Intramural Radiofrequency Ablation:. Effects of Electrode Temperature and Length

Radiofrequency ablation in snoring surgery: local tissue effects and safety measures

Radiofrequency ablation is now widely used in the treatment of patients with snoring and obstructive sleep apnoea. It is well suited to the multilevel approach typically required in snoring surgery and can be used interstitially or in cutting mode. However, no research has been performed to analyse the collateral local tissue damage or size of lesions induced by radiofrequency application. This is the first study of the histological effects of interstitial radiofrequency. We studied the size of tissue lesions generated at differing power settings with and without local anaesthetic application in human tonsils and chicken breast tissue. The size of the lesion was independent of power setting and local anaesthetic infiltration. The typical effect was an oval-shaped lesion centred on the active electrode, 6-7 mm in width and 7-8 mm in length. This was confirmed histopathologically, with smaller lesions generated in the submucosa (2 × 3 mm) but with no collateral tissue damage beyond this distinct zone of injury. To avoid complications, we recommend placing contiguous radiofrequency applications at least 8 mm apart. Post-nasal space packing raises and tautens the soft palate. The use of local anaesthetic increases interstitial volume, thereby reducing the risk of incorrect probe placement and resultant ulceration or fistulation.

Direct thermography-a new in vitro method to characterize temperature kinetics of ablation catheters

PURPOSE

For the treatment of increasingly complex cardiac arrhythmias, new catheter designs as well as alternative energy sources are constantly being developed. However, there is presently no in vitro method available for assessment of the temperature changes induced at various myocardial levels during energy delivery. Therefore, our study was aimed at developing an in vitro model to record and display the temperature kinetics during ablation in the entire muscle cross section.

METHODS AND RESULTS

A sapphire glass pane was inserted into one wall of the in vitro experimental set-up. Due to its thermodynamic properties, the temperature distribution in an adjacent cross section of the cardiac muscle can be measured exactly ( 1 °C) through this pane by means of a thermography camera. Computer-supported image processing enables the colour-coded and two-dimensional display of the temperature kinetics during the energy application at any location of the myocardial cross section (± 0.5 mm). This new measuring methodology was validated by direct temperature measurements utilizing several intramyocardial thermo elements.

CONCLUSION

This new method allows a temporal and spatial analysis of the temperature phenomena during ablation without the interference and spatial limitation of intramyocardial temperature probes. New ablation technologies can thus be evaluated, independent of the catheter configuration or source of energy used.

Comparison of unipolar versus bipolar ablation and single electrode control versus simultaneous multielectrode temperature control

BACKGROUND

Creation of linear lesions using multielectrode catheters may be effective at treating cardiac arrhythmias.

OBJECTIVE

We compared unipolar versus bipolar ablation, evaluated the effects of varying effective electrode areas, and compared single electrode versus multielectrode temperature control during multielectrode radiofrequency ablation.

METHODS

Intramural radiofrequency ablation was performed on five greyhounds at thoracotomy, from an epicardial approach using a 0.8 mm diameter bipolar electrode needle. Fifteen left ventricular ablations were performed per animal. Intramural ablation was performed to maintain a constant electrode-tissue interface. The distal and proximal electrodes measured 1.5 and 1.0 mm in length respectively with an interelectrode distance of 4 mm. Radiofrequency energy was applied to both electrodes simultaneously for 60 s using a target temperature of 80 degrees C. During bipolar ablation, the temperature was regulated from either the distal (BPA1.5) or proximal (BPA1.0) electrode only. During unipolar ablation (UPA), the temperature at both electrodes were simultaneously controlled. Lesions were assessed histologically.

RESULTS

During UPA, consistent target temperatures were achieved at both electrodes. In comparison to UPA, the temperature at both electrodes were significantly decreased during BPA1.0. During BPA1.5 a significant (p < 0.001) temperature increase (94.7 +/- 2.1 degrees C) was observed at the 1.0 mm electrode. BPA1.0 resulted in reduced (p = 0.008) lesion width at the 1.5 mm electrode and no change in lesion depth (p = 0.064) at both electrodes compared to UPA. Conversely, lesion dimensions increase significantly at both electrodes during BPA1.5.

CONCLUSION

Unipolar multielectrode ablation with simultaneous temperature control at both electrodes is more predictable and hence likely to be safer than bipolar ablation.

