Coronary artery injury related to catheter ablation of cardiac arrhythmias: A systematic review

Long-term Risk of Right Coronary Artery Injury Following Catheter Ablation of Cavotricuspid Isthmus-dependent Flutter

BACKGROUND

Radiofrequency ablation (RFA) of cavotricuspid isthmus (CTI)-dependent atrial flutter requires ablation of the tricuspid annulus overlying the right coronary artery (RCA). While considered safe, reports of acute and subacute RCA injury in human and animal studies raise the possibility of late RCA stenosis.

OBJECTIVE

To compare the incidence and severity of angiographic RCA stenoses in patients who have undergone CTI RFA to a control group to assess the long-term risk of RCA damage.

METHODS

A two-center retrospective case-cohort study was performed including all patients from 2002-2018 undergoing atrial fibrillation (AF) with CTI ablation (CTI+AF) or AF ablation alone with subsequent coronary angiography (CAG). The AF alone group served as controls due to anticipated similarity of baseline characteristics. Coronary arteries that are anatomically remote to the CTI were examined as prespecified falsification endpoints. CAG was scored by a blinded observer.

RESULTS

156 patients who underwent PVI with subsequent CAG (CTI+AF, n=81; AF alone, n=75) had no difference in baseline characteristics including age, sex, comorbidities, and medications. Mean time from ablation to CAG was similar (CTI+AF 5.0±3.7 years vs AF alone 5.4 ±3.9 years, p=0.5). The mid and distal RCA showed no difference in the average number of angiographic stenoses or lesion severity. In regression analysis, CTI ablation was not a predictor of RCA stenosis severity (p=0.6). There was no difference in coronary disease at sites remote to the CTI ablation (p=NS for all).

CONCLUSION

There was no observed relationship between CTI RFA and the number or severity of angiographically apparent RCA stenoses in long-term follow up.

Acute myocardial infarction following radiofrequency catheter ablation in a child: a case report on the mechanism of coronary artery occlusion assessed by cardiovascular imaging

Background

Radiofrequency ablation is a common treatment for atrioventricular nodal re-entrant tachycardia, even in paediatric patients weighing ≥15 kg, where outcomes are similar to those in adults. However, reports of acute coronary artery occlusion after radiofrequency ablation for atrioventricular nodal re-entrant tachycardia are rare.

Case summary

An 11-year-old girl with symptomatic atrioventricular nodal re-entrant tachycardia refractory to drug treatment underwent radiofrequency ablation. During the procedure, ST elevation was observed, and coronary angiography revealed occlusion of the right coronary artery at the segment 4 atrioventricular branch. Intravascular ultrasonography showed a narrowed lumen and an abnormal area of low echogenicity in the adjacent myocardium. After dilation with a 1.5 mm diameter balloon, blood flow was successfully restored. Follow-up coronary computed tomography angiography revealed residual stenosis in the right coronary artery at the segment 4 atrioventricular branch; however, blood flow to the distal occlusion was preserved. The patient was discharged without further complications.

Discussion

To the best of our knowledge, this is the first report of coronary artery occlusion following radiofrequency ablation for atrioventricular nodal re-entrant tachycardia, evaluated using intravascular ultrasonography and coronary computed tomography angiography. Based on the imaging findings, direct thermal injury was considered the cause of occlusion.

Electroanatomic visualization of coronary arteries: a case series to elicit safety, feasibility, and diagnostic value in complex ablation procedures

AIMS

The goal of this case series was to evaluate the feasibility, safety, and advantages of a wire-based approach for the live visualization of coronary arteries (CAs) in an electroanatomic mapping (EAM) system and to assess its diagnostic information.

METHODS AND RESULTS

For this single-centre case series, we included procedures in which close proximity of a possible ablation site to any epicardial vessel was suspected. An uncoated-tip guidewire was introduced into the relevant CAs after exclusion of critical CA stenosis by coronary angiography. By connecting this wire to the EAM system using a clip and pin connection, mapping and live visualization of the wire tip is possible, as well as the assessment of the local electrograms within the respective CAs. Procedural wire insertion and intracoronary mapping was performed by EP specialists and was assisted to judge the relevance of CA disease by an interventional cardiologist. A total of nine procedures in nine patients were included in this case series, four ventricular tachycardia ablation procedures and five procedures for the ablation of premature ventricular contractions. The left CAs were mapped in eight cases and the right CA was mapped in one case. In two cases, epicardial mapping was combined with visualization of the right or left CAs. There were no complications attributed to coronary wiring and mapping in this case.

