To our knowledge, this is the first prospective and randomized study to compare the effects of Cryo and RF ablation modalities on the atria, assessed using 3D echocardiography. We found that patients who were in SR at one year after maze procedure with concomitant mitral valve surgery presented with 1) a reduction of left atrial size which was more pronounced after RF ablation compared to Cryo; 2) better systolic left atrial function after RF ablation (expressed as booster and reservoir fractions compared to the Cryo cohort); and 3) a lower booster fraction compared to the NoMaze group.
The maze procedure is performed to electrically isolate the atria by generating linear scars through application of a warm or cold energy source. Its safety has been proven by several studies (15, 16, 17), and sinus rhythm restoration after this procedure has positive impacts on long-term clinical outcomes (18, 19), although further investigation is needed to determine the extent of LA reverse remodeling and atrial function improvement. Atria undergo a stunning phase directly after cardioversion (20) and lone (surgical or percutaneous) correction of MR results in LA reverse remodeling (21) during the early postprocedural period (22),with a tendency to subsequently return to baseline levels in patients with untreated AF (23).
The setting becomes more complex when adding surgical ablation distress. Atrial myocardial contraction restoration might be more vulnerable when ablation lines are added to the surgical scars. In patients with preoperative AF and mitral valve disease who undergo lone MV surgery, spontaneous SR recovery occurs in 10–20% of cases (24, 25).
In our cohort, SR was restored in 14% of patients who underwent surgery without AF treatment, compared to 65% in the maze group.
EFFECT OF RF VS. CRYO ABLATION ON LEFT ATRIAL SIZE AND FUNCTION. The two maze groups showed, from similar preoperatively indexed values, a significant reduction of LA size, with a greater reduction and better preserved atrial function in the RF group.
We tested the hypothesis that Cryo ablation has greater negative impact on atrial function compared to RF, which is supported by the fact that Cryo lesions seem to cause wider damage on the atrial wall (26).
The result might reflect the different breadth of atrial myocardial injury during the ablative procedure between the two energy sources. Direct application of a cooled probe on the atrial epicardium leads to formation of frozen tissue in which trapped cells become irreversibly damaged and are ultimately replaced by fibrous tissue. The width of this area can vary depending on the atrial wall thickness, the time of application, and the temperature achieved. Our Cryo probe reached temperatures of − 150°C. However, currently available devices that cool down to higher temperatures might have the advantage of causing less damage to the surrounding tissues.
The RF lesion produced by tissue heating is smaller and more distinct upon gross examination. The temperature reached is over 50°C, which generates a central zone of coagulative necrosis, with denaturation of most intracellular proteins. These areas develop inflammatory infiltrates, which are replaced by fibrosis (27).
Impaired atrial function could indicate an increased fibrotic burden of the atria, resulting from the larger lesions caused by Cryo ablation, which may irreversibly affect atrial mechanical properties. Kim et al. found a higher atrial contractility restoration rate after Cryo than after RF ablation (28). The authors used however, different lesion set in this retrospective study and atrial contraction recovery was analyzed on two dimensional echocardiographic parameters based only on atrial wave velocity.
In our study, the feasibility of LA acquisition and analysis on 3 D echocardiography was good (90%) and at one year after MV surgery and maze, LA volumes were still more than moderately enlarged. This enlargement likely reflects the structural remodeling process in response to the volume and pressure overload caused by the preoperative duration and severity of mitral disease, as well as the AF burden. Left ventricular diastolic function may impact LA size and function. Preoperative assessment of diastolic function is challenging in the presence of severe mitral regurgitation. Postoperatively, we found no difference in diastolic parameters expressed as E/E´, and the right ventricular systolic pressure was comparable between ablation groups. Furthermore, we found no difference in LA conduit function, which is closely related to passive LV filling and LV compliance (29).
BOOSTER FUNCTION IN NoMaze COMPARED TO MAZE PATIENTS. Compier et al. reported that atrial active transport function is not restored in approximately half of patients with post-procedural SR after concomitant surgical limited LA RF ablation, while it was restored in patients who underwent bare pulmonary vein isolation (30).
In parallel, our present study showed that compared to in ablated patients, non-ablated patients with spontaneous SR recovery showed better LA active contraction recovery, when the regurgitation was corrected, and no atrial wall distress was added by freezing or warming.
Despite the complex geometry and the thin atrial wall, the quantification of atrial myocardial deformation using strain analysis to evaluate phasic atrial function, was shown to be feasible, in line with other studies (31, 32).
However, we did not detect a difference in atrial function, expressed by reservoir and booster strain, between the RF and Cryo ablation groups.
LV function has been reported to improve after mitral surgery with concomitant ablation in patients with impaired function at baseline (33).
Most of our patients had good systolic ventricular function before the procedure, and they maintained comparable systolic function postoperatively.