Ventricular cryosurgery: short-term effects on intramural electrophysiology

Permanent and Transient Electrophysiological Effects During Cardiac Cryoablation Documented by Optical Activation Mapping and Thermal Imaging

OBJECTIVE

Cardiac catheter cryoablation is a safer alternative to radiofrequency ablation for arrhythmia treatment, but electrophysiological (EP) effects during and after freezing are not adequately characterized. The goal of this study was to determine transient and permanent temperature induced EP effects, during and after localized tissue freezing.

METHODS

Conduction in right (RV) and left ventricles (LV) was studied by optical activation mapping during and after cryoablation in paced, isolated Langendorff-perfused porcine hearts. Cryoablation was performed endocardially (n=4) or epicardially (n=4) by a cryoprobe cooled to -120 °C for 8 minutes. Epicardial surface temperature was imaged with an infrared camera. Viability staining was performed after ablation. Motion compensation and co-registration was performed between optical mapping data, temperature image data, and lesion images.

RESULTS

Cryoablation produced lesions 14.9 +/- 3.1 mm in diameter and 5.8 +/- 1.7 mm deep. A permanent lesion was formed in tissue cooled below -5 +/- 4 °C. Transient EP changes observed at temperatures between 17 and 37 °C during cryoablation surrounding the frozen tissue region directly correlated with local temperature, and include action potential (AP) duration prolongation, decrease in AP magnitude, and slowing in conduction velocity (Q10=2.0). Transient conduction block was observed when epicardial temperature reached <17 °C, but completely resolved upon tissue rewarming, within 5 minutes.

CONCLUSION

Transient EP changes were observed surrounding the permanent cryo lesion (<-5 °C), including conduction block (-5 to 17 °C), and reduced conduction velocity (>17 °C).

SIGNIFICANCE

The observed changes explain effects observed during clinical cryoablation, including transient increases in effective refractory period, transient conduction block, and transient slowing of conduction. The presented quantitative data on temperature dependence of EP effects may enable the prediction of the effects of clinical cryoablation devices.

Ablation technology for the surgical treatment of atrial fibrillation

The Cox maze procedure for the surgical treatment of atrial fibrillation has been simplified from its original cut-and-sew technique. Various energy sources now exist which create linear lines of ablation that can be used to replace the original incisions, greatly facilitating the surgical approach. This review article describes the anatomy of the atria that must be considered in choosing a successful energy source. Furthermore the device characteristics, safety profile, mechanism of tissue injury, and ability to create transmural lesions of the various energy sources that have been used in the Cox maze procedure, along with the strengths and weaknesses of each device is discussed.

Evaluation of a novel cryoablation system: in vivo testing in a chronic porcine model

OBJECTIVE

Cryoablation is commonly used at present in the surgical treatment of atrial fibrillation (AF). However, there have been few studies examining the efficacy of the commonly used ablation devices. This report compares the efficacy of two cryoprobes in creating transmural endocardial lesions on the beating heart in a porcine model for chronic AF.

METHODS

In six Hanford miniature swine, the right atrial appendage and the inferior vena cava were isolated using a bipolar radiofrequency clamp to create areas of known conduction block. A connecting ablation line was performed endocardially via a purse string with the novel malleable 10-cm Cryo1 probe for 2 minutes at -40°C. Additional ablation lines were created with the Cryo1 and the 3.5-cm 3011 Maze Linear probe on the right and the left atrial wall. Epicardial activation mapping was performed before and immediately after ablation as well as 14 days postoperatively. Histologic examination was performed 14 days postoperatively.

