Evaluation of an Unipolar RF Coagulation System for Epicardial AF Ablation in Chronic GLP Canine Models

DOI: 10.19102/icrm.2012.031206

^1,2H. EDWARD GARRETT, JR., MD, ²CHARLOTTE PORTER, RN, ³JAMES D. FONGER, MD, ⁴MIKE BRAVO, SRS and ⁴ROY MARTIN, DVM

¹University of Tennessee, Memphis, TN
²Cardiovascular Surgery Clinic, PLLC, Memphis, TN
³Saint Joseph's Translational Research Institute (SJTRI), Atlanta, GA
⁴The Integra Group, Brooklyn Park, MN

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ABSTRACT. Background: The Cox Maze III procedure has become the gold standard for the surgical treatment of atrial fibrillation, but cardiac arrest during cardiopulmonary bypass is required to create atrial incisions, which excludes a minimally invasive approach. The ability of radiofrequency technology to create transmural epicardial lesions on a beating heart during chronic good laboratory practice (GLP) canine studies with 1 week of recovery is reported. Methods: Six cardiac lesions were created in 10 canine subjects with a total of 60 lesions using an epicardial vacuum-irrigated coagulation device during two GLP preclinical studies performed at two sites. Epicardial lesions were created on the left atrial appendage, right atrial appendage, and right ventricle, accessed through a left thoracotomy. One week after surgery, the subjects were euthanized and histologic analysis performed. Results: No complications were observed during the coagulation procedures and recuperative period. All 33 atrial lesions were transmural by triphenyltetrazolium and hematoxylin and eosin staining. Seventeen of 26 ventricular lesions were transmural. The ventricular lesions measured an average±standard deviation length of 32.7±4.3 mm, width of 14.7±2.4 mm, and depth of 6.5±1.1 mm. Conclusion: Use of this epicardial coagulation system resulted in effective focal transmural ablation lesions on a beating heart without complication.

KEYWORDS. epicardial ablation, good laboratory practice study, transmurality.

Dr. Garrett reports he is a consultant for nContact, Inc. Dr. Fonger reports he currently serves on the Clinical Events Committee for IDE clinical trials being conducted by nContact, Inc. All other authors report no conflicts of interest for the published content. Financial support for study conduct was provided by nContact, Inc.

Address correspondence to: H. Edward Garrett, Jr., MD, Cardiovascular Surgery Clinic, PLLC, 6029 Walnut Grove Rd., Suite 401, Memphis, Tennessee, 38120. E-mail: egarrettmd@cvsclinic.com

Introduction

Atrial fibrillation (AF) is a common supraventricular arrhythmia affecting approximately 5 million Americans, resulting in significant risk of congestive heart failure and cerebrovascular accident.^1–3 AF is an independent predictor of mortality following coronary artery bypass and mitral valve repair in the general population.^4–9

The surgical Maze procedure was introduced for treatment of AF in 1987 by Dr. James Cox.¹⁰ The Maze procedure was designed to interrupt all macro-re-entrant circuits by creating incisions to electrically isolate the pulmonary veins and the posterior left atrium. The current version of this procedure, Cox Maze III, has become the gold standard for the surgical treatment of AF. Recent reports demonstrate that over 90% of patients who received this procedure are free from symptomatic AF at 1-year surveillance.¹¹

However, surgical Maze procedures have not gained widespread adoption because of their complexity and invasiveness. In an attempt to achieve the results of the open surgical procedure with a minimally invasive approach, epicardial radiofrequency (RF) ablation has been used to create electrical isolation on a beating heart.

RF ablation has been performed with both unipolar and bipolar delivery systems. Historically, the use of unipolar RF energy has not resulted in consistent transmural lesions when applied to the epicardial surface of the beating heart.¹² In contrast, the use of bipolar RF clamps has been suggested as more effective for the production of transmural lesions,⁷ primarily due to the improvement in contact pressure between clamp electrodes and tissue surface. However, the requirement of embedding tissue between the jaws of a bipolar clamp device limits its utility, especially during minimally invasive beating heart procedures. Additionally, electrophysiologic mapping of patients having recurrent atrial tachycardias after bilateral mini-thoracotomy AF surgery demonstrated bipolar energy delivery through clamps was associated with a 50% rate of pulmonary vein (PV) reconnection.¹¹

A unique unipolar RF ablation device has been developed by nContact, Inc (Morrisville, NC) which features a low-profile, flexible RF electrode configuration that provides consistent, directional contact with the tissue through constant suction delivered through the probe. In addition, the probe is continuously irrigated with saline to prevent the lateral spread of thermal injury. Saline perfusion is also used to visibly confirm tissue contact since saline only flows when a vacuum seal is obtained. Tissue impedance is measured to regulate energy delivery to prevent excessive desiccation or vaporization of tissue.

