Intracardiac pulsed field ablation: Proof of feasibility in a chronic porcine model.

BACKGROUND: Radiofrequency (RF) has become an accepted energy source for myocardial ablation but may result in discontinuous lesions and nontargeted tissue injury. We examined the feasibility and safety of lesion formation using high-amplitude, bipolar pulsed electric fields delivered from a multielectrode array catheter. OBJECTIVE: The purpose of this study was to compare duty-cycled radiofrequency ablation (RFA) to pulsed field ablation (PFA) in terms of acute electrical effects, 2-week lesion formation, and injury to nontargeted tissues. METHODS: Intracardiac ablations were performed in 6 pigs using a circular pulmonary vein ablation catheter. The energy source for ablation delivery was randomized to deliver either PFA or RFA to 3 atrial endocardial sites. Bipolar pace capture and electrogram amplitude measurements were recorded at each site. Histopathology and necropsies were performed after 2 weeks. RESULTS: The circular pulmonary vein ablation catheter was used to deliver pulsed electric fields to produce cardiac lesions without skeletal muscle stimulation. Evaluating all ablations in each site, electrogram amplitudes were reduced to <0.5 mV in 67.5% of PFA vs 27.0% of RFA deliveries (P <.001). Bipolar cardiac capture was lost after 100% vs 92.0% of PFA vs RFA (P = .005). At 2 weeks, PFA resulted in consistent transmural and homogeneous replacement fibrosis devoid of lingering myocyte "sequesters." RFA lesions showed a stronger inflammatory response extending to the epicardial fat, arterial injury, and thrombosis. Neither PFA nor RFA lesions showed endocardial thrombus. CONCLUSION: Intracardiac PFA can be feasibly delivered from a circular catheter to create fibrotic lesions that have acute electrical effects, without injury to nontargeted tissue.

Dosing of the second-generation cryoballoon using acute time-to-pulmonary vein isolation as an indicator of durable ablation in a canine model.

BACKGROUND: Rigid time-based dosing protocol(s) currently used in the clinic for cryoballoon ablation of atrial fibrillation may be inadequate to guide the circumferential and transmural cryothermal energy transfer across the pulmonary vein (PV) and may result in injury to collateral tissues or electrical gaps between the PV and left atrium (LA). OBJECTIVE: A physiologic endpoint (e.g., acute time-to-PV isolation a.k.a. time-to-effect; TTE) may be effective in the determination of a transmural lesion formation and may allow for individualized ablation dosing across each PV. METHODS: Thirty PVs from 15 dogs were randomized into five dosing protocols, including (1) TTE + 60 s, (2) TTE + 90 s, (3) TTE + 120 s, (4) TTE + 150 s, and (5) 2 × 180 s. Ablations were conducted with a 23-mm second-generation cryoballoon, and TTE was assessed during a freeze by pacing from an inner balloon-lumen circular diagnostic catheter to a quadripolar diagnostic catheter in the coronary sinus. After ablation, animals were survived for 30 to 34 days, and repeat electrophysiology assessment of PV isolation was conducted after which animals were euthanized for gross anatomy and histological examination. RESULTS: At study termination, efficacy endpoint evaluations were based on maintenance of PV electrical isolation, gross anatomy assessment of PV lesions, and histological examination of PVs. Five efficacy endpoint failures were noted, including the following: 1 PV in the TTE + 90 sec group; 2 PVs in the TTE + 120 sec group; 1 PV in the TTE + 150 s group; and 1 PV in the 2 × 180 s group. Regarding safety, one phrenic nerve injury was observed in the 2 × 180 s cohort. No other complications were observed. CONCLUSIONS: In a canine model, effective PV isolation could be found even in the shortest duration dosing cohort (TTE + 60 s). One complication (phrenic nerve injury) was observed in the longest duration dosing group (2 × 180 s). Further studies will be required to correlate these results to a 28-mm cryoballoon (more commonly used in the cryoablation of a human LA); however, to date, this is the first reporting of a successful cryoablation using TTE + 60 s dosing (approximately 90 s total duration of freezing).