RESULTS

Transmural lesions were confirmed in 83/84 cross-sections (99%) for the Cryo1 probe and in 40/41 cross-sections (98%) for the 3011 Maze Linear probe. There was no difference between the devices in lesion width (mean ± SD, Cryo1, 10.7 ± 3.5 mm; 3011, 10.0 ± 3.9 mm; P = 0.31), lesion depth (Cryo1, 4.5 ± 1.7 mm; 3011, 4.6 ± 1.5 mm; P = 0.74), or atrial wall thickness (Cryo1, 4.5 ± 1.8 mm; 3011, 4.7 ± 1.7 mm; P = 0.74). There was a conduction delay across the right atrial ablation line (20 ± 2 milliseconds vs 51 ± 8 milliseconds, P < 0.001) that remained unchanged at 14 days (51 ± 8 milliseconds vs 52 ± 10 milliseconds, P = 0.88).

CONCLUSIONS

The Cryo1 probe created transmural lesions on the beating heart, resulting in sustained conduction delay. Both probes had a similar performance in lesion geometry in this chronic animal model.

Evaluation of a Novel Cryoablation System: In vivo Testing in a Chronic Porcine Model

Objective

Cryoablation is commonly used at present in the surgical treatment of atrial fibrillation (AF). However, there have been few studies examining the efficacy of the commonly used ablation devices. This report compares the efficacy of two cryoprobes in creating transmural endocardial lesions on the beating heart in a porcine model for chronic AF.

Methods

In six Hanford miniature swine, the right atrial appendage and the inferior vena cava were isolated using a bipolar radiofrequency clamp to create areas of known conduction block. A connecting ablation line was performed endocardially via a purse string with the novel malleable 10-cm Cryo1 probe for 2 minutes at −40°C. Additional ablation lines were created with the Cryo1 and the 3.5-cm 3011 Maze Linear probe on the right and the left atrial wall. Epicardial activation mapping was performed before and immediately after ablation as well as 14 days postoperatively. Histologic examination was performed 14 days postoperatively.

Results

Transmural lesions were confirmed in 83/84 cross-sections (99%) for the Cryo1 probe and in 40/41 cross-sections (98%) for the 3011 Maze Linear probe. There was no difference between the devices in lesion width (mean ± SD, Cryo1, 10.7 ± 3.5 mm; 3011, 10.0 ± 3.9mm; P = 0.31), lesion depth (Cryo1, 4.5 ± 1.7 mm; 3011, 4.6 ± 1.5 mm; P = 0.74), or atrial wall thickness (Cryo1, 4.5 ± 1.8 mm; 3011, 4.7 ± 1.7 mm; P = 0.74). There was a conduction delay across the right atrial ablation line (20 ± 2 milliseconds vs 51 ± 8 milliseconds, P < 0.001) that remained unchanged at 14 days (51 ± 8 milliseconds vs 52 ± 10 milliseconds, P = 0.88).

Conclusions

The Cryo1 probe created transmural lesions on the beating heart, resulting insustained conduction delay. Both probes had a similar performance in lesion geometry in this chronic animal model.

Serial changes in epicardial electrograms during and after a coronary artery occlusion

OBJECTIVE

Suddenly occurring ventricular tachyarrhythmias are a complication during off-pump coronary artery bypass (OPCAB) surgery, potentially leading to the need for conversion to on-pump surgery. We examined serial changes in the spatial dispersion of the electrical activity and refractoriness at the myocardial ischemia border zones during and after coronary occlusion.

METHODS

Unipolar epicardial electrograms were continuously recorded from the anterior left ventricle at the border zones during and after a 10-min occlusion of the left anterior descending (LAD) coronary artery in 22 patients undergoing OPCAB. The local electrogram amplitude and local refractoriness were evaluated by the unipolar peak-to-peak amplitude (UPPA) and activation recovery interval (ARI), respectively. The spatial dispersion of the electrical activity and refractoriness were examined using the coefficient of variation of these parameters.

RESULTS

No sustained ventricular tachyarrhythmias occurred in any patients. The UPPA dispersion significantly increased up to 5 min after the LAD occlusion and then returned to a nonsignificant level and again increased after reperfusion. The ARI dispersion gradually increased after the LAD occlusion, reached a significantly increased level 3 min after the occlusion, and stayed at a significantly increased level for at least 5 min after the reperfusion.