CONCLUSION

We demonstrated the feasibility and safety of CA visualization and integration in an EAM. The live visualization of the CAs added valuable information without the need for preprocedural planning or the purchase of separate software. Electroanatomic visualization was achieved intraprocedurally in a safe and straightforward manner, adding critical diagnostic information without excessive costs or risks.

Complications of catheter ablation for ventricular tachycardia

With the increasing literature demonstrating benefits of catheter ablation for ventricular tachycardia (VT), the number of patients undergoing VT ablation has increased dramatically. As VT ablation is being performed more routinely, operators must be aware of potential complications of VT ablation. This review delves deeper into the practice of VT ablation with a focus on periprocedural complications.

Best Practices for the Catheter Ablation of Ventricular Arrhythmias

Techniques for catheter ablation have evolved to effectively treat a range of ventricular arrhythmias. Pre-operative electrocardiographic and cardiac imaging data are very useful in understanding the arrhythmogenic substrate and can guide mapping and ablation. In this review, we focus on best practices for catheter ablation, with emphasis on tailoring ablation strategies, based on the presence or absence of structural heart disease, underlying clinical status, and hemodynamic stability of the ventricular arrhythmia. We discuss steps to make ablation safe and prevent complications, and techniques to improve the efficacy of ablation, including optimal use of electroanatomical mapping algorithms, energy delivery, intracardiac echocardiography, and selective use of mechanical circulatory support.

Contact Force Guided Radiofrequency Current Application at Developing Myocardium: Summary and Conclusions

BACKGROUND

Catheter contact is one key determinant for lesion size in radiofrequency catheter ablation (RFA). Monitoring of contact force (CF) during RFA has been shown to improve efficacy of RFA in experimental settings as well as in adult patients. Coronary artery narrowing after RFA has been described in experimental settings as well as in children and adults and may be dependent from catheter contact. Value of CF monitoring concerning these issues has not been systematically yet.

OBJECTIVE

Value of high versus low CF during RFA in piglets was studied to assess lesion size and potential coronary artery involvement mimicking RFA in small children.

ANIMALS AND METHODS

RFA with continuous CF monitoring was performed in 24 piglets (median weight 18.5 kg) using a 7F TactiCath Quartz RF ablation catheter (Abbott, Illinois, USA). A total of 7 lesions were induced in each animal applying low (10-20 g) or high (40-60 g) CF. RF energy was delivered with a target temperature of 65 °C at 30 W for 30 seconds. Coronary angiography was performed prior and immediately after RF application. Animals were assigned to repeat coronary angiography followed by heart removal after 48 h (n=12) or 6 months (n=12). Lesions with surrounding myocardium were excised, fixated and stained. Lesion volumes were measured by microscopic planimetry.

RESULTS

A total of 148 RF lesions were identified in the explanted hearts. Only in the subset of lesions at the AV annulus 6 month after ablation, lesion size and number of lesions exhibiting transmural extension were higher in the high CF group compared to low CF. In all other locations CF had no impact on lesion size and mural extension after 48 h as well as after 6 months. Additional parameters as Lesion Size Index and Force Time Integral were also not related to lesion size. Coronary artery damage was present in 2 animals after 48 h and in 1 after 6 months and was not related to CF. This article is protected by copyright. All rights reserved.

Catheter ablation of outflow tract ventricular arrhythmia with intracardiac echocardiography assistance

As the indications for catheter-based electrophysiologic procedures become more diverse and complex, accurate assessment of the anatomy of intracardiac structure has become essential for the optimal clinical outcome. Since intracardiac echocardiography (ICE) was first introduced in 1980, it has become an integral part of various electrophysiologic procedures enabling accurate visualization of cardiac structures and continuous monitoring of catheter position, with integration of real-time images and electroanatomic mapping. Catheter ablation for outflow tract ventricular arrhythmias (OTVAs) has been actively performed in symptomatic patients. However, the anatomic complexity of OT serves as the biggest obstacle to obtain the optimal results. In OTVAs, ICE has played an invaluable role in precisely defining the anatomic shell of OT beyond fluoroscopy and in guiding catheter manipulation in relation to critical structures like the conduction system and coronary arteries. This review article provides detailed information on comprehensive application of ICE for catheter ablation of OTVAs.

