2.2 Assessment before ablation
Antiarrhythmic drugs, except for AV nodal slowing agents, and amiodarone, were discontinued at least five half-lives prior to ablation. Patients were studied in the fasting, post-absorptive state. A coronary sinus (CS) catheter was inserted through the left subclavian vein, and a multipolar circular right atrial catheter with alternating 2–10–2 mm interelectrode distance was positioned from the right groin, with the tip positioned immediately lateral to the planned position of the isthmus line and anterior to the crista terminalis. Heparin 100 U/kg was given and a further 5 U/kg given if the procedure lasted longer than 180 min.
If the patient was in sinus rhythm, isthmus conduction was confirmed by pacing. If flutter was present, entrainment was performed to confirm isthmus dependence, and the patient left in flutter. If AF was present, the patient was cardioverted after a transoesophageal echocardiogram, and then isthmus conduction confirmed. Absence of isthmus conduction or non-isthmus dependence was not seen in the selected patients. No induction of arrhythmia was attempted if patients had sinus rhythm.
Radiological assessment of the right atrial isthmus was made in a right anterior oblique (RAO) 30°, and a left anterior oblique (LAO) 45° view (each with 40 cc at 18 cc/s). Angiograms were acquired digitally to allow for post-hoc analysis. The treating physician was able to view the angiographic findings to optimize the planned ablation line. The isthmus length was assessed from the inferior hinge point of the tricuspid valve to the IVC at the end of atrial diastole (the frame before opening of the tricuspid valve) [
14]. Morphology was assessed visually as to the presence of a Eustachian valve or a recess, as well as to the general shape i.e. flat or concave.
2.3 Ablation procedure
The catheters were a 9Fr 8 mm tip catheter (FreezorMax, Cryocath Technologies Inc, Kirkland, Canada) with a cryoconsole for the cryoablation group, and a 7Fr 8 mm tip single sensor catheter (EPT Blazer II, Boston Scientific, Natick, MA, USA) with an EPT-1000XP generator for the radiofrequency group. A large curve was initially selected in both groups, with change out of catheter curve during the study only as necessary. Applications of −75°, for 4 min were given with cryothermy, and applications of 60 Watt, for 60 s, targeted at 60°C for RF. Lines were made with discrete applications between the tricuspid valve and the inferior vena cava at an approximately 6 o’clock position in LAO 45°, unless otherwise dictated. If termination of atrial flutter occurred, or if the patient was in sinus rhythm, continuous pacing from the proximal coronary sinus was employed to continually assess isthmus conduction. After the first line, a new assessment of conduction was performed. If conduction over the isthmus remained present, gaps were sought. If there was still isthmus conduction, a slightly more medial or lateral line was made. In no patient was an attempt made to perform a septal ablation line. Final assessment of acute block was confirmed after 30 min waiting.
The end point for successful ablation was induction of complete bidirectional isthmus block, defined as the presence of reversal of activation on the lateral and septal wall when pacing the CS os and low lateral RA, the presence of widely split potentials along the isthmus line, by activation mapping across the isthmus, and by differential pacing. All 4 were required before calling the ablation successful. In the case where bidirectional block was not achieved, ablation was stopped when no large, sharp signals could be identified over a broad area of the isthmus.
As pain perception was assessed, sedation was standardized. Before venous puncture 5 mg of diazepam was given intravenously, and repeated at the patient’s request. Fentanyl 50 μg intravenously was given when the patient requested pain control and the physician considered this necessary. This was repeated as needed. Dosages of both diazepam and fentanyl were recorded.
In the initial 40 patients creatine kinase (CK) and CK-MB were taken before the procedure, 2 and 24 h after the start of the procedure. For the final 22 patients the laboratory had changed the measurement to CK mass. We then modified the protocol to measure CK-MB mass, Troponin T, and Myoglobin at 4 and 24 h after the start of the procedure.
No crossover, other than in catheter curve, was allowed in an attempt to remove any possible bias. Change over to an irrigated tip ablation catheter was also not allowed. In patients in whom no block could be induced, a repeat procedure was scheduled not earlier than 6 weeks after the initial ablation, at the physicians’ discretion. The choice of energy source at that time was at the physicians’ discretion.
Patients were all questioned with regard to pain perception using a visual analogue score, where patients are shown a line from 0 to 10, where 0 is no pain, and 10 is the highest pain level imaginable, and were asked to point to the position on the line where their pain level during ablation was.