CONCLUSION

There were unique serial changes in the spatial dispersion of the electrical activity and refractoriness at the myocardial ischemia border zones during and after coronary occlusions. Continuous monitoring of these parameters may be useful for predicting the critical electrophysiological conditions prone to the occurrence of ventricular tachyarrhythmias in patients undergoing OPCAB.

An overview of energy sources in clinical use for the ablation of atrial fibrillation

Recent years have seen many developments in the field of alternative energy sources for arrhythmia surgery. The impetus behind these advances is to replace the traditional, "cut-and-sew" Cox maze III procedure with lesion sets that are simpler, shorter, and safer but just as effective. There is demand for technology to make continuous, linear, transmural ablations reliably with a versatile energy source via an epicardial approach. This would make minimally invasive endoscopic surgical ablation of atrial fibrillation (AF) without cardiopulmonary bypass and with a closed chest feasible. These advances would shorten cardio-pulmonary bypass and improve outcomes in patients having surgical ablation and concomitant cardiac surgery. This review summarizes the technology behind alternative energy sources used to treat AF. Alternative energy sources include hypothermic sources (cryoablation) and hyperthermic sources (radiofrequency, microwave, laser, ultrasound). For each source, the biophysical background, mode of tissue injury, factors affecting lesion size, and advantages and complications are discussed.

Single cryothermia applications of less than five minutes produce permanent cavotricuspid isthmus block in humans

OBJECTIVES

The aim of this study was to compare single-3-minute (single-3) with double-3-minute (double-3) cryothermia applications for treatment of atrial flutter (AFL).

BACKGROUND

Previous animal studies have indicated the need for a double 5-minute cryothermal application to create large and permanent lesions.

METHODS

Forty patients (56 +/- 13 years old) with typical AFL (cycle length 229 +/- 35 ms) were randomized to single-3 (n = 20) or double-3 (n = 20) cryothermia applications at each site along the cavotricuspid isthmus (CTI). Cryoablation was performed with the CryoCor cryoablation system. A successful procedure was defined as noninducibility of AFL with the concomitant presence of bidirectional CTI conduction block under isoproterenol infusion.

RESULTS

All but 1 patient (95%) of the single-3 group and all patients (100%) of the double-3 group were successfully ablated. The number of sites needed to create isthmus conduction block was 9 +/- 4 (single-3) and 8 +/- 2 (double-3) (NS). Fluoroscopy time did not differ between the two groups (single-3: 31 +/- 14; double-3: 36 +/- 17 min, NS). The procedure time of the single-3 group was significantly shorter compared to the procedure time of the double-3 group (mean procedure duration 132 +/- 64 vs 159 +/- 50 min, P < .04). After a mean follow-up of 11.7 +/- 4.7 months, two recurrences of AFL occurred in the double-3 group.

CONCLUSIONS

Single cryothermia applications of 3 minutes produce permanent CTI conduction block in patients with typical AFL and significantly reduce procedure duration.

Randomized study comparing radiofrequency ablation with cryoablation for the treatment of atrial flutter with emphasis on pain perception

BACKGROUND

Radiofrequency ablation (RF) of atrial flutter (AFL) has a high procedural efficacy, a low recurrence rate, and reports of procedure-related pain. The aim of the present study was to compare RF with cryoablation (cryo) for the treatment of AFL, with emphasis on pain perception during application of energy.

METHODS AND RESULTS

Fourteen patients (55+/-11 years, 11 males) with AFL were randomized to receive ablation of the cavotricuspid isthmus (CTI) by either RF or cryo. Cryothermia was delivered with the CryoCor Cryoablation System (10F, 6-mm tip), and radiofrequency energy was delivered with the use of an 8-mm-tip catheter. Pain was evaluated according to a visual analogue scale (VAS; 0 to 100). All patients in the cryo group were successfully ablated with a mean of 18 applications (9 sites), and RF was successful in 6 of 7 patients (not significant) with 13 applications (not significant). The mean temperature was -82 degrees C and 55 degrees C for cryo and RF, respectively. One patient in the cryo group perceived pain, versus all 7 patients in the RF group (P<0.05). The proportion of painful applications averaged 75.3% in the RF group and 2.0% in the cryo group (P<0.05), whereas the corresponding VAS for pain was 38.3+/-25.3 and 0.32+/-0.86, respectively (P<0.05). At 6-month follow-up, there were no recurrences of atrial flutter.

