Discussion

Transseptal puncture has been performed since the 1950s. The procedure requires positioning, repositioning, and visualisation of the sheath, dilator, needle and guidewire. Traditionally, visualisation has been done by fluoroscopy. ^{Reference Ross11} With the advent of three-dimensional mapping systems and their ability to minimise the radiation exposure, there has been an interest in non-fluoroscopic methods for crossing the interatrial septum. Since the sheath, dilator, guidewire, and needle cannot be readily viewed on three-dimensional mapping systems, an alternative means of guiding the procedure is required. Ultrasound is the standard tool to guide transseptal puncture without fluoroscopy. This may be intracardiac, transesophageal, or transthoracic echocardiography. Since there is not yet a perfect alternative to fluoroscopy for transseptal puncture, many physicians still choose to utilise fluoroscopy for the procedure.

In this study, we evaluated the radiation exposure associated with transseptal puncture. This is the first study to directly compare the procedural outcomes in patients requiring transseptal puncture with those not requiring transseptal puncture, during ablation procedures utilising three-dimensional electroanatomic mapping. Our findings demonstrate that patients undergoing transseptal puncture have longer procedure durations and are exposed to more fluoroscopy, compared to patients who do not require transseptal puncture.

In order to ensure a direct comparison of procedures in which transseptal puncture was performed to those without transseptal puncture, we limited our patient population to those with a left-sided accessory pathway and selected patients with a structurally normal heart undergoing transseptal puncture as the study group and patients with a patent foramen ovale not requiring transseptal puncture as the control group. There may be occasions when a transseptal puncture is still performed, even in the presence of a patent foramen ovale, as in atrial fibrillation ablation. However, the patients in the current study who had a patent foramen ovale did not undergo transseptal puncture. By evaluating left-sided pathways only, we eliminated confounding variables such as different types of arrhythmias. Specifically, we wanted to avoid comparing ablations of right-sided accessory pathways and atrioventricular nodal reentrant tachycardia to those of left-sided pathways, as these may require different amounts of fluoroscopy to ablate successfully and thus affect the total fluoroscopy time. Kean et al. retrospectively evaluated ablation procedures in which non-fluoroscopic navigation was employed and compared those requiring high fluoroscopy time (≥10 minutes of fluoroscopy) to those requiring low fluoroscopy time (<10 minutes). High fluoroscopy time was associated with transseptal puncture but when atrioventricular nodal reentrant tachycardia was excluded, this was no longer true. The authors hypothesised that there was no association between high fluoroscopy and transseptal puncture because less fluoroscopy is required to ablate left-sided substrates. ^{Reference Kean, LaPage, Yu, Dick and Bradley12} By using only left-sided accessory pathways in the current study, we aimed to evaluate the outcomes associated specifically with the performance of transseptal puncture.

Transseptal puncture is an extra procedure performed during catheter ablation; therefore, it was expected that ablations requiring transseptal puncture would take longer, as was found in our study. There are multiple steps involved in performing a transseptal puncture, and it is important to be in the correct spot at the fossa ovalis in order to increase success and avoid complications. The correct location represents only about 20% of the atrial septum and failure to localise/puncture it is the most common cause of uncompleted transseptal puncture procedures. ^{Reference Sharma, Nalamasu, Gopinathannair, Vasamreddy and Lakkireddy8} Traditionally, fluoroscopy has been used to confirm the location of the needle at the fossa ovalis and in the left atrium once the puncture is made. In recent years, three-dimensional electroanatomic mapping systems have been used to reduce and even eliminate the need for fluoroscopy. However, even with the use of electroanatomic mapping, some cases still require the use of fluoroscopy. This includes patients with complex anatomy, prior septal repair, implantable cardiac devices, and any challenging transseptal puncture cases. ^{Reference Sawhney, Breitenstein and Watts10,Reference Baykaner, Quadros and Thosani13,Reference Clark, Sumihara, McCarter, Berul and Moak14} Therefore, it was expected that transseptal puncture would be associated with a higher rate of fluoroscopy use and overall longer fluoroscopy times, as was found in our study. Given the possibility of other indications for fluoroscopy use, it is also not surprising that some patients in the patent foramen ovale group required the use of fluoroscopy.

