The present two-tiered animal and clinical study yielded four major findings. First, in swine hearts, higher vs. lower power RF applications were more effective in creating larger and deeper necrosis yet shallower lesions, while lower power was associated with deeper lesions without basophilic changes in connective tissue. Second, in patients with paroxysmal atrial fibrillation, high-power ablation was associated with lesser duration procedures and lesser total radio frequency energy delivery, especially on the left anterior wall. Third, ablation index (AI), the novel marker incorporating time, power and contact force, reliably predicted the degree of necrosis in RF delivery. Fourth, higher power combined with AI increased PVI effectiveness with more frequent first-pass isolation, decreased acute reconnection, and favorable 12-month outcomes.
Myocardial lesion creation starts at 45°C, being partially reversible below 50°C (transient stunning), and definitive above 50°C (durable necrosis)(18). Delivery of radio frequency energy to the tissue is a complicated interaction(1). Thermal injury induced by electrical current delivery with an irrigated-tip comprises resistive and the conductive phases. Resistive heating is a relation between the actual current supplied to the tissue, with the resistance of the radio frequency generator which probably occurs relatively early in the radio frequency application. By using lower resistance or greater radio frequency power, better resistive heating can be achieved. For instance, with standard power (25–30W), temperature already rises above 50°C, but tissue necrosis is confined to the first 1 to 1.5 mm from the ablation catheter tip(19). Conductive heating, which is a secondary passive heating of deeper tissue increases with the period of RF applications. An electrically silent scar is obtained when the tissue is heated to 50°C or higher for several seconds thereby obtaining irreversible coagulation necrosis. Force sensing and stability monitoring have considerably facilitated the reproducibility of heat transmission to the tissue(20,21).The balance between power and duration parameters, respectively involved in resistive and conductive heating, therefore has a growing influence on lesion creation. By increasing resistive heating size, high-power (40–50W) may theoretically favor the creation of durable lesions (temperature above 50°C) whose dimensions might be particularly suitable for PVI because antral thickness is consistently below 4 mm(22).
In the present study in swine left ventricular myocardium, 50W power ablations for short durations at 10g of CF created larger and deepernecrosis than that observed with 40W and 30W of power; however, use of 30W resulted in deeper lesions but without basophilic changes which might render them more susceptible to tissue recovery. Higher power, shorter duration applications were more effective in creating larger and deeper necrosis yet shallower lesions may be due to reduced temperature rise in deeper tissue relative to standard lesions. However, the area of tissue necrosis near the surface of the ablation catheter tip is larger because it is more dependent on resistive heating, which is proportional to power. The larger diameter of necrosis in study group may lead to complete encirclement of pulmonary veins, due to more effective contiguity with adjacent necrosis. At the same time, it was found that higher power (50W) ablation may lead to more complete cell necrosis. However, nuclear pyknosis was observed only more superficially with low-power ablation, and only cell edema was present in the deep part, which may lead to cell reactivation and consequent acute and late PV reconnection and AF recurrence.
In the present study, high-power ablation was associated with shorter procedural duration due to shorter time required for lesion creation, more first-pass PVI, and fewer acute pulmonary vein reconnections. Nilsson et al(23) reported that ablation with 45W for 20 seconds vs. 30W for 120 seconds was associated with shorter pulmonary vein isolation period, mean fluoroscopy time, radiation dose, and total time of radio frequency application. In ventricles from freshly killed pigs, Goyal et al(24) demonstrated that for 20 g of CF, the time required to generate a 4-mm deep lesion decreased from about 20s at 20W to 6–7 s for 50W. In the present study, under the same pressure, the ablation time per point of the left atrial posterior wall in the high-power group was 8-9s shorter than that in the control group, and 10-13s shorter for the left atrial anterior wall. Because catheter instability in a constantly beating heart may also account for the difficulty to transmit heat to the tissue, RF application time shortening probably optimizes lesion creation by increasing the likelihood of catheter stability throughout the entire RF application, particularly for ablation of the left arterial anterior wall by the appendage ridge on the left artery. The significantly smaller pressure of <10g (60%) used for the left atrial anterior wall than for the other three surfaces helps avoid catheter slippage（Figure 6）. However, low power use requires longer time which increases catheter slippage, and the additional time spent adjusting the catheter may cause discontinuous ablation tissue edema, lower first-pass PVI rate, and increased risk of acute PV reconnections. In the present study, ablation time of the left anterior wall was longer than that of other parts of the left atrium in both study and control groups（Figure 7）; however, it showed lesser time in the study when compared to the control group likely secondary to shorter time required to adjust the catheter or the ineffective point of ablation(procedure time minus total ablation time).