CONCLUSION

Cryo, as compared with RF, produces significantly less pain during application. Although in the present study there was no significant difference in efficacy, larger studies will be needed to definitively compare efficacy.

New cryotechnology for electrical isolation of the pulmonary veins

INTRODUCTION

Creation of radiofrequency lesions to isolate the pulmonary veins (PV) and ablate atrial fibrillation (AF) has been complicated by stenosis of the PVs. We tested a cryoballoon technology that can create electrical isolation of the PVs, with the hypothesis that cryoenergy will not result in PV stenosis.

METHODS AND RESULTS

Lesions were created in 9 dogs (weight 31-37 kg). Cryoenergy was applied to the PV-left atrial (LA) interface. Data collected before and after ablation included PV orifice size, arrhythmia inducibility, electrogram activity, and pacing threshold in the PVs. Tissue examination was performed immediately after ablation in 3 dogs and after 3 months (4.8 +/- 1.0) in 6 dogs. After ablation there was no localized P wave activity in the ablation zone and no LA-PV conduction. Before ablation, the pacing threshold was 1.9 +/- 1.1 mA in each PV. After ablation, the pacing threshold increased significantly to 7.2 +/- 1.8 mA, or capture was not possible. Burst pacing did not induce any sustained arrhythmias. Most dogs had hemoptysis during the first 24 to 48 hours. Acute tissue examination revealed hemorrhagic injury of the atrial-PV junction that extended into the lung parenchyma. After recovery, the lesions were circumferential and soft with no PV stenosis. Histologic examination revealed fibrous tissue with no PV-LA interface thickening.

CONCLUSION

This new cryoballoon technology effectively isolates the PVs from LA tissue. No PV stenosis was noted. Acute tissue hemorrhage and hemoptysis are short-term complications of this procedure. After 3 months of recovery, cryoablated tissue exhibits no collagen or cartilage formation.

The technology in use for the surgical ablation of atrial fibrillation

The purpose of this article is to present and evaluate the various technologies recently developed for the surgical treatment of atrial fibrillation as alternatives or adjuncts to the traditional Maze III procedure and other "cut and sew" techniques. The discussion contains a detailed consideration of the biophysical background of the most common ablation techniques, their mode of tissue injury, the methods of use, and the related complications through a review of the existing literature and analysis of experimental results. All of the current technologies presented are still being tested to augment the success rates and reduce the incidence of complications, although all are not available for clinical use. Radiofrequency and cryoablation have been used clinically on large numbers of patients with varying results. Microwave technology has been used in small groups of patients, and the results are to be evaluated. Laser technology is still in an experimental phase, and the clinical results are forthcoming. True transmurality, reduction of operative time, friendly use of ablation devices, and substantial reduction of complications appear to be the key factors for broad adoption of alternative energy sources for surgical ablation.

The importance of cryoablation of the coronary sinus during the Maze procedure

Although the Maze procedure has proven to be very effective in the treatment of atrial fibrillation, some authors have chosen to delete some of the important steps of the technique. Both our experimental and clinical experiences with the Maze procedure indicate that 1 of the most important principles is to interrupt conduction across the posterior-inferior portion of the left atrium. This is accomplished by creating a transmural lesion in the myocardium and then creating a circumferential lesion at the same site in the coronary sinus. We have used surgical incisions in the atrium and a cryolesion in the coronary sinus to block conduction in this area. If either fails, there is a high rate of arrhythmia recurrence.

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.

