The OASIS trial: A critical appraisal

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Stefan Spitzer
Stefan G Spitzer

Recent results of the first randomised trial—OASIS trial—to compare rotor-only ablation with two other ablation strategies in non-paroxysmal atrial fibrillation patients found poor outcomes in terms of arrhythmia recurrence with rotor-only ablation. However, at CARDIOSTIM-EHRA EUROPACE (8‒11 June, Nice, France), Stefan G Spitzer (Dresden, Germany) addressed various “limitations and omissions” of the trial, which he said “are worthy of discussion”. In this commentary, Spitzer writes his views in detail.

Although the STAR AF II trial1 demonstrated that there is no significant benefit associated with substrate ablation beyond pulmonary vein isolation (PVI) for patients with persistent atrial fibrillation, the results were nevertheless sub-optimal, therefore indicating that a “better” ablation approach is required for these patients.

Several approaches have been suggested for the ablation of persistent atrial fibrillation, including rotor ablation and extended PVAI (conventional PVAI plus non-PV trigger and poster wall ablation) advocated by the group from Austin, Texas. The purpose of the OASIS (Outcome of different ablation strategies in persistent and long-standing persistent atrial fibrillation) trial2 was to evaluate the long-term clinical outcomes of three different treatment approaches, namely: 1) FIRM-guided rotor-ablation only without conventional PVAI (Group 1, n=29); 2) rotor ablation in combination with conventional PVAI (the technique utilised for virtually all publications evaluating the effectiveness of rotor ablation3-10) (Group 2, n=42); and 3) the extended PVAI procedure advocated by the Austin group combined with posterior wall ablation and ablation of non-PV triggers) (Group 3, n=42). The results indicated that the long-term outcomes of Group 3 were significantly better. Moreover, although the outcomes of Group 1 are poor, these investigators should be congratulated for conducting a study to evaluate a new treatment approach for which there was no previously reported outcome data. This was truly unchartered territory.

After a careful review of the publication, we are left with the impression that several relevant limitations and omissions are not completely addressed and are therefore worthy of discussion, specifically:

  • As previously indicated, although the results for Group 1 are of significant interest, one can argue that it is clinically relevant to be aware of the fact that rotor-only ablation is rarely performed and is a significant deviation from the approach utilised in all published data evaluating the benefits of FIRM-guided rotor ablation for non-paroxysmal atrial fibrillation.3-10
  • Regarding the procedure times reported for the group undergoing rotor ablation in combination with conventional PVI (Group 2), the publication states that isoproterenol was given for 15‒20 minutes to evaluate acute PV reconnection. As reported, this is applicable to Group 2 (rotors + conventional PVI) and Group 3 (the extended PVAI procedure from Austin group). However, when comparing the procedure time for Group 1 (rotor-only ablation) to Group 2 where conventional PVAI was added to rotor-only ablation, the mean procedure time only increased by 11 minutes. It would have been of tremendous value for the authors to explain how they were able to perform conventional PVI, including an isoproterenol challenge, in a mean of 11 minutes.
  • Further to the above, it was reported that mean radiofrequency time increased by seven minutes when conventional PVI was performed in combination with rotor ablation (Group 2), as compared to rotor-only ablation (Group 1). However, if the mean total procedure time increased by 11 minutes, and a mean of seven of those minutes are accounted for by the increase in radiofrequency ablation time, there are only four minutes remaining for isoproterenol. An explanation is warranted.
  • In a study of >2,100 patients, Santangeli et al11 reported that the incidence of non-PV triggers in patients with non-paroxysmal atrial fibrillation is 11%. However, in the OASIS trial, it was reported that 100% of patients had non-PV triggers. An explanation for this discrepancy between the incidence of non-PV triggers reported in the OASIS trial versus the results reported by others would have been beneficial.
  • The acute endpoint for the study was clearly defined as termination of atrial fibrillation and/or cycle length prolongation, these acute results were not provided for Group 3. An explanation for omitting these results for the acute endpoint is warranted.
  • In Groups 1 (rotors only) and 2 (rotors in combination with conventional PVAI), if atrial fibrillation converted to atrial tachycardia no further ablation was performed and the patient was cardioverted. However, it appears that this is not necessarily true for Group 3. Based on data reported by Ammar et al12, failing to ablate atrial tachycardia in the groups undergoing rotor ablation may have significantly biased the results, in that these investigators reported a 13% success rate when patients were terminated to atria tachycardia followed by cardioversion to sinus rhythm. Although one can argue that this was the appropriate protocol for the intended purpose of the OASIS trial, a discussion of the impact of this protocol would be of clinical interest.
  • In Group 2, rotors were ablated in combination with traditional PVAI, and yet there was no control group (conventional PVAI alone) to assess the incremental benefit of rotor ablation. However, the Austin group recently published their conventional PVAI results13. In the absence of a control group in the OASIS trial, it is reasonable to utilise these recently published results as a sort of “historical control”, and in so doing it is apparent that the OASIS trial demonstrates that there is in fact a substantial benefit associated with adding rotor ablation to their PVAI ablation procedure. Specifically, using the same endpoint as the OASIS trial (freedom from any arrhythmia off antiarrhythmic medication), the Austin group’s reported one-year success of conventional PVAI for persistent atrial fibrillation was 20%, and yet the OASIS trial reported a success rate of 52% when rotors were ablated in combination with that group’s same conventional PVAI approach. Therefore, based on these data it appears that rotor ablation does in fact provide a substantial benefit. A discussion of these facts would have been beneficial in that it speaks directly to the clinical relevance of FIRM-guided rotor ablation.