Despite the higher fluoroscopy times and thus ionising radiation exposure associated with transseptal puncture, utilisation of electroanatomic mapping during the procedure has been shown to reduce fluoroscopy use. Troisi et al. retrospectively evaluated 145 patients who underwent atrial fibrillation ablation using electroanatomic mapping guidance between June 2018 and April 2019 and 145 patients who underwent atrial fibrillation ablation using standard fluoroscopic guidance before June 2018. The group using electroanatomic mapping guidance had significantly shorter fluoroscopy times (3.6 ± 2.5 versus 13.5 ± 10.5 minutes, p < 0.001) and a lower dose area product (13 ± 11 Gy*cm ^{Reference Smith and Clark2} versus 28 ± 27 Gy*cm ^{Reference Smith and Clark2} , p < 0.001). ^{Reference Troisi, Quadrini and Di Monaco9} Another study by Sawhney et al. retrospectively evaluated 20 patients (32 transseptal punctures) who had a transseptal puncture using three-dimensional mapping systems alone and 14 patients (25 transseptal punctures) who had a transseptal puncture using fluoroscopic guidance. The authors reported a significantly lower mean fluoroscopy time and dose in the group using three-dimensional mapping (0.75 ± 0.50 versus 5.32 ± 3.23 minutes, p < 0.001; 92.5 ± 60.7 versus 394.3 ± 182.7 cGy/cm ^{Reference Smith and Clark2} , p < 0.001). ^{Reference Sawhney, Breitenstein and Watts10} In our study, transseptal puncture was performed on 62% of the patients without the use of fluoroscopy. These findings highlight the benefit of reduced fluoroscopy use associated with electroanatomic mapping.

Use of electroanatomic mapping has emerged as a tool to reduce radiation exposure. This is an important topic in electrophysiology for both patients and staff, as both can be exposed to high cumulative levels of ionising radiation which may be associated with adverse health effects, including an increased risk of cancer. ^{Reference Journy, Dreuil and Rage15–Reference Macle, Weerasooriya and Jais23} This is especially important in CHD patients as they may need to undergo multiple diagnostic and interventional procedures throughout their lifetime which can expose them to high levels of cumulative radiation which can exceed the recommended limit. ^{Reference Glatz, Purrington and Klinger24} Therefore, the goal should be to eliminate any radiation whenever possible, as it is difficult to predict what procedures patients will need in the future and when they will reach their limit.

Efforts should be undertaken to further reduce radiation exposure during catheter ablation procedures. In the current study, excluding one institution that is entirely fluoroless, transseptal puncture resulted in radiation exposure in 98% of procedures, compared to 24% when transseptal puncture was not performed. These data are from institutions with a very aggressive use of three-dimensional mapping. The data suggest that transseptal puncture is likely the number one reason that fluoroscopy is still used in the paediatric electrophysiology lab. While the radiation times are low, there is no lower limit of dose that is known to be safe. Visualising most sheaths, dilators, and guidewires is not possible with current three-dimensional mapping systems. Therefore, the easiest way to perform transseptal puncture is with fluoroscopy. However, providing data to demonstrate this fact can be the impetus for industry to provide better tools for fluoroscopy reduction.

Ultrasound is typically used as an adjunctive tool to try to reduce fluoroscopy use. All procedures in the current study in which non-fluoroscopic transseptal puncture was performed utilised some form of ultrasound, either transesophageal or intracardiac echocardiography. However, both of these modalities have potential drawbacks. The use of transesophageal echocardiography is limited by the need for an additional provider to perform the test as well as requiring the patient to go under general anaesthesia instead of deep sedation. The use of intracardiac echocardiography is limited by its expense as well as difficulty with vascular access, especially in small children and patients who have had multiple previous vascular accesses. Therefore, this is less commonly used in the paediatric population. Both transesophageal and intracardiac echocardiography also increase the procedure duration. In order to use electroanatomic mapping alone to reduce or eliminate fluoroscopy use, there is a need for guidewires, sheaths, and dilators that are visible on electroanatomic mapping systems. These advancements would reduce the total procedure time, making the use of adjunctive transesophageal or intracardiac echocardiography more acceptable. These tools might also make it more feasible to use transthoracic echocardiography instead of transesophageal or intracardiac echocardiography. With improved tools, fluoroscopy-free electroanatomic mapping-guided transseptal punctures may be promising.

Compared to a traditional fluoroscopic approach, three-dimensional electroanatomic mapping has greatly reduced fluoroscopy use during ablations, even in those requiring transseptal puncture. However, transseptal punctures are still associated with longer procedure durations and higher radiation exposure compared to procedures not requiring transseptal puncture. Although 2.4 minutes of fluoroscopy time in the whole group, or 3.7 minutes in the subset, may seem like a reasonable amount of radiation exposure, the goal should be to eliminate any radiation whenever possible, as it can contribute to a patient’s lifetime cumulative radiation exposure. There remains much room for improvement, and radiation times can be further decreased or eliminated. This can be accomplished with the help of industry, by providing the electrophysiology community with improved tools.