The histological effects of cryocoagulation on the myocardium and coronary arteries

Cryosurgery for Atrial Fibrillation: Physiologic Basis for Creating Optimal Cryolesions

BACKGROUND

Although cryosurgery has been used to treat cardiac arrhythmias for nearly 5 decades, the mechanism of action and the surgical technique that produces optimal cryolesions for the treatment of atrial fibrillation are still poorly understood. This has resulted in surgical outcomes that can be improved by a better understanding the mechanisms of cryothermia ablation and the proper surgical techniques that take advantage of those mechanisms.

METHODS

The cryobiology underlying cryosurgical ablation is described, as are the nuances of cryosurgical techniques that ensure the reliable creation of contiguous, uniformly transmural atrial cryolesions. The oft-misunderstood "2-minute rule" for the application of cryothermia is clarified in detail, along with its variations that depend on target myocardial temperature.

RESULTS

The creation of optimal cryolesions depends on cryoprobe temperature, the temperature of the target myocardium, the duration of cryothermia application, and the presence or absence of a "heat sink" or "cooling sink" created by intracavitary blood circulation. Cryothermia kills myocardial cells during both the freezing and thawing phases of cryoablation cycle. The critical lethal temperature for myocardium is -30°C. The slower the target tissue thaws, the higher the percentage of cell death.

CONCLUSIONS

The availability of cryosurgical techniques has revolutionized the surgical treatment of atrial fibrillation. By utilizing modern cryosurgical devices and adhering to the technical principles described, surgeons can now perform surgical procedures for atrial fibrillation that are quicker, safer, and as effective as the standard Maze-III/IV procedure.

Irrigated Microwave Catheter Ablation Can Create Deep Ventricular Lesions Through Epicardial Fat With Relative Sparing of Adjacent Coronary Arteries

BACKGROUND

Radiofrequency ablation depth can be inadequate to reach intramural or epicardial substrate, and energy delivery in the pericardium is limited by penetration through epicardial fat and coronary anatomy. We hypothesized that open irrigated microwave catheter ablation can create deep myocardial lesions endocardially and epicardially though fat while acutely sparing nearby the coronary arteries.

METHODS

In-house designed and constructed irrigated microwave catheters were tested in in vitro phantom models and in 15 sheep. Endocardial ablations were performed at 140 to 180 W for 4 minutes; epicardial ablations via subxiphoid access were performed at 90 to 100 W for 4 minutes at sites near coronary arteries.

RESULTS

Epicardial ablations at 90 to 100 W produced mean lesion depth of 10±4 mm, width 18±10 mm, and length 29±8 mm through median epicardial fat thickness of 1.2 mm. Endocardial ablations at 180 W reached depths of 10.7±3.3 mm, width of 16.6±5 mm, and length of 20±5 mm. Acute coronary occlusion or spasm was not observed at a median separation distance of 2.7 mm (IQR, 1.2-3.4 mm). Saline electrodes recorded unipolar and bipolar electrograms; microwave ablation caused reductions in voltage and changes in electrogram morphology with loss of pace-capture. In vitro models demonstrated the heat sink effect of coronary flow, as well as preferential microwave coupling to myocardium and blood as opposed to lung and epicardial fat phantoms.

CONCLUSIONS

Irrigated microwave catheter ablation may be an effective ablation modality for deep ventricular lesion creation with capacity for fat penetration and sparing of nearby coronary arteries because of cooling endoluminal flow. Clinical translation could improve the treatment of ventricular tachycardia arising from mid myocardial or epicardial substrates.

Experimental cryosurgery investigations in vivo

Cryosurgery is the use of freezing temperatures to elicit an ablative response in a targeted tissue. This review provides a global overview of experimentation in vivo which has been the basis of advancement of this widely applied therapeutic option. The cellular and tissue-related events that underlie the mechanisms of destruction, including direct cell injury (cryolysis), vascular stasis, apoptosis and necrosis, are described and are related to the optimal methods of technique of freezing to achieve efficacious therapy. In vivo experiments with major organs, including wound healing, the putative immunological response following thawing, and the use of cryoadjunctive strategies to enhance cancer cell sensitivity to freezing, are described.

Easy Pulmonary Vein Isolation Using Epicardial Cryoablation

The maze procedure can be accomplished by several techniques with varying degrees of success. These techniques include cut-and-sew, endocardial cryoablation, and epicardial ablation using heat-producing devices. We describe our surgical modification of pulmonary vein isolation with cryoablation using an epicardial and endocardial approach. This simple and reliable technique, which does not need any special device, will assist in mitral valve surgery for patients with chronic atrial fibrillation.

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.

Safety and efficacy of epicardial cryoablation in a canine model

OBJECTIVES

The purpose of this study was to evaluate the safety and efficacy of cryoablation in a closed chest canine epicardial ablation model.

BACKGROUND

Limitations of radiofrequency energy in the epicardial space warrants investigation of alternative energy sources.

