Dr Shinsuke Miyazaki of Tsuchiura Kyodo Hospital in Tokyo, Japan, explored the upper limits of contact force through the existing literature and the experience of his research colleagues.
The first study to investigate the catheter tip force required for mechanical perforation was performed on ex vivo porcine hearts. Researchers found that the perforation force was significantly lower in the RA than LA. In addition, the perforation force was significantly lower through the ablated tissue than through unablated tissue. The minimal perforation force was found to be 40 g. In another study using swine atrial chambers, 111 perforations were created in the RA and LA wall, with or without preceding RA delivery. Perforation of the atrial wall was seen over a wide range of contact force values, and the lowest perforation force was 77 g. RF ablation reduced the perforation force by 23 %.35
Another study investigated the perforation force of atrial tissue using a human heart and a porcine cardiac specimen and found similar results: the perforation force was lower in the RA than the LA; perforation after RF delivery required lesser force than after cryoablation and in ablated tissue; the use of larger tip size catheters significantly decreased the risk of perforation. Most importantly, the human atrial perforation force differed from those in porcine hearts: 38 g in the RA and 63 g in the LA, even in healthy human atrial tissue.36
Studies on the incidence of steam pop have found that, under constant RF power and RF duration, lesion size correlates with the contact force, and the incidence of steam pop increases with increasing contact force.37 Other studies have shown that, without knowledge from realtime contact force monitoring, transient high-contact force of >100 g was commonly observed, even among the experienced operators.38 In other trials, results indicate that >40 g of contact force should be avoided during RF ablation to prevent cardiac tamponade.39
After describing these published results, Dr Miyazaki turned to the experience at his institution, where in 3,483 patients and 5,222 procedures from 2002 to 2016, there have been 51 episodes of cardiac tamponade (0.98 %). CF-sensing catheters were used in 526 cases, 11 of which had cardiac tamponade (2.1 %). In all cases, two patients required surgical repair of tamponade (0.038 %). Based on this experience and the literature, Dr Miyazaki’s institution was able to determine parameters potentially associated with the risk of cardiac tamponade:
- CF during RF applications (power, duration) >40g;
- ablated tissue by RF;
- regional variations (tissue composition, underlying tissue);
- wall thickness and stiffness;
- catheter tip configuration and size, and type of catheter;
- catheter-tissue orientation;
- patient background, underlying atrial disease, scar tissue; and
- heart movement associated with contractions and respirations.
“We need to control the lesion size to avoid collateral damage, such as oesophageal injury,” said Dr Miyazaki. “And predicting the lesion sites in the vicinity of our already-ablated areas seems to be difficult due to the tissue oedema. So, we do not know exactly how deep of a lesion is necessary to obtain a durable lesion. There are multiple variable factors that impact on the risk of a perforation and steam pops during AF ablation. Therefore, an optimal CF range should be carefully adjusted considering these factors in individual situations.”