the circulating blood pool, which results in less-efficient tissue heating. Conversely, good tissue contact results in a larger area where catheter touches the tissue and less amount of current will be lost to the blood pool. In case of low CF, a higher power might be necessary to reach an optimal degree of tissue heating. On the other hand, similarly, good lesion formation can be produced even with smaller CF in case of high-power applications [5]. A few years ago, contact force-sensing ablation catheters were introduced and nowadays, their use is a part of everyday practice. They allow to reach better durability of lesions and thus facilitate the procedure in terms of achieving better safety and efficacy [3, 14–16].

Finally, an essential determinant of lesion size is the duration of RF application. The rate of tissue heating at the electrode-tissue contact point is rapid, and steady-state temperatures are reached within a few seconds in the resistive heating zone. Conversely, deeper tissue sites have a much slower rate of temperature rise due to the time required for conductive heating. Thus, the lesion growth is rapid in the first few seconds but much slower thereafter. Studies have demonstrated that the half-time of lesion growth is approximately 7–10 s, and maximum lesion size is achieved after 30–40 s of RF energy delivery [5].