Cardiology at Westmead Hospital from 1990 to 2007

Professor John Uther was the Director of Cardiology at Westmead Hospital from 1979 to 1990. Professor David Ross and Dr Pramesh Kovoor followed in this capacity subsequently. Networking between Westmead and metropolitan hospitals was established by conjoint appointment of cardiologists across the facilities. Westmead has maintained its excellence in electrophysiology with leadership in operative/catheter ablation of atrial fibrillation, development of catheter for mapping tricuspid annulus, multi-electrode mapping and intramural ablation of ventricular tachycardia and paediatric electrophysiology. Dr. Hugh Paterson became the Director of Cardiothoracic Surgery in 2006. The previous Directors were Dr. David Johnson, Dr. Graham Nunn and Associate Professor Richard Chard. Westmead established an area-wide acute infarct angioplasty service for all patients presenting to any facility in Western Sydney along with triage of chest pain in the ambulance in 2004. Collaborative sessions with vascular surgeons for non-coronary interventions commenced in 2005. In the future, Westmead will continue its excellence in vascular and electrophysiological interventions. Imaging (echocardiography, computerised tomography and magnetic resonance imaging) will be a major part of the service. Innovation in basic science is likely. Overall, it will be an exciting time to be a cardiologist, vascular surgeon or cardiothoracic surgeon at Westmead.

Surgery and ablative therapy for ventricular tachycardia

In the absence of acute ischaemia, ventricular tachycardia (VT) is the most common arrhythmia leading to cardiac arrest and death. This paper will describe the history of research into VT and the therapies that evolved. The contributions of John Uther and other members of the Department of Cardiology at Westmead Hospital will be outlined and placed into perspective.

Epicardial catheter mapping and ablation of ventricular tachycardia

Percutaneous entry into the pericardial space allows epicardial catheter mapping and ablation in the electrophysiology laboratory, opening a perspective on cardiac electrophysiology that previously was limited largely to the operating room. Scar-related reentry after myocardial infarction usually involves the subendocardium but in some patients can only be ablated from the epicardium. Epicardial, scar-related reentry also is an important cause of ventricular tachycardia in nonischemic cardiomyopathies. Rare supraventricular tachycardias and idiopathic ventricular tachycardia that cannot be defined from the endocardium sometimes can be ablated from the epicardium. With appropriate precautions the procedural risks are low. Epicardial catheter techniques expand the options for investigating cardiac electrophysiology and treating arrhythmias in humans and may lead to insights into transmural properties influencing repolarization and the genesis of arrhythmias.

Comparison of radiofrequency ablation in normal versus scarred myocardium

INTRODUCTION

Reentrant circuits causing ventricular tachycardia are closely associated with previously scarred myocardium. The presence of scar has been blamed for the poor success rate of radiofrequency ablation (RFA) in that context. This article investigates the in vivo effects of radiofrequency ablation in myocardium scarred from acute myocardial infarction.

METHODS AND RESULTS

Anterior myocardial infarction was induced in five dogs by ligating the left anterior descending artery. The mean left ventricular ejection fraction after infarction was 38%. At a mean of 15 weeks following myocardial infarction, 50 RFA lesions were created in random order, 25 in scarred and 25 in normal myocardium using a needle electrode (21 gauge, 5 mm in length) introduced from the epicardium of the left ventricle at thoracotomy. During unipolar temperature-controlled RFA (90 degrees C for 60 seconds), intramural temperatures were measured by thermistors at distances of 1, 2, 3, 4, and 5 mm from the ablating electrode. The margins of the lesions were clearly discernible in scar at histological examination in 64% of ablations where the scarring was patchy. There were no significant differences between lesion sizes, intramural temperatures at different distances, total energy required for ablation, or mean impedance during ablation of normal versus scarred myocardium.

CONCLUSIONS

Scar does not affect lesion size or intramural temperature profile during RFA if electrode size, tissue contact, and tip temperature are controlled. More radiofrequency energy is not required to maintain tip temperature at 90 degrees C in scar compared to normal myocardium.

Change in Size of Lesions Over 3 Weeks After Radiofrequency Ablation of Left Ventricle

INTRODUCTION

The initial success or failure of radiofrequency ablation (RFA) does not always reflect the long-term outcome that can lead to complications such as late atrioventricular block or recurrence of accessory pathways. We hypothesize that these occurrences may be due to a change in lesion size over time.

