Asian Cardiovasc Thorac Ann 2001;9:115-118
© 2001 Asia Publishing EXchange Pte Ltd
Power Versus Temperature-Controlled Ablation of Supraventricular Tachycardia
Tan Ru San, MBBS,
Ruth Kam Ming Li, MBBS,
Teo Wee Siong, MBBS
Department of Cardiology
National Heart Centre
Singapore, Republic of Singapore
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For reprint information contact: Tan Ru San, MBBS Tel: 65 436 7546 Fax: 65 227 3562 email: tan_ru_san{at}nhc.com.sg Department of Cardiology, National Heart Centre, 17 Third Hospital Avenue, Mistri Wing, Singapore 168752, Republic of Singapore.
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Abstract
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Temperature-controlled radiofrequency catheter ablation was prospectively compared with the power-controlled technique in 53 patients with atrioventricular reciprocating tachycardia or atrioventricular nodal reentrant tachycardia. Patients were randomly assigned to either power-controlled (n = 26) or temperature-controlled (n = 27) ablation after electrophysiologic studies. The groups were comparable in terms of mean age (40 ± 16 versus 44 ± 15 years, p = 0.60), sex (54% versus 52% males, p = 0.88), and type of tachycardia (38% versus 52% atrioventricular reciprocating tachycardia, p = 0.91). Successful ablation was achieved in all patients, and the number of radiofrequency applications required were similar. There were no significant differences between groups in mean fluoroscopy time for initial success (2.1 ± 2.3 minutes versus 1.5 ± 1.2 minutes, p = 0.21), for ablative plus booster doses (6.9 ± 4.7 minutes versus 6.0 ± 3.5 minutes, p = 0.42), or for the entire procedure (13.1 ± 6.9 minutes versus 11.6 ± 4.5 minutes, p = 0.35). It was concluded that power-controlled and temperature-controlled methods of radiofrequency ablation were equally efficacious.
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Introduction
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Radiofrequency catheter ablation (RFCA) for either accessory pathway ablation or atrioventricular node modification is a highly effective treatment modality for atrioventricular reciprocating tachycardia (AVRT) and atrioventricular nodal reentrant (AVNRT) tachycardia.1–4 The primary mechanism is thermally mediated myocardial conduction tissue injury that causes altered impulse propagation (irreversible myocardial tissue damage requires heating to approximately 50°C).5,6 In the established power-controlled method, the radiofrequency energy pulse is delivered with manual titration of power output. The recent development of radiofrequency ablation catheters with a temperature-monitoring capability enables energy delivery with temperature regulation. This newer temperature-controlled method has the theoretical advantage of allowing delivery of more energy without excessive heating.7 Several observational studies have underscored the safety and efficacy of temperaturecontrolled RFCA.6,8,9 However, the few available com-parative studies have failed to demonstrate any apparent clinical superiority of temperature-controlled RFCA over power-controlled RFCA.10–12 We undertook a singlecenter prospective randomized comparison of the powercontrolled and temperature-controlled RFCA for AVRT and AVNRT.
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Patients and Methods
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Between June 1997 and January 1998, 53 consecutive patients with AVRT or AVNRT documented on electrophysiologic studies were recruited into the study. Informed consent was obtained from every patient and they were randomly assigned to either power-controlled (group 1) or temperature-controlled RFCA (group 2). A comprehensive diagnostic electrophysiologic study was performed, followed by definitive RFCA therapy during the same session. Indicators of procedural success were predefined. Immediately after successful ablation, an additional 1 or 2 radiofrequency pulses (booster doses) were applied to ensure against recurrence. Procedural complications were actively monitored and recorded. These included vascular access site complications (hema-toma necessitating surgical intervention, arteriovenous fistula, puncture site sepsis), new conduction abnormalities (new first-, second-, or third-degree atrioventricular block, new bundle branch block), and myocardial puncture. The parameters measured were the number of radiofrequency ablations required for initial success, fluoroscopy time for initial success, fluoroscopy time for RFCA plus booster radiofrequency applications, and fluoroscopy time for the entire procedure. The various fluoroscopy times were used to compare the efficacy of the 2 methods because they reflect the ease with which successful ablation was achieved. In addition, they are indicative of the amount of radiation exposure of the patients and operators.
In cases of AVRT, the accessory pathway was mapped to the site of the earliest ventricular activation during sinus rhythm or atrial pacing. In the case of a concealed pathway, the earliest retrograde atrial activation during ventricular pacing or induced reentrant tachycardia was determined. A retrograde aortic approach was used for left-sided accessory pathways. Radiofrequency energy pulses were applied to the accessory pathway for up to 120 seconds at a time. The procedure was successful if AVRT was not inducible 30 minutes after ablation. For persistent ventriculoatrial conduction, selective pharmacological blockade of the atrioventricular node (with intravenous adenosine, verapamil, and/or propranolol) was used to rule out accessory pathway conduction.
In cases of AVNRT, slow atrioventricular node pathways were targeted for ablation. Radiofrequency energy pulses were applied for up to 60 seconds at a time. Junctional beats during radiofrequency application were indicative of an accurate catheter position. The procedure was successful if AVNRT could not be induced despite an increase in the rate of isoprenaline infusion over that required to induce AVNRT at baseline and/or after 1.2 mg atropine intravenously. Residual atrioventricular dual pathways or single echo beats after ablation were permissible.
