High-Speed Visualization of Steam Pops During Radiofrequency Ablation1 (2014)

Atrial fibrillation (AF) affects up to 5 million people in the United States, and as the population continues to age, the incidence is expected to further increase. The overall cost associated with treatment for AF was estimated to be in excess of $6.5 billion dollars annually [1]. Currently, radiofrequency (RF) ablation is routinely used for the treatment of cardiac arrhythmias. In RF ablation, alternating current is applied to heat the myocardial tissues, inducing temperatures of 50 °C or higher that lead to the creation of a myocardial scar which is hopefully therapeutic [2].However, when excessive and/or rapid tissue heating occurs, at temperatures of greater than 50 °C, cellular content may be vaporized and in some cases, an audible steam pop is heard. Although temperatures measured at the electrode tip may be considerably less than 50 °C, steam pop generation can result from high endocardial tissue temperatures. Importantly, it should be considered that temperatures recorded from the catheter tip may not be indicative of the tissue temperatures because convective flow of blood or irrigation cools the electrode tip. Nevertheless, these induced steam pops can be quite damaging; potentially causing perforations of the atrial wall, clot formations on the catheter which could dislodge and form emboli, and/or air bubble generation [3].To visualize the generation of steam pops, porcine Yorkshire Cross animals (n = 14) with weights of 84.5 ± 9.8 kg were reanimated using Visible Heart® methodologies previously described by Chinchoy and colleagues [4]. The tricuspid annuli of these reanimated hearts were aggressively ablated using a 7 Fr Marinr® catheter (Medtronic, Inc., MN) introduced through the superior vena cava and guided via fluoroscopic imaging. The incidences of steam pops were then recorded using a high-speed camera (666 frames/s) and videoscopes from within these hearts.The occurrence of a steam pop causes the ablation catheter to jump away from the lesion site, i.e., due to the explosive generation of gas bubbles (Fig. 1). Interestingly, the typical duration of the pops was less than 10 ms, although in the case shown in Fig. 1, air bubbles were observed to linger for a longer period of time around the lesion site. In this case, the catheter tip temperature reached 76 °C before the explosion, and dropped significantly after because of the lack of wall contact: the void created during the explosion was approximately 0.7 cm in diameter and 0.4 cm deep (Fig. 2).In this series of studies, the relative occurrences of steam pops were 5% at energies of 40–50 W (n = 59), 25% at energies of 50–60 W (n = 73), and 26% at energies of 60–100 W (n = 69) with average catheter tip temperatures of 52 °C, 64 °C, and 65 °C, respectively. Additionally, steam pops happened 63% of the time when power rapidly increased, at a rate of 3 W/s, from 40 W to 100 W (n = 38). Applied power significantly affected steam pop generation (p < 0.01). Steam pops occurred 33.3 ± 17.2 s after the start of the ablation without any significant differences between power settings (p > 0.05).AF is a serious health concern that affects a large number of individuals. The decrease in quality of life and cost associated with the disease are also a significant issue. One of the treatment options, RF ablation, can be accompanied by a relative risk of steam pop occurrences that can in turn cause severe damages to the myocardium. Noteworthy, there is the possibility for complete atrial rupture, which would then lead to cardiac tamponade. Additionally, released gas bubbles and tissue emboli created during the elicitation of such an event may critically lead to myocardial infarction and/or stroke if this occurs on the left side of the heart or pulmonary emboli on the right side. High-speed video was used to visualize the steam pop explosion at a visually perceptible rate.It is important to note that using fluoroscopy alone as an imaging modality is likely difficult for operators to detect the occurrences of steam pops. In other words, the only indications of these events being perhaps the visual displacement of the catheter if fluoroscopy is running at the time of the pop and/or a detected rapid reduction in tip temperatures (Fig. 3). Interestingly, the explosions are not always audible suggesting that operators may have difficulty recognizing the incidence of steam pops while using fluoroscopy alone [5]. We have shown here one example of high-speed footage of a steam pop and the resulting myocardial damage created, i.e., visualized in vitro using Visible Heart® methodologies. This is the first documented instance to the authors' knowledge, of visualization within a beating heart using a videoscope or high-speed camera.We consider here, that this study highlights the difficulty the physicians may encounter in recognizing steam pop incidences. In other words, these events may be occurring unnoticed until the related complications arise. For this reason, developing a database of parameters effecting steam pop formation and a system for steam pop detection requires investigation and is underway by our laboratory. With the increasing prevalence of AF, understanding how to reduce the occurrences of steam pops may have important clinical insights.