The importance of sustained junctional tachycardia following cessation of radiofrequency current delivery in slow pathway ablation

BACKGROUND

Apart from junctional rhythms during slow pathway ablation, there is limited knowledge about the junctional tachycardia persisting following ablation cessation. This study is conducted to determine the characteristics and significance of this rare arrhythmia.

METHODS AND RESULTS

This study was done on 487 patients with AVNRT undergoing the radiofrequency ablation. The RF delivery-induced Supraventricular Tachycardia, persisting for a few minutes following the termination of ablation (post-ablation SVT) was investigated in this research. Atrial Overdrive Pacing (AOP) was applied to the post-ablation SVT to distinguish AVNRT from Junctional Tachycardia(JT). A total of 2337 RF-current deliveries were applied, and post-ablation SVT was observed in 81 of them. According to the electrophysiological studies, five of them (in five separate cases) were definitely diagnosed as JT. The overall incidence of post-ablation JT was about 1% of all patients. In these cases, RF energy was applied to the posteroseptal region and roof of the proximal coronary sinus. The mean Cycle Length (CL) of JTs was equal to 446 ±67ms. Following post- ablation JT termination, four cases met endpoints of successful ablation, demonstrating a positive predictive value of 80%. Atrioventricular (AV) block did not occur in any of the cases and reappearance of JT was not observed during procedure or mean follow-up period of 19.8 ± 8.4 months.

CONCLUSIONS

Post-ablation JT is probably a transient Ischemia-induced arrhythmia that does not require further ablation. Thus, it is recommended to differentiate between the AVNRT and JT in post-ablation arrhythmias to avoid unnecessary RF application.

Mutual Arrangements of Coronary Blood Vessels within the Right Atrial Appendage Vestibule

Background: The aim of our study was to investigate the presence and mutual relationships of coronary vessels within the right atrial appendage (RAA) vestibule. Methods: We examined 200 autopsied hearts. The RAA vestibule was cross-sectioned along its isthmuses (superior, middle, and inferior). Results: The right coronary artery (RCA) was present in 100% of the superior RAA isthmuses but absent in 2.0% of hearts within the middle isthmus and in 6.5% of hearts within the inferior RAA isthmus. Its diameter was quite uniform along the superior (2.6 ± 0.8 mm), middle (2.9 ± 1.1 mm), and inferior (2.7 ± 0.9 mm) isthmuses (p = 0.12). The location of the RCA varied significantly, and it was sometimes accompanied by other accessory coronary vessels. In all the isthmuses, the RCA ran significantly closer to the endocardial surface than to the epicardial surface (p < 0.001). At the superior RAA isthmus, the artery was furthest from the right atrial endocardial surface and this distance gradually decreased between the middle RAA isthmus and the inferior RAA. Conclusions: This study was the most complex analysis of the mutual arrangements and morphometric characteristics of coronary blood vessels within the RAA vestibule. Awareness of additional blood vessels within the vestibule can help clinicians plan and perform safe and efficacious procedures in this region.

Evaluation of Motion Compensation Methods for Noninvasive Cardiac Radioablation of Ventricular Tachycardia

PURPOSE

Noninvasive cardiac radioablation is increasingly used for treatment of refractory ventricular tachycardia. Attempts to limit normal tissue exposure are important, including managing motion of the target. An interplay between cardiac and respiratory motion exists for cardiac radioablation, which has not been studied in depth. The objectives of this study were to estimate target motion during abdominal compression free breathing (ACFB) and respiratory gated (RG) deliveries and to investigate the quality of either implanted cardioverter defibrillator lead tip or the diaphragm as a gating surrogate.

METHODS AND MATERIALS

Eleven patients underwent computed tomography (CT) simulation with an ACFB 4-dimensional CT (r4DCT) and an exhale breath-hold cardiac 4D-CT (c4DCT). The target, implanted cardioverter defibrillator lead tip and diaphragm trajectories were measured for each patient on the r4DCT and c4DCT using rigid registration of each 4D phase to the reference (0%) phase. Motion ranges for ACFB and exhale (40%-60%) RG delivery were estimated from the target trajectories. Surrogate quality was estimated as the correlation with the target motion magnitudes.