METHODS AND RESULTS

Intramural RFAs were performed on five greyhounds at thoracotomy using an epicardial approach into the left ventricular (LV) wall. Twenty-one gauge needle electrode ablations were created in the anterior aspect of the left ventricle. Radiofrequency energy was delivered at 600 Hz for 60 seconds and at an electrode temperature of 90 degrees C. Eight ablations were created in each greyhound and the chest was closed. After 3 weeks, a further eight ablations were created under the same conditions in the lateral aspect of the LV, ensuring they were well away from the chronic lesions, and the dogs were sacrificed an hour later. All lesions were removed, stained with Gomori Trichrome and measured. There was no significant difference in lesion size detected in the 1-hour-old lesions compared with 3-week-old lesions. Acute lesions were well demarcated by an area of fibrous scar and a central necrotic region. Chronic lesions showed chronic inflammatory cells and strands of collagen.

CONCLUSIONS

This study shows no change in lesion dimension over time and hence a change in size may not contribute to a change in RFA outcome over time.

Effect of inter-electrode distance on bipolar intramural radiofrequency ablation

OBJECTIVES

We aimed at evaluating bipolar radiofrequency ablation by correlating inter-electrode distance (ILD) with lesion dimensions and continuity.

BACKGROUND

Previous reports indicated that bipolar radiofrequency (RF) current applied to two adjacent sites in vitro, synergistically increased lesion sizes greater than that observed for unipolar RF current delivery using the same electrodes.

METHODS

Ablations were performed intramurally to ensure that each electrode surface (radius = 0.4 mm, area = 3.52 mm(2)) provided consistent contact with the myocardium. Ninety-six ablations were performed in four greyhounds using bipolar ablation needles with ILDs of 1, 2, 3, and 4 mm. An epicardial approach was used to ensure accurate positioning of the needles within the myocardium. Lesions were created using temperature-controlled RF delivery for a duration of 60 seconds to achieve 90 degrees C at the electrode proximal to the needle base. Lesion dimensions were determined histologically.

RESULTS

Increasing the ILD, decreased lesion width (P = 0.003) but increased lesion depth (P = 0.001). Lesions remained continuous with ILDs of 1-3 mm but became discontinuous at 4 mm. Energy requirements during ablation increased with increasing ILDs.

CONCLUSION

Using the above parameters (electrode radius, RF power delivery, time) during bipolar ablation, lesion continuity was critically dependent on the ILD. The maximum ILD threshold to create contiguous overlapping lesions was 3 mm. Lesions of greater width were created using shorter ILDs. Clinically, greater control over lesion dimensions can be obtained by manipulating the ILD distance.

Cooled Needle Catheter Ablation Creates Deeper and Wider Lesions Than Irrigated Tip Catheter Ablation

OBJECTIVES

To design and test a catheter that could create deeper ablation lesions.

BACKGROUND

Endocardial radiofrequency (RF) ablation is unable to reliably create transmural ventricular lesions. We designed an intramural needle ablation catheter with an internally cooled 1.1-mm diameter straight needle that could be advanced up to 14 mm into the myocardium. The prototype catheter was compared with an irrigated tip ablation catheter.

METHODS

Ablation lesions were created under general anesthesia in 14 male sheep (weight 44 +/- 7.3 kg) with fluoroscopic guidance. Each of the catheters was used to create two ablation lesions at randomly allocated positions within the left ventricle. The irrigation rate, target temperature, and maximum power were: 20 mL/min, 85 degrees C, 50 W for the intramural needle catheter and 20 mL/min, 50 degrees C, 50 W for the irrigated tip catheter, respectively. All ablations were performed for 2 minutes. After the last ablation, blue tetrazolium (12.5 mg/kg) was infused intravenously. The heart was removed via a left thoracotomy after monitoring the sheep for one hour.

RESULTS

There was no evidence of cardiac tamponade in any sheep. The intramural needle catheter lesions were significantly wider (10.9 +/- 2.8 mm vs 10.1 +/- 2.4 mm, P = 0.01), deeper (9.6 +/- 2.0 mm vs 7.0 +/- 1.3 mm, P = 0.01), and more likely to be transmural (38% vs 0%, P = 0.03).

CONCLUSIONS

Cooled intramural needle ablation creates lesions that are significantly deeper and wider than endocardial RF ablation using an irrigated tip catheter in sheep hearts. This technology may be useful in treating ventricular tachycardia resistant to conventional ablation techniques.