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J Thorac Cardiovasc Surg 2005;129:477
© 2005 The American Association for Thoracic Surgery
Letters to the Editor |
Papworth Hospital, Cambridge, United Kingdom
To the Editor:
We read with interest the article by Gaynor and associates1 concerning the results of their prospective study of a consecutive group of 40 patients with 100% follow-up in which they reported a 91% freedom from atrial fibrillation (AF) at 6 months.
However, the denominator of patients at 6 months was only 23; therefore, 43% (17 patients) remained unaccounted for at that time point (presumably because they had not yet reached the 6-month interval). The authors used "at last follow-up" analysis to declare 100% follow-up, but usually the percentage follow-up is reported as the number of patients at the last time point (in studies with a pre-planned stopping point). In this case, some would consider the result of reporting only 23 of the 40 potential patients to have the same degree of inaccuracy as a loss to follow-up of 17 of 40 (43%).
A 91% freedom from AF at 6 months is based on the assumption that the remaining 17 patients will not alter this percentage when followed up to 6 months (note the 71% freedom from AF in the first month with 38 patients). However, in an extremely pessimistic situation (should AF subsequently develop in all 17 patients), the results could potentially be 21/40 (53%) freedom from AF at 6 months and 11/40 (28%) freedom from AF and antiarrhythmic medication at 6 months. We do acknowledge, however, that the true estimate would probably lie somewhere between the best- and worst-case scenarios.
Moreover, 10 (43%) of the 23 patients were receiving antiarrhythmic medication at 6 months. Unless the authors prescribed prophylactic antiarrhythmic therapy, it seems natural to assume that the 10 patients were having AF up to and including the 6-month interval. The authors also included 5 patients (13%) who were in paroxysmal AF at the start of the study, and 6 patients (15%) required pacemakers postoperatively due to sick sinus syndrome. Should freedom from AF be attributed to surgery in these patients?
A Kaplan-Meier analysis would have been more suitable to account for the unavailable/censored numbers that increased from 2 to 7 to 17 by 6 months if (any) AF was counted as evidence of an event (regardless of subsequent rhythm). We note that in this study, patients had different AF status at different follow-up times (evident from the increasing numerator between the first and third months), and perhaps more sophisticated methods needed to be employed (recurring time-to-event analysis) to quantify the uncertainty in the estimation of the time-dependent results.
In the same vein, Figure 6 in the manuscript is somewhat misleading. The denominator at the 4 time points decreased from 38 to 33 to 23; therefore, the apparent improvement could still be distorted by the yet-to-be completed follow-up.
Bearing in mind the lower limit of the confidence interval of 21/23 (95% confidence interval, 72% to 99%), we respectfully express our reservations to the conclusions of this otherwise novel and exciting modification of the Cox procedure.
References
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