RESULTS

Mean (range) target motion across patients from r4DCT was as follows: left/right (LR), 3.9 (1.7-6.9); anteroposterior (AP), 4.1 (2.2-5.4); and superoinferior (SI), 4.7 (2.2-7.9) mm. Mean (range) target motion from c4DCT was as follows: LR, 3.4 (1.0-4.8); AP, 4.3 (2.6-6.5); and SI, 4.1 (1.4-8.0) mm. For an ACFB, treatment required mean (range) margins to be 4.5 (3.1-6.9) LR, 4.8 (3-6.5) AP, and 5.5 (2.3-8.0) mm SI. For RG, mean (range) internal target volume motion would be 3.6 (1.1-4.8) mm LR, 4.3 (2.6-6.5) mm AP, and 4.2 (2.2-8.0) mm SI. The motion correlations between the surrogates and target showed a high level of interpatient variability.

CONCLUSIONS

In ACFB patients, a simulated exhale-gated approach did not lead to large projected improvements in margin reduction. Furthermore, the variable correlation between readily available gating surrogates could mitigate any potential advantage to gating and should be evaluated on a patient-specific basis.

[Pulmonary vein isolation in atrial fibrillation using pulsed field ablation]

Atrial fibrillation is the most common supraventricular arrhythmia with increasing incidence and prevalence. Until now, thermal energy sources such as radiofrequency or cryoablation have been used for pulmonary vein isolation of atrial fibrillation but these have led to indiscriminate tissue destruction in the target area. Pulsed field ablation (PFA) is an energy modality that does not utilize thermal effects. An ultrarapid electric field produces irreversible changes in cell membrane pores (irreversible electroporation) culminating in cell death. The myocardium is very sensitive to PFA compared to the esophagus, the pulmonary veins or the phrenic nerve. Consequently, it is possible to perform effective ablation of the pulmonary veins in a very short time and to make the treatment time more effective without causing relevant collateral damage. The treatment offers a potential paradigm shift from catheter ablation of cardiac arrhythmia.

High-power, short-duration ablation in the coronary sinus: clinical cases and preliminary observations on swine hearts

Purpose

Coronary sinus-related arrhythmias are common; however, it is difficult to perform radiofrequency (RF) ablation at these sites efficiently and safely. High-power, short-duration ablation (HPSD) is a proven alternative strategy for pulmonary vein isolation (PVI); whether it can be applied to ablation of the coronary sinus is unknown. The purpose of this preliminary study was to evaluate the feasibility and safety of HPSD ablation in the coronary sinus.

Methods

Firstly, we demonstrated 4 clinical cases of 3 types of arrhythmias who had unsuccessful ablation with standard power initially, but received successful ablations with HPSD. Secondly, RF ablation was performed in the coronary sinus ostium (CSO) and middle cardiac vein (MCV) of 4 in vitro swine hearts. Two protocols were compared: HPSD (45 W/5 S×5 rounds) and a conventional strategy that used low-power, long-duration ablation (LPLD: 25 W/10 S ×5 rounds). The total duration of HPSD protocol was 25 s, and which of LPLD was 50 s.

Results

A total of 28 lesions were created. HPSD can produce longer, wider, deeper, and larger lesions than LPLD. This difference was more pronounced when the ablation was in the MCV. One instance of steam pop occurred during LPLD in the MCV.

Conclusions

HPSD is an effective alternative strategy for ablation in coronary sinus according to clinical applications and preliminary animal study. However, the safety needs to be further evaluated based on more animal and clinical studies.

Coronary artery dissection following radiofrequency ablation for atrial fibrillation: Case report and review of the literature

Coronary artery injury following catheter ablation for cardiac arrhythmias is very rare. We present a case of left circumflex (LCx) coronary artery dissection causing inferoposterior ST-elevation myocardial infarction following radiofrequency (RF) ablation for atrial fibrillation (AF) in a 39-year-old male with no cardiovascular risk factors. This was confirmed on coronary angiography and intracoronary vascular ultrasound (IVUS). The likely etiology is thermal injury during RF ablation for AF, due to the close proximity of the left atrial appendage and left pulmonary veins to the LCx. He was successfully treated with primary percutaneous coronary intervention with good outcome. This is, to our knowledge, the first reported case of proven acute coronary dissection secondary to RF ablation for AF reported in the literature, and highlights the importance of considering this as a mechanism for coronary occlusion in these patients.