A load-independent in vivo model for evaluating therapeutic interventions in injured myocardium

Although cardiomyocyte damage is normally irreversible, gene therapy and somatic cell transfer offer potential for improving function in damaged regions of the heart. However, in ischemic models of injury, variability in depth, size, and location of damage compromises statistical evaluation of in vivo function. We have adapted cryoablation to create a reproducible, posterior, transmural lesion within rabbit myocardium in which small changes in function are measurable in vivo. Before and at 2 and 6 wk postinjury, in vivo left ventricular intracavitary pressure and myocardial segment length were measured. Regional indexes of performance, segmental stroke work (SW), and percent systolic shortening (SS) were significantly decreased (P < 0.001) postcryoinjury as was the slope (Mw) of the linear preload recruitable SW relationship between SW and end-diastolic segment length (P = 0.0001). Decreased SW, SS, and Mw correlated with wall thinning, loss of myocytes, presence of fibroblasts, and transmural scar formation. Reproducible changes in regional myocardial performance in vivo postcryoinjury suggest that this is a reasonable model for evaluating novel therapies for cardiovascular disease.

Transvenous cold mapping and cryoablation of the AV node in dogs: observations of chronic lesions and comparison to those obtained using radiofrequency ablation

INTRODUCTION

Radiofrequency (RF) is the most commonly used energy source for the treatment of cardiac arrhythmias. Surgical experience has shown that cryoablation also is effective for ablating arrhythmias. The aims of this study were to (1) investigate the feasibility of inducing permanent complete AV block (CAVB), (2) investigate the value of cold mapping to select the cryoablation site to produce permanent CAVB, (3) study the macro- and microscopic lesion characteristics 6 weeks later, and (4) compare them to those produced with RF energy.

METHODS AND RESULTS

A new steerable 8.5-French bipolar electrode catheter having a thermocouple with a 3-mm tip using N2O as the refrigerant controlled by a cryoconsole was used. Six mongrel dogs were anesthetized, and the catheter was positioned via the femoral vein across the tricuspid valve to record a large low right atrial and a small His-bundle potential. After cold mapping (-15 degrees to -20 degrees C tip temperature) resulted in ECG modifications, cryothermia (-70 degrees C) was given twice, lasting 5 minutes each, to create permanent CAVB (Cryo group). Additionally, RF catheter ablation of the AV node was performed in two anesthetized mongrel dogs (RF group). In the Cryo group, a permanent proximal CAVB was created in four dogs (block occurred within 10 to 20 sec of cryothermia). Permanent right bundle branch block was obtained in one dog and transient CAVB in the remaining dog. In both dogs of the RF group, permanent CAVB was obtained. The cryolesions consisted of well-circumscribed, homogeneous areas of fibrotic tissue without viable cardiomyocytes. Lesions produced with RF were less circumscribed and inhomogeneous, with clear evidence of viable cardiomyocytes and cartilage formation (patchy lesions).

CONCLUSIONS

(1) Permanent CAVB can be created by using a steerable cryoablation catheter. (2) Histologically, cryoablated sites were homogeneous and showed fibrotic tissue without signs of chronic inflammation and no evidence of viable myocytes. (3) Lesions created with RF were less homogenous and still contained viable myocytes within the lesion and cartilage formation. (4) The arrhythmogenic significance of these differences requires further study. (5) The technology of using reversible cold mapping has the potential to identify the successful ablation site and warrants further clinical study.

Combined radiofrequency ablation-cooling catheter for reversible cryothermal mapping and ablation

Reversible cryothermal mapping of cardiac arrhythmias has been performed intraoperatively. However, a steerable cooling catheter for reversible mapping has not yet been developed. We therefore developed and tested a cooling system consisting of a -15 degrees C hypertonic saline reservoir and a 7F steerable catheter also capable of radiofrequency (RF) ablation. Using excised ovine hearts placed in a 37 degrees C circulating saline bath, we measured the temperatures at depths of 0 mm, 1 mm, and 2 mm. The temperature after 90 seconds of cooling was 16.5 +/- 2.1 degrees C at 0 mm compared to 23.9 +/- 4.1 degrees C at 1 mm and 31.1 +/- 3.9 degrees C at 2 mm depth (p < 0.01). These data suggest that a 7F steerable combined RF ablation-cooling catheter may achieve temperatures suitable for mapping arrhythmias such as atrial tachycardias and right ventricular outflow tract tachycardias. Further enhancements to achieve lower temperatures at depth may be needed to reversibly map other arrhythmias such as left ventricular tachycardias.