METHODS

A linear-tip catheter with a 3-cm freezing element and a 6-mm-tip catheter were used to create epicardial atrial and ventricular cryolesions. Epicardial coronary arteries were targeted to evaluate the effects of cryoablation on epicardial vessels. Cryoablation was performed at -90 degrees C for 4 minutes per lesion. Pathologic examination of the hearts was performed. Lesions were stained with tetrazolium chloride, analyzed grossly, and examined histologically.

RESULTS

Ten of 11 linear catheter atrial lesions were transmural (average depth 1.5 +/- 1.3 mm). Only three of 13 6-mm-tip atrial lesions were transmural (average depth 1.1 +/- 1.2 mm). Ventricular lesions were continuous and not transmural (average depth of lesion for the linear and 6-mm-tip catheters: 2.7 +/- 1.3 mm and 1.6 +/- 0.7 mm, respectively). Angiographic stenosis (20-100%) during freezing was detected in 9 of 28 lesions, with TIMI III flow present in all vessels 5 minutes following thaw. Neointimal proliferation was present in 13 vessels, with no evidence of damage in vessels with internal diameters greater than 0.7 mm. Occlusive injury was identified in one small branch vessel.

CONCLUSIONS

Creation of transmural lesions was possible in the atria but not in the ventricles. Cryothermal ablation can cause neointimal proliferation, with the probability that damage will be directly proportional to lesion depth and inversely proportional to vessel diameter.

Atrial fibrillation surgery simplified with cryoablation to improve left atrial function

BACKGROUND

The Maze procedure restores atrial fibrillation to normal sinus rhythm. However, concurrent left atrial functional recovery is not always achieved. To address this limitation, a modification using linear cryoablation is described.

METHODS

Between July 1997 and December 1999, 83 patients received atrial fibrillation surgery in association with mitral valve surgery with or without additional concurrent procedures by either the conventional technique, group I (n = 30) or the modified technique, group II (n = 53). Onset of sinus conversion and echocardiographic assessment of postoperative left ventricular function, left atrial size, and mitral A-wave velocity were compared in the early postoperative period and 6 months after surgery.

RESULTS

Sinus conversion occurred significantly earlier in group II, 2.4 +/- 5 days versus group I, 7.0 +/- 10 days. The mean transmitral A-wave velocity and the incidence of A-wave appearance in the early postoperative period and 6 months postoperatively were greater in group II than group I.

CONCLUSIONS

With the current modification, restoration of sinus rhythm and superior left atrial contractile function occurred earlier than with the standard Maze III technique.

Cardiogenic shock due to coronary narrowings one day after a MAZE III procedure

A MAZE III procedure was performed on a patient with a small body surface area. On the first postoperative day, the patient developed severe dysfunction of the left ventricle, due to significant narrowings of the right and circumflex coronary arteries in the areas that were cryoablated during the MAZE III procedure. The coronary narrowings were treated by percutaneous transluminal coronary angioplasty (PTCA). At discharge the coronary anatomy was normal again with an almost normal left ventricular function.

Cryothermal ablation: mechanism of tissue injury and current experience in the treatment of tachyarrhythmias

Cryosurgery has been an integral part of the surgical management of cardiac arrhythmias since the late 1970s. With the recent development of intravenous cryocatheters, the use of cryothermy in the treatment of cardiac arrhythmias will increase in the near future. The following discussion includes a detailed consideration of the mode of tissue injury associated with cryothermy and a comprehensive review of cryosurgery in the management of a variety of cardiac arrhythmias. Cryosurgical management of supraventricular and ventricular tachycardias has proven to be both safe and effective. Cryothermal tissue injury is distinguished from hyperthermic injury by the preservation of basic underlying tissue architecture and minimal thrombus formation. Such differences will be particularly important in settings requiring extensive lesion formation, such as catheter-based maze procedures for the treatment of atrial fibrillation.

Effects of cryothermia during cold cardioplegia on epicardial and intramural coronary arteries

Cryosurgery is playing an increasingly important role in the surgical treatment of various supraventricular and ventricular tachyarrhythmias. The short-term and long-term effects of cryogenic injury on epicardial and intramural coronary arteries were studied in 22 dogs. Transmural cryolesions encompassing a posterolateral branch of the circumflex coronary artery were produced in the inferior left ventricular wall during extracorporeal circulation and cold cardioplegic arrest. The mean epicardial cryolesion area +/- standard deviation was 10.4 +/- 1.8 cm2. The mean epicardial coronary artery diameter +/- standard deviation measured 1.2 +/- 0.4 mm. At 6 hours, no important structural changes were noted in any of 6 dogs. At 48 hours, 1 of 2 epicardial coronary arteries showed recent thrombus. At 1, 3, and 6 months, the epicardial coronary arteries were occluded due to thrombosis and intimal hyperplasia in 13 of 14 dogs. A limited degree of recanalization was observed. At all follow-up intervals, the intramural coronary arteries exhibited a histologic pattern similar to that of the epicardial coronary arteries. It is concluded that the exposure of major epicardial coronary arteries to cryoinjury during cold cardioplegic arrest should be avoided where possible.