Topographical anatomy of the right atrial appendage vestibule and its isthmuses

INTRODUCTION

The right atrial appendage (RAA) vestibule is an area located in the right atrium between the RAA orifice and the right atrioventricular valve annulus and may be a target for invasive transcatheter procedures.

METHODS AND RESULTS

We examined 200 autopsied human hearts. Three isthmuses (an inferior, a middle, and a superior isthmus) were detected. The average length of the vestibule was 67.4 ± 10.1 mm. Crevices and diverticula were observed within the vestibule in 15.3% of specimens. The isthmuses had varying heights: superior: 14.0 ± 3.4 mm, middle: 11.2 ± 3.1 mm, and inferior: 10.1 ± 2.7 mm (p < .001). The superior isthmus had the thickest atrial wall (at midlevel: 16.7 ± 5.6 mm), the middle isthmus had the second thickest wall (13.5 ± 4.2 mm), and the inferior isthmus had the thinnest wall (9.3 ± 3.0 mm; p < .001). This same pattern was observed when analyzing the thickness of the adipose layer (superior isthmus had a thickness of 15.4 ± 5.6 mm, middle: 11.7 ± 4.1 mm and inferior: 7.1 ± 3.1 mm; p < .001). The average myocardial thickness did not vary between isthmuses (superior isthmus: 1.3 ± 0.5 mm, middle isthmus: 1.8 ± 0.8 mm, inferior isthmus: 1.6 ± 0.5 mm; p > .05). Within each isthmus, there were variations in the thickness of the entire atrial wall and of the adipose layer. These were thickest near the valve annulus and thinnest near the RAA orifice (p < .001). The thickness of the myocardial layer followed an inverse trend (p < .001).

CONCLUSIONS

This study was the first to describe the detailed topographical anatomy of the RAA vestibule and that of its adjoining isthmuses. The substantial variability in the structure and dimensions of the RAA isthmuses may play a role in planning interventions within this anatomic region.

Technical Note: Cardiac synchronized volumetric modulated arc therapy for stereotactic arrhythmia radioablation - Proof of principle

PURPOSE

Ventricular tachycardia (VT) is a rapid, abnormal heart rhythm that can lead to sudden cardiac death. Current treatment options include antiarrhythmic drug therapy and catheter ablation, both of which have only modest efficacy and have potential complications. Cardiac radiosurgery has the potential to be a noninvasive and efficient treatment option for VT. Cardiac motion, however, must be accounted for to ensure accurate dose delivery to the target region. Cardiac synchronized volumetric modulated arc therapy (CSVMAT) aims to minimize the dose delivered to normal tissues by synchronizing beam delivery with a cardiac signal, irradiating only during the quiescent intervals of the cardiac cycle (when heart motion is minimal) and adjusting the beam delivery speed in response to heart rate changes.

METHODS

A CSVMAT plan was adapted from a conventional VMAT plan and delivered on a Varian TrueBeam linear accelerator. The original VMAT plan was divided into three interleaved CSVMAT phases, each consisting of alternating beam-on and beam-off segments synchronized to a sample heart rate. Trajectory log files were collected for the original VMAT and CSVMAT deliveries and the dose distributions were measured with Gafchromic EBT-XD film.

RESULTS

Analysis of the trajectory log files showed successful synchronization with the sample cardiac signal. Film analysis comparing the original VMAT and CSVMAT dose distributions returned a gamma passing rate of 99.14% (2%/2 mm tolerance).

CONCLUSIONS

The film results indicated excellent agreement between the dose distributions of the original and cardiac synchronized beam deliveries. This study demonstrates a proof of principle cardiac synchronization strategy for precise radiation treatment plan delivery and adjustment to a variable heart rate. The cardiac synchronized technique may be advantageous in radioablation for VT.

Successful cryoablation of left ventricular summit premature ventricular contractions via the coronary sinus

The left ventricular summit (LVS) is a challenging location for catheter-based percutaneous ablation due to its anatomical location. There have been case reports of cryoablations performed in this region, but the technique may be underutilized when radiofrequency ablation fails. A 45-year-old male was found to have 25 000 premature ventricular contractions (PVCs) a day despite previous ablation and a reduced ejection fraction of 40% despite medical therapy. Coronary sinus epicardial mapping revealed the coronary sinus distal region generated activations earlier than the QRS onset by 28 ms. Two separate, 4-minute cryoablations were delivered that suppressed the PVCs within 5 seconds. Alternate energy modalities such as cryo may offer a safer and more viable approach for ablation of LVS in select patients.

