To realize the epicardial maze procedure, the major drawback is how to make the transmural lesion on the beating atrial free wall under the condition of existence of inner warm blood flow which weakens heating/cooling effect of the ablation device.
Nath et al. demonstrated that hyperthermia induced by radiofrequency energy causes significant changes in the electrophysiological properties of myocardiocytes, including membrane depolarization, reversible and irreversible loss of excitability, and abnormal automaticity, in an in vitro isolated guinea pig right ventricular papillary muscle model. They observed reversible loss of cellular excitability and tissue injury after exposure to temperatures in the 42.7 °C to 51.3 °C range (median, 48.0 °C) for 60 s and irreversible loss of cellular excitability and tissue injury after exposure to temperatures > 50 °C for 60 seconds [1].
Bulava et al. assessed the efficacy of epicardially created lesions produced with bipolar radiofrequency (RF) energy in 70 patients who had persistent, longstanding atrial fibrillation [2] and reported achieving complete isolation of the posterior left atrial wall in only 22.9% of the patients. The success rates for creating conduction block across the inferior pulmonary veins (PVs) and across the roof line connecting the two superior PVs were only 58.0 and 24.3%, respectively. Right PVs were found to have been isolated in a significantly higher proportion of patients (91.4%) than the left PVs (75.7%) were. The low efficacy of epicardial RF ablations in creating a transmural irreversible electrophysiological block, especially on the “unclampable free wall” of the atrium, represent that heat sinking effect of the inner blood flow that weaken the thermal effect of the RF.
We previously reported the fundamental results of using an infrared coagulator in animal models [3,4,5]. The results of the series of experiments in animal models revealed that an infrared coagulator enables creation of a transmural lesion on the canine beating right ventricle to a maximum depth of 10.3 mm, a conduction block on an arrested heart, and a conduction block on a beating right atrium. A successful epicardial maze procedure and successful electrical isolation of the right atrial appendage with an infrared coagulator and concomitant on-pump beating coronary artery bypass grafting have been reported in a clinical case [6, 7].
A cuboid probe was newly developed to create a linear lesion and conduction block on the atrial free wall. To demonstrate the conduction block on the beating heart, encircled lesions e.g. the both atrial appendage, PV cuffs, or box lesion are easy because the ineffective overdrive pacing from inside the lesion can prove the exit block. By contrast, it is difficult to demonstrate a conduction block on a beating atrial free wall. Intraoperative epicardial mapping with multi-electrodes is in common use, but it requires a large-scaled mapping system. Furthermore, because the multi-electrodes are just placed on the epicardium, not attached to it, they slip, and it is difficult to be estimate the exact electrical conduction time. A polygraph with two channels was used in our patient, and it was possible to measure the exact conduction time in every heart beats. We hypothesized that an increase in the translesion stimulus-excitation delay indicates a continuous, transmural, linear lesion, and that the translesion stimulus-excitation delay is directly proportional to path length in the viable tissue around the lesion. Himel et al. demonstrated that complete lesions with RF in rabbit hearts increase translesion stimulus-excitation delay, whereas incomplete lesions do not increase the delay [8]. As far as we investigated, there is no report that proved the stimulus excitation delay on the ablated epicardial atrial free wall in a clinical case. The right atrial free wall was selected for examination. It is easy to apply the cuboid-shaped probe to the right atrial free wall, and it was thought that it would be more difficult to make a transmural lesion in the right atrium than in the left atrium, because the right atrium has a complicated inner structure due to its thick trabecular muscle.
In our patient the myocardium was ablated intermittently at 2-s intervals. Intermittent ablations are more effective than long continuous ablations, because the intervals prevent rapid temperature rise, and prevent to make charring that blocks the photo-energy radiation deep inside the myocardium.
The actual conduction prolongation ratios in the present study were smaller than the predicted ratios, and the main reason for the smaller ratios is thought to be that it was difficult to determine the exact distance between the ablation line and alligator clip electrodes. To determine the more acculate distance, using small bipolar needle electrodes is better. Marginal non-transmural lesions and atrial tissue shrinkage are considered to be other factors that affect conduction time; both factors may shorten the conduction distance and reduce the prolongation ratio.
Pathological examination of the tissue obtained from our patient confirmed the presence of transmural degeneration, however, it was not homogeneous. It may represent the heat sinking effect of the inner blood flow. Considering the result of the presented electrophysiologocal study, there may be a discrepancy between the histopathological transmurality of the lesion and its electrophysiological transmurality. The histopathological change may not exactly represent the electrophysiological change but instead underestimate it by overlooking the pathologically normal but electrophysiologically remodeled lesion of the myocardium. Chronic reversibility of the prolonged conduction time was not investigated in this study. However, in a previous animal study we verified hemosiderin deposition, macrophage invasion, increased capillary vessels, and increased juvenile elastic fibers in the right atrial free wall 3 months after ablation. The myocardium did not revive, the endocardium became thickened, and elastic fibers appeared.
Theoretically, same as RF, this new technology can be applied not only to the atrium but also to the ventricle. Modifying the shape and flexibility of the probe may enable safe and effective minimally invasive endoscopic ablation to create a box lesion on the beating left atrium and to treat ventricular tachycardia based on the same tissue photocoagulation principle as described above.