The purpose of these good laboratory practice (GLP) studies was to investigate the safety and efficacy of this unipolar ablation device when applied to the epicardial surface of a normal beating heart in chronic canine models.

Materials and methods

Ten healthy dogs weighing 20–27 kg were enrolled in two GLP preclinical studies at two sites. All animals received humane care under the supervision of attending veterinarians, in accordance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health (NIH publication no. 85–23, revised 1985).

In the study conducted at Saint Joseph's Translational Research Institute (SJTRI), Atlanta, GA, a facility fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC), the animals were sedated with intravenous (IV) thiopental sodium (10 mg/kg). In the study conducted at Integra, a USDA registered facility, the animals were sedated by an intramuscular (IM) injection of acepromazine (0.05 mg/kg or to effect) and buprenorphine (0.01 mg/kg or to effect) and anesthetized by IV injection of propofol (2–6 mg/kg or to effect). After endotracheal intubation, a surgical plane of anesthesia was maintained with isoflurane through a vaporizer with oxygen in a rebreathing gas anesthesia machine.

The heart was exposed through a left thoracotomy. Using the nContact RF ablation system (Figure 1) lesions were created on the left atrium, right atrium, and right ventricle. Location and details for the test device application are summarized in Table 1. In the first subject, 216A, an attempt was made to create a lesion on the left ventricle, resulting in ventricular fibrillation after the third application to the same position on the left ventricle totaling 95 s. Post-mortem examination did not reveal any coronary stenosis or occlusion, although the lesion was near a coronary artery. This lesion was not included in the data analysis. Cardiac resuscitation was successfully performed and the procedure was continued. Left ventricular ablation lesions were not created for any of the other subjects at either site to avoid ventricular fibrillation. Preset RF generator settings were used to create all the lesions in the study at both sites, as well as suction for attachment of the device to the ventricle and saline irrigation through the probe. When a 1-cm device was used, 15 watts was delivered for 120 s. With a 3-cm device, 30 watts was delivered for 90 s. With the 5-cm probe, 60 watts was delivered for 60 s. Standard ablation protocols for lesion creation were followed at both sites. The depth of fat on the tissue was not measured since locations with minimal or no fat on the atria and ventricle were selected for lesion creation. After the ablation procedures were performed, the thoracic cavities were closed in routine fashion and all animals were recovered from anesthesia.

One week after the epicardial ablation procedure, the animals were euthanized and macroscopic examination was performed. The thorax was examined for any cardiac, mediastinal, or pulmonary lesions. At the SJTRI study site the heart was excised, perfused with triphenyltetrazolium chloride (TTC), and immersed in TTC solution for 40 min. At the Integra study site the heart was excised, rinsed with saline, and immersed in TTC for a minimum of 20 min. The length and width of each coagulation lesion were measured. Each lesion was cut transmurally at four sites, and the depth of each lesion was measured. The entire heart was then immersed in formalin for histologic analysis. For each lesion, all four sections were embedded in paraffin, sectioned at 4–5 μm, and stained with hematoxylin and eosin (H&E) or Masson's trichrome. At the Integra study site, sections were stained with both H&E and Masson's trichrome. To be scored as histologically transmural, the lesion had to include contiguous sections of coagulated epicardium, endocardium, and intervening myocardium.

Results

All lesions were evaluated macroscopically with TTC staining and microscopically with H&E or Masson's trichrome staining after 1 week of reperfusion and washout of any mitochondrial enzymes that could erroneously signal viable tissue. The size of the coagulation lesions were measured after immersion in TTC solution (Table 1). The extent of transmurality was assessed by histologic staining. All atrial lesions were transmural (Figure 2) with an average±standard deviation length of 34.7±5.5 mm, width 9.7±2.4 mm, and depth 2.9±1.2 mm (Table 2). Seventeen of the 26 ventricular lesions (67%) were transmural. The ventricular lesions measured an average±standard deviation length of 32.7±4.3 mm, width of 14.7±2.4 mm, and depth of 6.5±1.1 mm (Table 3). The average depth of the non-transmural ventricular lesion was also 6.5 mm.

Histologic analysis showed the classic lesion of focal coagulative necrosis as a consistent finding in all ablated tissue segments. Although the coagulated myocardium retained basic fiber morphology, the cells were eosinophilic and hyalinized. The nuclei of the coagulated cells were either small and shrunken or fragmented, which are the characteristic findings of pyknosis and karyorrhexis as seen in coagulation necrosis. A fibrous tissue reaction was consistently observed at the margins of the coagulated myocardium. The fibrous tissue was seen as variably wide bands that separated the coagulated myocardium from non-coagulated (normal and viable) myocardium (Figure 3).

The 10 animals in the two studies tolerated the coagulation procedure, and survived until scheduled euthanasia without any complications. At necropsy, a coagulation lesion was identified on the left ventricular wall right behind the left atrial appendage (LAA) in subjects 217A and 218A. Since the coagulation device was not applied on the left ventricle, it was suspected that the lesions resulted from application of the device on the LAA. No other abnormalities were identified in the heart or in the surrounding tissues.