AI, a novel marker in a weighted formula utilizing time, contact force and power in a weighted formula. The use of contact force targets and ablation output markers such as Force-Time Integral diminishes recurrence and complication rates in cohorts of patients with AF undergoing PVI(3,25,26). Single-center studies on AI-guided ablation have demonstrated a very high first pass PVI rates of 97% to 98% and 2% to 6% pulmonary vein Reconnection rates, which are very low(26,27). In the present study on AI-guided high-power ablation, numbers of first-pass PVI were elevated at 87% albeit lesser when compared to earlier reports. Equivalently, when compared to controls the numbers for acute pulmonary vein reconnection were lower (11%). The targets of AI were arbitrary, with ranges for posterior walls at 380 to 400 while for other areas it was 550. ST catheters delivering high power was the novel combination used in this study. With reference to this, the AI target of 400 for posterior wall and 500 for other places has contributed to higher First Pass PVI rates and lower acute Pulmonary Vein Reconnection rates.
First-pass pulmonary vein isolation
In AI guided centered evaluation of High Power Ablation, Dhillon G(28) reported more frequent first-pass PVI . Leshem E(29)compared ablation using 90W for a duration of four seconds to 25W for a duration of twenty seconds. The 90W four second ablation resulted in lesions with full thickness and no gaps in all cases, while the 25W twenty second ablation yielded partially thick lesions with several gaps between the lesions. At 25W for a 20 second duration, “endocardial sparing” seemed to occur due to the catheter tip irrigation, which was perceived to be a scar creation failure since there was not enough resistive heating as a result of cooling due to irrigation, for the tissue to be destroyed. In the present animal study experiment, use of higher power was associated with larger maximum diameter and more thorough tissue necrosis. Greater size and better consistency of the created tissue necrosis may explain why high power increases procedure efficiency by ensuring more first-pass PVI and fewer reconnections at 20 minutes.
Acute pv reconnections
An important requirement for the success of the procedure is the durability of PVI; Late as well as acute pulmonary vein reconnection have been known to cause atrial fibrilation (30,5). In a study using an open irrigated-tip catheter, patients undergoing ablation at 50W vs. 35W had greater freedom from AF (82% vs. 66%)(31). The concept of the “weakest link” indicating locations of reconnection was explained in the recent past by El Haddad and colleague(32), who discovered that the absence of contiguous lesion sets and the insufficiency of lesion depth cause the sites of reconnection. Consistent with other studies, the present one documented fewer acute PV reconnections in the high- vs. standard power group, and in an animal model, higher power caused more thorough tissue necrosis thereby precluding cell restoration and reducing acute pulmonary vein connection.
By avoiding tissue damage caused by distant conductive heating, which becomes perceptible at later stages of longer duration radio frequency applications, and providing solutions by way of shorter duration, but higher power radio frequency energy delivery, wherein tissue destruction happens at the earlier portion of the Radio Frequency Application . In porcine ventricles of freshly killed pigs, Goyal et al(24) demonstrated that to create a 4 mm deep ablation the time taken was twenty seconds at 20W, whereas at 5W it was a mere 6-7 seconds, thereby implying that collateral injury can very much be reduced by utilizing the higher power lesser duration radio frequency applications approach. Bhaskaran et al(10) demonstrated that a 5 second procedure at 50W achieved transmural lesions which were inherently safer than a 30 second procedure at 40W. While there was a 8% incidence of steam pops in a 30 second procedure at 40W, there were none in a 5 second procedure at 50W. Winkle et al(7) created a comparison between a 3 to 10 second 50W short duration procedure using open tip irrigated catheters and a 25 to 40 second lower power application. There was no increase in complications and there were much greater incidence of freedom from atrial fibrillations long term, with shorter procedural and fluoroscopy times. Winkle(33) compared the incidences of complications at four experienced centers between atrial fibrillation ablations performed at 45W to 50W power intensity over 2 to 15 second durations per lesion, against reduced power 30W procedures over 20 second durations, and concluded that ablations performed at 45W to 50W over short durations yielded results with very low complication rates. Our clinical results cannot provide definitive information (for example using esophageal temperature monitoring) on whether high-power, short-duration lesions were safer than low-power energy delivered for a longer time. However, our histological results confirmed shallower tissue lesions with high-power than low-power ablation, with shorter ablation time reflecting shorter time for catheter to attach to the atrial wall tissue, thus leading to higher safety. In addition, the extremely low complication rate may be reassuring for using 40W in left atrium at short duration, and may encourage considering use of high power short time radio frequency ablations to enjoy the benefit of reduced procedural time, fluoroscopy, and total RF energy delivery times, even on the posterior wall.
Despite this observational study demonstrating encouraging results, their robustness is limited by several factors. First, degree of esophageal injury during ablation is unclear because all patients were treated under local anesthesia and could not tolerate esophageal temperature monitoring, and no postoperative endoscopic examination was performed. Second, the relatively fixed AI values used may be insufficient or excessive for some patients, especially for thin women. Third, because ST were used instead of SmartTouch Surround Flow (STSF) catheter, there is a small amount of scab at the catheter tip during the 50W ablation inanimal study, for which, 50W was not selected for ablation in clinical research. Fourth, after a year, there was similar sinus rhythm maintenance rate after ablation with 30W and 40W which may because of the numbers in each group were probably too small to look at a real difference in these two techniques.However, use of 50W during ablation may further improve long-term prognosis.