High density mapping and catheter ablation of atrial tachycardias in adults with congenital heart disease

AIMS

We used a new grid-style multi-electrode mapping catheter (Advisor™ HD Grid, Abbott) and investigated its use for high density mapping of atrial tachycardias in adult patients with congenital heart disease.

PATIENTS AND METHODS

All patients with congenital heart disease who had mapping of atrial tachycardias using the new grid-style catheter between March 2018 and April 2019 were included.

RESULTS

A total of 24 adult patients had high density mapping of atrial tachycardias using the grid-style multi-electrode catheter. Mean procedure duration was 207 ± 72 min., mean fluoroscopy time was 7.1 ± 7.9 min. In patients with right atrial substrates, fluoroscopy time was shorter compared to biatrial or left atrial substrates (0.9 ± 2.2 min for right atrial substrates, n = 19 vs. 6.3 ± 8.3 min for left atrial substrates, n = 2 and 7.5 ± 4.3 min for biatrial substrates, n = 3, p = 0.01). A mean number of 14.814 ± 10.140 endocardial points were collected and 2.319 ± 1244 points were finally used to characterize the tachycardia. Procedural success was achieved in 21/24 (88%) subjects and partial success in 2/24 (8%) patients. Recurrence rate was low (12.5%). In one patient, radiofrequency ablation within the cavotricuspid isthmus resulted in occlusion of a branch of the right coronary artery. No complications related to the use of the mapping catheter itself occurred.

CONCLUSION

High density mapping of AT using the grid-style catheter showed promising results with respect to procedural and midterm outcome and fluoroscopy time. Using the grid-style catheter might offer advantages compared to other multi-electrode catheters used for high density mapping of AT in patients with CHD.

Double cryoenergy application (freeze-thaw-freeze) at growing myocardium: Lesion volume and effects on coronary arteries late after energy application. Implications for efficacy and safety in pediatric patients

INTRODUCTION

Cryoenergy is accepted as an alternative to radiofrequency ablation (RFA) in childen for ablation of supraventricular tachycardia substrates. Single cryoenergy application has been shown to be inferior to RFA. Double cryoenergy application has therefore been introduced into clinical practice, but experience concerning efficacy is limited. Coronary artery stenosis has been reported as serious complication after RFA for arrhythmia substrates but not after single cryoablation. The purpose of the study was to assess lesion volume (efficacy) and risk of coronary artery damage (safety), late, that is, 6 months, after double cryoenergy application in a piglet model.

METHODS

Two sequential cycles of cryoenergy were delivered at -75°C for 4 minutes at the atrioventricular groove in five piglets. Animals were restudied after 6 months by coronary angiography and intracoronary ultrasound (ICUS). Ablation lesions were examined histologically and lesion volume was determined by three-dimensional morphometric analysis.

RESULTS

Cryolesion volume was 174.04 ± 67.18 mm³ for atrial and 238.69 ± 112.1 mm³ for ventricular lesions (P > .05). Ventricular lesions, 4.06 ± 1.05 mm, were significantly deeper than atrial lesions, 3.58 ± 0.78 mm, (P < .05). In two of the 29 lesions, cryoenergy induced minor coronary artery injury with mild medial and adventitial thickening as well as minimal intimal proliferation, which had neither been detected by coronary angiography nor by ICUS.

CONCLUSION

Late after double cryoenergy application at growing myocardium, subclinical minor affection of the coronary artery wall could be detected with minimal intimal proliferation. As lifetime sequelae of this finding remains unknown, further studies are warranted to address safety of repeated cycles of cryoenergy application for tachycardia substrates in children.

"A Song of Ice and Fire"-another verse from the world of ablation

The debate between the use of radiofrequency (RF) or cryoenergy for ablation near the atrioventricular (AV) conducting system or small coronaries has been fueled by the relative efficacies and risks of the two technologies, particularly in smaller hearts. The manuscript by Schneider et al adds another chapter to that ongoing debate.