Surgical management of ventricular tachycardia

BACKGROUND

Ventricular tachyarrhythmias are the leading cause of death from coronary artery disease. A small percentage of these arrhythmias originate in chronically ischemic myocardium, rather than acutely ischemic myocardium, and can be refractory to medical management. Epicardial mapping and focal cryoablation of foci demonstrating early activation may provide definitive therapy when pharmacologic management fails. We report a series of 42 consecutive patients with refractory ventricular tachycardia (VT) who were treated with open epicardial mapping and focal cryoablation after pharmacologic management failed.

METHODS

We retrospectively reviewed the records of patients who underwent surgical treatment of malignant VT. For patients not recently seen in the clinic, we conducted telephone interviews. At the time of operation, epicardial mapping was performed to locate foci of early electrical activation. These foci were then cryoablated, using 2-minute applications of liquid nitrogen-cooled probes. All patients underwent postoperative electrophysiologic studies to test for inducible VT.

RESULTS

Of these 42 patients, 34 (81%) were male, 8 (19%) female. Average age was 62.9 +/- 10.6 years; ejection fraction, 0.20 (range, 0.04 to 0.50); and number of foci ablated, 2.1 +/- 1.1 (range, 1 to 6). At the time of cryoablation, all patients underwent additional procedures, including aneurysmectomy, coronary artery bypass, or valve replacement. The 30-day operative mortality was 9.5% (4 of 42). Of the 38 survivors, 36 (94.7%) were clinically free of VT; the remaining 2 had spontaneous or inducible VT.

CONCLUSIONS

Open cryoablation of foci propagating VT appears to be safe and effective. It may be the most definitive treatment for malignant VT.

Postoperative influences of surgical cryoablation for Wolff-Parkinson-White syndrome--a analysis of myocardial enzymes and function

We evaluated postoperative myocardial enzymes and function associated with cryoablation in 20 patients with Wolff-Parkinson-White syndrome undergoing surgical treatment for a single left-sided accessory conduction pathway. Ten patients underwent endocardial atrial incision with cryoablation using CO2 at -60 degrees C for 120 sec (group A), while the remaining 10 patients did not receive cryoablation (group B). Levels of aspartate aminotransferase (GOT), lactate dehydrogenase (LDH), and creatine kinase (CK-MB) on postoperative days 1, 2, and 3 were higher in patients in group A than in group B (p < 0.05). However, mean values remained low (GOT, 120.5 IU/L; LDH, 1105.1 IU/L; CK-MB, 76.3 IU/L). No electrocardiographic changes were detected. Parameters of cardiac function, including cardiac index, stroke volume index, systemic vascular resistance, and ejection fraction, remained unchanged during the postoperative period in both groups. Furthermore, 201Tl cardiac scintigraphy demonstrated no evidence of myocardial perfusion defects due to cryoablation in group A. In conclusion, myocardial damage induced by cryoablation is very minor and is not associated with any clinical impairment of cardiac function.