Echocardiographic assessment of aortic valve annular and left ventricular outflow tract diameter in conscious and anesthetized adult sheep prior to prosthetic aortic valve implantation.

OBJECTIVE: Prosthetic heart valve implantation is commonly performed in patients that have valvular heart disease. Prior to clinical evaluation of newly developed prostheses, preclinical animal studies are performed for the assessment of both acute and chronic valvular function. Commonly, one size of valve is used in these preclinical studies, which can present difficulties with the implantation procedure and assessing valve function. Due to these potential problems, we developed a preoperative screening assessment in potential ovine candidates for prosthetic aortic valve implantation. By determining if there is a correlation between conscious and anesthetized echocardiographic examinations, an improvement in surgical confidence can predict that the animal is deemed a suitable candidate for a particular size of prosthetic valve for implantation prior to subjecting the animal to anesthesia and surgery. METHODS: A total of 53 crossbred sheep (Ovis aries), male and female, 10-37 months of age, weighing between 41 and 77 kg, underwent conscious echocardiography and a subset of 29 of these animals underwent echocardiographic assessment under anesthesia for a preoperative valve size comparison in these animals prior to surgical prosthetic aortic valve implantation. Using 2D echocardiographic assessment, left ventricular outflow tract (LVOT) dimensions were assessed. RESULTS: The mean paired difference between anesthetized and conscious LVOT diameter measurement was -0.87 mm (p = 0.0066, standard deviation 1.598, 95% confidence interval, -0.4796, -0.26378, n = 29). CONCLUSION: This pilot study evaluation revealed that conscious echocardiographic assessment can play a role preoperatively in selecting potential candidates for surgical prosthetic aortic valve implantation, thereby minimizing the potential in prosthetic-native annular mismatching, which can contribute to altered LVOT function.

Chronic aortic root pressure-loading assessment model.

PURPOSE: Percutaneous placement of transcatheter prosthetic aortic valves without cardiopulmonary bypass (CPB) continues to gain clinical acceptance. However, information on pressure-loading characteristics of the aortic root/annular areas is limited. For this reason, we designed a preclinical model, implanting an aortic root load transducer with a power source/telemetry system for chronic, conscious, loading data acquisition. This research study was conducted to determine whether an animal model could accurately measure in vivo loading. METHODS: Preoperatively, echocardiography and magnetic resonance imaging were used to determine both aortic annular and sinotubular junction dimensions, as well as ascending aortic length. Six adult sheep were placed on CPB, aortic root and ascending aorta were skeletonized and the origins of both coronary ostia were identified. Cardiac arrest with myocardial protection with cold coronary blood cardioplegia was instituted. A properly sized aortic root load-sensing device, consisting of a transcatheter aortic valve with a ring load transducer was implanted via a left apical ventriculotomy. Verification of position was determined before closure of the ventriculotomy. Each animal was weaned from CPB, and closed in routine fashion with the power source of the device placed in a subcutaneous pocket. RESULTS: There were no operative deaths or significant postoperative complications. Serial pressure-load sensing assessments in a conscious state produced reproducible proprietary data. CONCLUSIONS: This animal model allowed successful serial pressure-load sensing assessment of the aortic root/annular areas, providing a better physiological understanding of these anatomical inter-relationships. This added information could aid in future device designs with potential improved clinical outcomes.

Effects of freezing and cryopreservation on the mechanical properties of arteries.

