INTRODUCTION

Percutaneous epicardial catheter ablation is being increasingly utilized for treatment of cardiac arrhythmias. Cooled-tip radiofrequency (RF) ablation creates larger epicardial lesions than standard RF, particularly when ablating areas with overlying epicardial fat.¹ Furthermore, the absence of blood flow in the pericardial space can lead to premature heating of the catheter tip, thereby limiting delivery of power.² Historically, during epicardial ablation, most operators have used conventional endocardial irrigation flow rates (IFRs), ranging from 13–30 mL/min. However, ablation at high IFR (≥ 10 mL/min) may not be necessary for delivery of optimal epicardial RF applications. Moreover, this approach can result in significant intrapericardial fluid (IPF) accumulation not only requiring continuous drainage, but it may adversely impact RF lesion formation. As steam pop and coagulum during epicardial ablation are believed to be of minimal clinical concern, reduced IFRs may instead be used within this space to perform high-energy ablation. In this chapter, we will review ex vivo and in vivo lesion characteristics, lesion formation, and ablation outcomes during cooled-tip epicardial RF catheter ablation as related to variations in IFR and IPF.

IMPACT OF IRRIGATION FLOW RATES

Prior ex vivo and in vivo studies have illustrated the benefits of saline-irrigated versus non-irrigated RF catheter ablation, as a result of reduction/elimination of impedance rises during RF delivery despite near-boiling temperatures at the electrode–myocardial interface, higher RF energy deposition in the underlying myocardium and attenuation/eradication of coagulum at the electrode tip and also steam pops.^3–5 Furthermore, the temperature profile at the tip of the electrode has been shown to vary inversely as a function of IFR without significant impact on the maximum temperature at the catheter–tissue interface.^6,7 Yet certain biophysical differences between epicardial and endocardial ablation may differentiate RF ablation inside the normally fluid-free pericardial space from the endocardium.

The authors previously conducted a study to examine the impact of IFR and IPF accumulation during epicardial RF catheter ablation.⁸ Altogether, 452 ex vivo unipolar RF applications were delivered to the epicardial surface of bovine ventricular myocardium in a tissue bath with fixed conductivity (6.5 ± 0.4 mS/cm), temperature (37 ± 1° C), contact force (10 g), and duration (60 seconds) at low (≤ 3 mL/min), reduced (5–7 mL/min) or high (≥ 10 mL/min) IFRs, using intermediate (25–35 W), or high (35–45 W) power with a variety of open-irrigated RF ablation catheters (Figure 40.1). Specifically, 10 g of contact force was selected based on reports^9,10 indicating that epicardial contact force in the range of 8–10 g is often accompanied by optimal catheter vector orientation (orientation directed toward the myocardium) and that forces in excess of this may be associated with inadequate catheter vector. Consistent with prior reports,¹¹ the lesion surface diameter correlated well with IFR (P = 0.03) such that they were in general larger with low and reduced IFR as compared to high IFR (Table 40.1). However, the maximum lesion diameters and depths were relatively similar when comparing low, reduced, and high IFRs for all the tested ablation catheter and power setting combinations. Similarly, the maximum lesion diameter (P = 0.22) and depth (P = 0.50) versus IFR were nonsignificant when examining the results from various ablation catheters and power settings (Figure 40.2). Although lesion volume did significantly vary with IFR for most of the catheter and power setting combinations, the overall trend for lesion volume versus IFR was again nonsignificant (P = 0.19). This was observed because in some cases the lesion volume was largest with low IFR and in other instances largest with reduced IFR, whereas high IFR yielded either similar or smaller lesion volumes. Meanwhile, no charring was observed during any of the ex vivo applications. But as expected, the incidence of steam pops was overall observed to be greater with high versus intermediate power and with the use of low IFR (≤ 3 mL/min). Having said that, the incidence of tissue disruption did not vary by IFR (P = 0.52).

IMPACT OF IPF

The authors also examined the role of IPF on epicardial RF ablation in an in vivo study.⁸ After obtaining percutaneous subxiphoid epicardial access using a conventional approach,¹² ventricular RF applications were delivered epicardially in healthy adult swine using fixed contact force (9 ± 2 g), power (39 ± 1 W) and duration (60 seconds), using reduced (5 mL/min) or high (15 mL/min) IFRs, in the presence or absence of IPF. Specifically, in one group of swine, epicardial RF applications were delivered in presence of 125 mL of normal saline (IPF group) introduced directly into the pericardium, whereas in the other group the same was performed in the absence of IPF (Figure 40.3). Attempts were made particularly to drain the irrigated saline infused during RF ablation from the pericardial space in the group without IPF (no-IPF group). Briefly, no steam pops or coagulum were detected. Also, no differences in impedance drop during ablation using 5 mL/min versus 15 mL/min (19 ± 5% vs. 18 ± 6%; P = 0.46) were observed. While there was a trend, there was no significant difference in the impedance drop when comparing applications delivered in presence or absence of IPF (17 ± 6% vs. 19± 6%; P = 0.16). Lesion analysis (Table 40.2) once again revealed no significant differences in lesion size as a function of IFR (5 vs. 15 mL/min). Figure 40.4 depicts correlation plots and 95% confidence interval curves for in vivo epicardial lesion dimensions as a function of IFR. As seen, none of these correlation coefficients or P values reached significance. On the other hand, when using a constant force, power, and duration, all lesion dimensions, including surface diameter, maximum diameter, depth, and volume, were found to be significantly smaller in presence versus absence of IPF (Table 40.3).

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