Variation in cryolesion penetration due to probe size and tissue thermal conductivity

The purpose of this study is to present data comparing the penetration of cryolesions created by various sizes and shapes of cryoprobes in human cadaveric myocardium, fat, and tissue of the central fibrous body. Ten cryolesions were made for each combination of tissue and cryoprobe studied. All cryolesions enlarged most rapidly during the first minute of cryothermia (p less than 0.01). Maximal cryothermic penetration into nontrabeculated myocardium was 8.5 +/- 0.5 mm (15-mm flat probe) and 6.1 +/- 1.0 mm (5-mm small probe). Maximal cryothermic penetration into trabeculated myocardium was 9.4 +/- 1.0 mm (10-mm cone-tipped probe) and 7.4 +/- 0.5 mm (10-mm flat probe). Maximal cryothermic penetration into fat was 4.7 +/- 0.7 mm (15-mm flat probe) and 3.9 +/- 0.7 mm (5-mm flat probe). The deeper penetration of cryothermia into myocardium as compared with fat (p less than 0.05) is related to the lower thermal conductivity of fat. Maximal cryothermic penetration of the central fibrous body was similar to that of the myocardium with transmural freezing of the central fibrous body after 4.4 +/- 0.3 minutes of cryothermia. These data can be used when determining the optimal cryothermic exposure for ablation of arrhythmogenic tissue.

Transvenous cryoablation of the bundle of His

Cardiac dysrhythmias are a prominent cause of morbidity and mortality. Pharmacological treatment is ineffective in a large number of patients and is associated with many serious side effects. Thus, direct treatment of cardiac arrhythmias has been used with increasing frequency. Each form of direct treatment, such as surgical ablation, DC catheter ablation, radiofrequency catheter ablation, laser catheter ablation suffer serious drawbacks. Thus, we investigated the utility of transvenous catheter cryoablation of the bundle of His in five miniature swine, 40-60 lbs. in weight. Complete atrioventricular block was produced in each animal during cryothermia and persisted for 1 hour of observation in four out of five swine. In the fifth animal, 2:1 atrioventricular block within the atrioventricular node persisted for 1 hour of observation. Morphological and histologic examination revealed no dysfunction of capillaries and myofibriles in the atrioventricular node and proximal bundle of His. This potential mode of transcatheter therapy deserves further investigation.

Cardiac cryolesions: factors affecting their size and a means of monitoring their formation

Twenty-seven endocardial cryolesions were created in mongrel dogs and analyzed to determine the effects on cryolesion size of both the initial myocardial temperature (37 degrees C versus 12 degrees C) and the pressure within the nitrous oxide delivery line (tank pressure of more than 700 pounds per square inch [psi] versus tank pressure of less than 700 psi). In addition, local myocardial temperatures were monitored to determine their utility in the intraoperative determination of the extent of cryothermic cell death. Cryolesion volume was significantly affected by both the initial myocardial temperature (p less than 0.001) and the line pressure (p = 0.014). In a 37 degrees C myocardium, the mean lesion volume ranged from 0.501 +/- 0.183 cc at line pressures lower than 700 psi to 0.839 +/- 0.258 cc at line pressures greater than 700 psi. In a 12 degrees C myocardium, the mean volume was 1.151 +/- 0.436 cc at line pressures lower than 700 psi and 1.361 +/- 0.288 cc at line pressures higher than 700 psi. A myocardial temperature of 0 degrees C occurs at the edge of the area of cell death. When analyzing the range from -5 degrees to +5 degrees C, the probability of a point at or lower than 0 degrees C falling inside the cryolesion is 84.2%. Monitoring intramyocardial temperature will predict the border of a cryolesion.

Treatment of absent pulmonary valve syndrome with homograft

We report the successful treatment of absent pulmonary valve syndrome in an infant. The treatment consisted of intracardiac repair of pulmonary regurgitation with a homograft valve, which allowed for early extubation and survival.