In conclusion, while the OASIS trial results provide a major contribution to the body of knowledge related to the treatment of non-paroxysmal atrial fibrillation, one may be left with the impression that the publication is with respect to several important clinically relevant questions incomplete.

To further assess the benefits of FIRM-guided rotor ablation in conjunction with conventional PVI, two large randomised controlled trials are underway. The first is a prospective, multicentre randomised controlled trial to assess the safety and effectiveness of FIRM-guided rotor ablation with conventional ablation as compared to conventional ablation alone for the treatment of persistent atrial fibrillation (clinicaltrials.gov identifier NCT02274857). The second is a recently initiated prospective, multicentre randomised controlled trial to assess the safety and effectiveness of FIRM-guided rotor ablation in combination with conventional PVI as compared to conventional PVI alone for “redo” ablation procedures (clinicaltrials.gov identifier NCT02799043).

References

  1. Verma A, Jiang CY, Betts TR, et al. Approaches to catheter ablation for persistent atrial fibrillation. N Engl J Med 2015;372:1812‒1822
  2. Mohanty S, Gianni C, Mohanty P, et al. Impact of rotor ablation in nonparoxysmal atrial fibrillation patiets: Results from the randomized OASIS Trial. J Am Coll Cardiol 2016;68(3):274‒282
  3. Sommer P, Kircher S, Rolf S, et al. Successful catheter ablation of recurrent longstanding persistent atrial fibrillation with rotor elimination as the procedural endpoint: A case series. J Cardiovasc Electrophysiol 2016;27:274‒280
  4. Narayan SM, Krummen DE, Shivkumar K, et al. Treatment of atrial fibrillation by the ablation of localised sources: CONFIRM (Conventional ablation for atrial fibrillation with or without focal impulse and rotor modulation) trial. J Am Coll Cardiol 2012;60:628–636
  5. Miller JM, Kowal RC, Swarup V, et al. Initial independent outcomes from focal impulse and rotor modulation ablation for atrial fibrillation: Multicenter FIRM registry. J Cardiovasc Electrophysiol 2014;25:921–929
  6. Tomassoni G, Duggal S, Muir M, et al. Long-term follow-up of FIRM-guided ablation of atrial fibrillation: A single-center experience. J Innov Card Rhythm Management 2015;6:2145‒2151
  7. Rashid H, Sweeney A: Approaches for focal impulse and rotor mapping in complex patients: A US private practice experience. J Innov in Card Rhythm Management 2015;6:2193‒2198
  8. Tilz RR, Lin T, Rillig A, et al. Nine month outcomes following focal impulse and rotor modulation for the treatment of atrial fibrillation using the novel 64-pole basket catheter. Europace 2015;17:iii16
  9. Miller JM, Das MK, Dandamudi G, et al. Single-center experience with rotor mapping and ablation for treatment of atrial fibrillation in 170 patients. Heart Rhythm 2016; PO01‒50
  10. Houmsee M, Hummel JD, Hamam I, Daoud EG. Long-term outcome of FIRM guided rotor ablation for atrial fibrillation: Heart Rhythm 2015; PO03‒120
  11. Santangeli P, Zado ES, Hutchison MD, et al. Prevalence and distribution of focal triggers in persistent and long-standing persistent atrial fibrillation. Heart Rhythm 2016;13(2):374‒382
  12. Ammar S, Hessling G, Reents T, et al. Importance of sinus rhythm as endpoint of persistent atrial fibrillation ablation. J Cardiovasc Electrophysiol 2013;24:388‒395
  13. Bai R, Di Biase L, Mohanty P, et al. Proven isolation of the pulmonary vein antrum with or without left atrial posterior wall isolation in patients with persistent atrial fibrillation. Heart Rhythm 2016;13:132‒140

Stefan G Spitzer is cardiologist and electrophysiologist; head of the Department of Rhythmology/Electrophysiology at the Praxisklinik Herz und Gefäße Dresden, Germany. Further he is honorary professor for cardiovascular medical technology at the Institute of Medical Technology, Brandenburg University of Technology Cottbus – Senftenberg, Germany.

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