All RFCA procedures were performed using 7F deflectable quadripolar 4-mm tip catheters (Mansfield-Webster, Watertown, MS, USA). In group 2 subjects, Mansfield-Webster thermocouple ablation catheters were used. These have an additional thermocouple embedded in the tip, which serves to maintain the temperature of the catheter tip at a preset value. Radiofrequency generators from one manufacturer (Radionics, Burlington, MS, USA) were used in both groups. In the power-controlled method (group 1), the initial energy delivered was set at 20 W. This could be continually and incrementally titrated at 5-W intervals up to a maximum of 50 W. The impedance was continuously monitored and radiofrequency delivery was immediately terminated when impedance exceeded 200 ohms. In the temperature-controlled method (group 2), the initial temperature was set at 50°C. The set value could be increased by 5°C or 10°C intervals up to a maximum of 70°C.
All patients were discharged one day after the procedure, and reviewed at a specialist electrophysiology outpatient clinic 2 to 4 weeks later. Repeat electrocardiograms were performed and the patients were screened for symptomatic recurrence of palpitations. Thereafter, they returned to their primary cardiologist or physician for follow-up.
Continuous variables were expressed as mean ± standard deviation and compared using the two-tailed Student's t test. Discrete variables were compared using the chi-squared test. A p value of < 0.05 was considered significant.
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Results
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There were 26 patients in group 1, and 27 in group 2. There were no significant differences between the groups in terms of mean age (40 ± 16 versus 44 ± 15 years, p = 0.60), sex distribution (54% versus 52% males, p = 0.88), or type of tachycardia (38% versus 52% AVRT, p = 0.91). In all patients, ablation of reentrant tachycardia was successful as defined by the predetermined criteria. There was no significant difference between groups 1 and 2 in the number of radiofrequency applications required for initial success. The number of radiofrequency applications required ranged from 1 to 9 in group 1, and from 1 to 11 in group 2; the median number in either group was 2. The fluoroscopy times for initial success, for radiofrequency ablation and booster doses, and for the entire procedure are shown in Table 1
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Table 1. Fluoroscopy Times in Power-Controlled (Group 1) and Temperature-Controlled (Group 2) Radiofrequency Catheter Ablation
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No major procedural complication was recorded in any of the study subjects during follow-up. All patients had an uneventful postprocedural recovery and were discharged from hospital the next day. At the first outpatient electrophysiology review, none of the patients reported symptomatic recurrence of palpitations.
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Discussion
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The premise of temperature monitoring during RFCA is that it enhances the precision of energy delivery, allowing accurate modulation of the amount of thermally mediated injury. Delivery of power may be safely prolonged, thus optimizing lesion formation while minimizing the complications of overheating. However, the true relation-ship between catheter tip temperature and therapeutic efficacy is not so straightforward. In in-vitro experiments using porcine ventricle, Kongsgaard and colleagues13 demonstrated significantly higher temperatures at tissue level compared to the catheter tip readings. This disparity increased with the duration of contact, the flow passing the catheter site, and the length of the tip electrode, and it depended on the catheter tip orientation with respect to the endocardium. Furthermore, the application of a radiofrequency pulse may cause a tissue temperature rise that persists after cessation of the pulse. This thermal latency may account for the discrepancy between eventual lesion size and the preset catheter tip temperature.14 Superior clinical efficacy of temperature-controlled ablation over the established power-controlled method remains unproved. Although small-scale in-vivo studies showed that closed-loop temperature control significantly reduced the incidence of coagulum formation during RFCA, the outcome in terms of procedural success was unaffected.10,15
Because of the prospective nature of this study, patients with either AVRT or AVNRT were included, reflecting the clinical situation in which electrophysiologic diagnosis is often attained at the same sitting as definitive therapeutic intervention with RFCA. Power-controlled and temperature-controlled RFCA were successful, safe, and comparable in performance. Short-term follow-up revealed uniformly favorable results with no symptomatic recurrence in either group at 2 to 4 weeks after RFCA. This was not unexpected given the acknowledged efficacy of this therapy in such patients. Long-term clinical follow-up was not addressed in this study, but it would be interesting to know whether the clinical outcomes diverge with longer follow-up.
The findings in this study are consistent with previous observations (Table 2). Strickberger and colleagues11 found no significant advantage with temperature monitoring during RFCA of AVRT in their prospective study of 132 patients. Kavesh and colleagues12 compared detailed electrophysiologic parameters and acute and long-term results of two groups of nonrandomized subjects who had undergone power-controlled or temperature-controlled RFCA of AVNRT. Although both methods gave equally excellent clinical results, temperature monitoring resulted in better elimination of dual atrioventricular pathway function: a subtle physiologic, albeit clinically inapparent, difference. The inability to demonstrate any incremental clinically meaningful benefit with temperature control may be due to the consistently excellent outcomes of RFCA, regardless of temperature monitoring. To determine conclusively whether there is a significant difference, a very large study population would be needed, which is beyond the scope of a single-center trial. In spite of this limitation, the prospective nature of this study is an advantage, and the accrued information may be combined with other published data in meta-analysis.
Although this study and other similar series have not demonstrated any significant clinical advantage of temperature control, this technique remains valuable. The ability to achieve the required temperature with relatively low power output implies good tissue contact, which is an important determinant of procedural success. On the other hand, a high and constant temperature reading with very low power output may indicate coagulum formation at the catheter tip. Whether a subtle difference in electrophysiologic parameters after temperature-controlled ablation, such as that detected by Kavesh and colleagues,12 will translate into better long-term clinical efficacy remains to be seen.
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References
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Abstract
Introduction