Cryoablative techniques in the treatment of cardiac tachyarrhythmias

Of 175 patients treated surgically for potentially lethal or refractory cardiac tachyarrhythmias, 53 underwent mapping and definitive operation using cryoablative techniques as the primary or adjunctive method. Included were 16 patients with supraventricular tachycardia caused by accessory pathways (Kent bundle) in the right anterior or posterior paraseptal location. Cryoablation was successful in abolishing tachycardia in 93.7% (15 of 16). Six patients (100%) with permanent junctional reciprocating tachycardia were cured by cryoablation. Eighteen of 19 patients with atrial ectopic tachycardia were treated by cryoablation alone or in combination with excision of the atrial appendage, with success in 15 (83.3%). Five of these were left atrial foci cured by cryoablation. Fourteen right atrial foci were treated by excision of the appendage only (1 patient), excision of the appendage and local cryoablation (8 patients), and cryoablation alone (5 patients). Three of these underwent partial (2 patients) or complete (1 patient) atrial disconnection after excisional and cryoablative techniques failed to control the tachycardia. Multiple ectopic atrial foci were common (9 patients), and successful cryoablation was accomplished in 100% of the patients with a single atrial ectopic focus (10 patients) but in only 66% of those with multiple foci. Thirteen of 19 infants with critical ventricular tachycardia were treated by cryoablation at the site of the ectopic focus, either alone or in combination with excision of the area. Elimination of tachycardia was accomplished in 13 patients (100%). Myocardial hamartoma (Purkinje cell tumor) was the histological diagnosis in 11 of the infants with ventricular tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)

Laser catheter ablation of simulated ventricular tachycardia

Catheter-directed laser injury of the left ventricular endocardium for ablation of ventricular tachycardia was studied in a canine model of simulated ventricular tachycardia. Bipolar plunge electrodes were placed at thoracotomy into the left ventricular endocardium in nine anesthetized dogs. Ventricular tachycardia was simulated by pacing at 200 beats per minute. After four days of recovery, catheter-directed neodymium:yttrium-aluminum-garnet laser injury was produced at the site of earliest recorded electrical activation during pacing tachycardia as detected by endocardial catheter mapping. Immediately after laser injury and again five days after injury, pacing tachycardia was attempted. Failure to pace after laser injury was defined as successful arrhythmia ablation. In 3/9 (33%) experiments, the laser-injured tissue surrounded the tachycardia source (pacing wires), and pacing-simulated ventricular tachycardia was prevented. When the laser injury did not involve the tachycardia source, 6/9 animals, due to limitations of the mapping system, pacing was not prevented. One animal developed sustained ventricular tachycardia during laser injury, with conversion to sinus rhythm by lidocaine. One animal, without recognized ventricular arrhythmia, died five days after laser injury. No unusual findings were noted at autopsy. These preliminary data suggest that catheter-directed laser-induced injury can ablate arrhythmia sources. Further studies are indicated in a more physiologic model, and the safety and risks of the procedure need further evaluation.

Nonarrhythmogenicity of therapeutic cryothermic lesions of the myocardium

There is increasing interest in the application of surgical methods to the treatment of refractory ventricular tachyarrhythmias (VT). Cryothermic ablation is one of the more promising techniques. However, there is clinical concern that a cryothermic lesion may lead to later arrhythmias. Previous studies have shown that dogs with nonhomogeneous, transmural infarctions are susceptible to VT initiation using programmed electrical stimulation (PES). The purpose of this study was to compare the incidence of inducing VT in dogs with transepicardial cryothermic myocardial damage (Group A) versus dogs with nonhomogeneous transmural infarctions resulting from 2-hr occlusion of the left anterior descending coronary artery (LAD) and subsequent reperfusion (Group B). Twelve dogs in each group were studied 10-14 days later using PES with unipolar cathodal ventricular pacing and two ventricular extrastimuli. Initiation of VT was attempted from at least six normal intramyocardial sites in each dog along the distribution of the LAD and in close proximity (less than or equal to 1 cm) to areas of chronically cryoablated damaged tissue. All dogs survived the initial procedure. VT was not inducible in any dog in Group A. Histological as well as electrophysiological evaluation, including determination of regional excitability thresholds and refractory periods employing strength-interval curves, revealed that all of the Group A dogs had homogeneous transmural infarcts with variable subendocardial sparing. In conclusion (1) cryothermal injury produces homogeneous damage; and (2) the lesion produced is not arrhythmogenic at 7-14 days.