Discussion

During the past decade, the technology for catheter and surgical ablation of AF has evolved rapidly. The attempts to reduce the invasiveness of AF treatment from the original highly invasive cut and sew Cox Maze III procedure has replaced incisions with devices capable of rendering viable tissue unable to propagate electrical impulses through linear thermal ablation lesions.

With respect to ablation devices, unipolar RF ablation has not been consistently successful in creating transmural lesions on the epicardial surface of a beating heart. ^6,12 However, the ablation device tested in this study appears to be more effective because consistent contact with the epicardial surface is maintained, resulting in more consistent transmural lesions. In addition, the device enables conformation to all surface regions of the heart and provides confirmation of contact and energy delivery.

In preliminary sheep experiments utilizing an early version of the device, the device was tested only on the LAA and around the left pulmonary vein orifice. ^13,14 Although all LAA (n = 5) and left pulmonary vein (n = 5) lesions were complete and transmural, evidenced by acute pathologic evaluation and conduction exit block across the lesions, the precise depth of the lesions was not measured. Furthermore, the animals were euthanized immediately after the coagulation procedure, which has been shown to be less than optimal timing for determining the extent of tissue necrosis.

It has been shown that TTC staining was not suitable for detecting infarcted myocardial tissue less than 4–5 h after infarction, ^12,15 and preferably requires 3 days of reperfusion to allow time for the mitochondrial enzymes to sufficiently wash out when quantifying the extent of tissue injury. In this study, to ensure that the full coagulation effects were properly delineated by the TTC staining, the subjects survived for 1 week after the ablation procedure, allowing the staining of cardiac tissue to more accurately illustrate the full extent of the tissue coagulation created by the device under observation.

Prasad et al ⁷ and Schuessler et al ⁸ have previously compared the transmurality of several available unipolar RF ablation devices including an early nContact prototype device. Animals were acutely sacrificed and tissue stained with TTC immediately after the lesions were created. No additional histologic evaluation was performed. Because of the subjective identification of poorly demarcated lesions with TTC in acute models, the reported percentage of atrial transmurality was far less than in these chronic GLP studies.

The lesions in these chronic GLP studies were further reviewed with histologic staining to compare the microscopic tissue to the macroscopic assessment made under TTC staining. The results showed that the unipolar ablation device was able to produce transmural lesions in atrial tissue with a 100% success rate, and in the ventricular tissue with a 67% success rate. Transmural lesions were created in the ventricular wall with an average depth of 6.7 mm and a maximum depth of 9.3 mm. H&E staining demonstrated that tissue damage was not fully represented by TTC staining, even after 1 week, as evidenced in Figure 3.

It has been demonstrated that thermal damage in ablation lesions evaluated during acute experiments extend an additional 3–6 mm beyond the nitro blue tetrazolium-stained lesion border when evaluated with electron microscopy. ^9,16 The latent ultrastructural alterations result in further gross and histologic signs of necrosis in the surrounding tissue. In addition, evaluation of microvascular perfusion after acute cardiac ablation has demonstrated a significantly larger cross-sectional area (48.4±6.3 mm) of poorly perfused microvasculature beyond that for the acute-stained ¹⁵ pathologic lesion (19.3±4.7 mm). ^14,17 This prolonged ischemic response has been identified as a potential mechanism for late lesion progression. One explanation is that the demarcation of the acute ablation lesion may be purely associated with protein denaturation, whereas ultrastructural or microvascular damage requires more time when pathologically evaluating the full extent of the lesion. The animals in these chronic GLP studies were euthanized 1 week after the procedure to allow for this lesion development. In some cases, the lesions may become progressive, leading to ongoing myocellular necrosis and loss of conduction. In other cases, the injury may be transient, resulting in recovery of regional perfusion and restoration of electrophysiological function.

Another perceived weakness of RF ablation with the standard unipolar probe is possible collateral cardiac or extracardiac damage. ^15,18 However, no such collateral or extracardiac damage was seen in this study other than the lesion on the left ventricle in subjects 217A and 218A. The LAA tissue is very thin; therefore, a sponge should be placed under the LAA prior to initiating RF ablation. The suction and irrigation features of this device are postulated to achieve cooling of the insulation covering on the probe and prevent collateral spread of energy. Adjacent and non-targeted anatomic structures are shielded.

These studies demonstrate that this unipolar RF ablation device could be effective for epicardial cardiac ablation. The device has the potential to electrically isolate cardiac tissue in a definitive, safe and expeditious manner without the use of cardiopulmonary bypass. In addition, the technology may be used endoscopically, allowing for a more minimally invasive procedure. Although these initial results are promising, clinical studies are needed to determine the effectiveness of these lesions in treating AF in humans.

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