Cryoplasty, a freezing therapy, is being used for the treatment of restenosis in peripheral arteries. In addition, cryo-preserved arteries are increasingly used in vascular grafts. While studies are being performed to establish the efficacy of such treatments, very little is known about the postcryosurgical or postcryo-preservation changes in mechanical properties of the arteries. Few studies have examined the effect of freezing in the absence of cryoprotective agents (CPAs), and the several studies done in the presence of CPAs have given mixed results. To examine this issue further, we froze pig femoral arteries in a controlled rate freezer, using an aluminum probe, both in the presence at (-80 degrees C to 1 degrees C/min) and absence (at -20 degrees C for 2 or 5 mins) of CPA and Fetal bovine serum (FBS). Following freezing, artery samples were subjected to uniaxial tensile testing. The weights of the tissue were measured before and after freezing. Our results suggest that freezing does have an effect on stress-strain properties, particularly in the low stress region corresponding to physiological conditions. The mechanisms of this change in mechanical properties may include the loss of smooth muscle cell viability, damage to extra cellular matrix (ECM), bulk redistribution of water, or changes in alignment caused by ice crystal growth. In the case of samples frozen in the absence of CPA or FBS, the results indicated a drastic reduction in weight of the tissue suggesting the importance of bulk water redistribution as one underlying mechanism. To further examine potential mechanisms, we subjected cryopreserved vessels to the same uniaxial tests. The extent of changes in mechanical properties and bulk water redistribution was greatly attenuated; reinforcing that water movement might play a role in the changes observed with freezing.

A cryoinjury model using engineered tissue equivalents for cryosurgical applications.

Cryosurgery is emerging as a promising treatment modality for various cancers, but there are still challenges to be addressed to improve its efficacy. Two primary challenges are determining thermal injury thresholds for various types of cell/tissue, and understanding of the mechanisms of freezing induced cell/tissue injury within a cryolesion. To address these challenges, various model systems ranging from cell suspensions to three-dimensional in vivo tissues have been developed and used. However, these models are either oversimplifications of in vivo tissues or difficult to control and extract precise experimental conditions from. Therefore, a more readily controllable model system with tissue-like characteristics is needed. In this study, a cryoinjury model was developed using tissue engineering technology, and the capabilities of the model were demonstrated. Engineered tissue equivalents (TEs) were constructed by seeding and culturing cells in a type I collagen matrix. Two different cell lines were used in this study, AT-1 rat prostate tumor cells and LNCaP human prostate cancer cells. The constructed TEs underwent a freeze/thaw cycle imitating in vivo cryosurgery. Thermal conditions within TEs during freeze/thaw cycles were characterized, and the responses of TEs to these thermal conditions including freezing induced cellular injury and extracellular matrix damage were investigated at three different time points. The results illustrate the feasibility to establish thermal thresholds of cryoinjury for different cell/tissue types using the presently developed model, and its potential capabilities to study cell death mechanisms, cell proliferation or migration, and extracellular matrix structural damage after a freeze/thaw cycle.

In vitro model systems for evaluation of smooth muscle cell response to cryoplasty.

Restenosis is a major health care problem, with approximately 40% of angioplasties resulting in restenosis. Mechanisms related to elastic recoil, cell proliferation, and extracellular matrix (ECM) synthesis are implicated. In vivo studies have demonstrated the potential for cryotherapy to combat the process of restenosis, but the mechanisms whereby freezing and/or cooling can reduce or eliminate smooth muscle cell (SMC) proliferation and ECM synthesis are not well known. While in vivo testing is ultimately necessary, in vitro models can provide important information on thermal parameters and mechanisms of injury. However, it is important to carefully choose the model system for in vitro work on cryoinjury characterization to adequately reflect the clinical situation. In this study, we examined the differences in response to cryoinjury by SMCs from different species (rat, pig, and human) and in different cellular environments (suspension vs. tissue equivalent). Tissue equivalents, composed of cells embedded in collagen or fibrin gel, provide a 3-D tissue-like environment, while allowing for controlled composition. As reported here, all SMCs showed similar trends, but rat cells appeared less sensitive to cooling at faster cooling rates in suspension, while human SMCs were less sensitive to temperatures just above freezing when embedded in collagen. In addition, the SMCs were less sensitive in suspension than they were in collagen. Cells in suspension exhibited 70% viability at -11 degrees C, whereas cells in the tissue equivalent model showed only 30% survival. Future studies will aim to more adequately represent the conditions in restenosis by providing inflammatory and proliferative cues to the cells.