Extensive cryoablation of the left ventricular posterior papillary muscle and subjacent ventricular wall

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.

Evaluation of collagenase-induced mitral valve regurgitation in dogs

OBJECTIVE

To investigate the hemodynamic changes induced by injecting collagenase into the mitral valve to induce mitral valve regurgitation (MVR) in dogs.

ANIMALS

9 healthy Beagles.

PROCEDURE

Dogs were randomly assigned to 3 groups: control (saline [0.9% NaCl] solution; n = 3), single collagenase injection (C1; 3), and 2 collagenase injections (C2; 3). Open-heart surgery was performed, and saline or collagenase solutions were injected into the mitral valve. Before and weekly for 11 weeks after surgery, radiography, echocardiography, and phonocardiography were performed. Mean pulmonary arterial pressure and mean pulmonary arterial wedge pressure (mPAWP) were measured before and 11 weeks after surgery. Postmortem examinations were performed after dogs were euthanatized.

RESULTS

No changes were detected in the control group during the 11-week follow-up period. A systolic murmur and MVR developed 1 week after surgery in groups C1 and C2. The murmur changed from a protosystolic to a pansystolic murmur, and left atrial diameter and the left atrial-to-aortic root diameter ratio increased with time. Mean pulmonary arterial pressure and mPAWP were greater 11 weeks after surgery in groups C1 and C2, compared with presurgery values. During necropsy, tissue loss was detected in the mitral valve at the site of collagenase injection. Degree of regurgitation corresponded to lesion size.

CONCLUSIONS AND CLINICAL RELEVANCE

Injection of collagenase into the mitral valve of healthy dogs induced MVR, and dogs with MVR developed progressive hemodynamic changes without acute overload. Collagenase-induced MVR may be an appropriate model for evaluation of prognostic markers of idiopathic MVR in dogs.

Mitral valve repair vs replacement. Current recommendations and long-term results

Techniques now exist to correct abnormalities of all components of the mitral valvular apparatus except extensive loss of pliable leaflet area. Thus, paradoxically, myxomatous valves with redundant leaflets represent the ideal candidates for mitral valve repair. Repair for mitral insufficiency can be performed for some rheumatic valves, but patient selection is critical. Loss of leaflet area, leaflet thickening, and extensive calcification of the leaflets or commissures are contraindications to repair. The abnormalities of the subvalvular apparatus are less important because a complete set of new chordae can be reconstructed using PTFE suture material. Some cases of endocarditis are ideal for repair using localized débridement and pericardial patch repair with or without PTFE chordal replacement. True ischemic mitral regurgitation of the Carpentier type I category is still something of a surgical enigma. Because it is a restrictive leaflet motion problem, annuloplasty alone is not always effective, and the outcome of any given repair attempt is less predictable. Repairs in patients with small annuli and multiple leaflet defects requiring complex series of maneuvers have a low probability of success. Furthermore, such patients with small left ventricular cavities are more prone to experience SAM. Several factors contributing to which therapy is chosen for mitral valve disease are summarized in Table 1. Patient selection, accurate evaluation of the cause or causes of mitral regurgitation, and well-executed application of the appropriate techniques for repair are all critical factors in the early and late success of mitral valve repair.

Coronary artery involvement early and late after radiofrequency current application in young pigs

Radiofrequency current (500 kHz) was delivered by temperature guidance (75 degrees C) over a 30-second period in 10 young piglets with a steerable 6F electrode catheter equipped with a thermistor at the 4 mm tip electrode. Lesions were created at the right atrial aspect of the tricuspid valve anulus, at the left ventricular myocardium under the lateral mitral valve anulus, and at the left ventricular apex. After 48 hours, five animals were randomly sacrificed. Lesions in the five animals appeared as transmural gray-white coagulation necrosis. Lymphocytic infiltration around the right atrial lesions extended into the layers of the right coronary artery in four of five animals. After 6 months, lesions consisted of compact fibrous tissue in the remaining five animals. Right atrial lesions extended to the layers of the right coronary artery in four of five pigs. In two animals the lumen of the right coronary artery was narrowed because of intimal thickening by 25% and 40%, respectively. No increase in the lesion size was observed with the growth of the animals. Effects on the right coronary artery as a late sequela after radiofrequency current application may also be possible in human beings and should be considered when radiofrequency current ablation procedures are proposed in infants and young children.

Is there space for surgery in the treatment of tachyarrhythmias?

Curative treatment of both supraventricular and ventricular tachyarrhythmias started with the introduction of surgical therapy. Surgical treatment modalities were often very successful and associated with low mortality and morbidity, especially in patients with various supraventricular tachyarrhythmias. However, results were acceptable in patients with ventricular tachyarrhythmias, with often a very complex and extended arrhythmogenic area associated with structural heart disease. Because of the development and proven effectiveness of catheter ablation and defibrillator implantation, the role of surgical therapy became limited. In the treatment of supraventricular arrhythmias, surgical therapy is an option after failure of catheter ablation. Since His-bundle catheter ablation is only a palliative treatment for atrial fibrillation, the potentially curative Maze operation may be an acceptable alternative. However, its potential against formation of intracavitary thrombi has not yet been proven. In the treatment of ventricular tachyarrhythmias, ischemia related polymorphic ventricular tachycardia and ventricular fibrillation can be treated very effectively by revascularization. Map-guided surgery is an appropriate treatment modality for patients with monomorphic ventricular tachycardia and an extended arrhythmogenic area. However, patients with very poor left ventricular function may have an unacceptable perioperative risk. In patients with congenital long QT syndrome who are refractory to beta blocking agents, left-sided sympathectomy is the most appropriate choice.

Change in delay of atrioventricular conduction after radiofrequency catheter ablation for atrioventricular nodal re-entry tachycardia

OBJECTIVE

To monitor atrioventricular conduction after radiofrequency ablation for atrioventricular nodal re-entry tachycardia.

DESIGN

Measurement of PR interval from 12 lead surface electrocardiograms before; at 0, 24, 48, 72, and 96 hours; and at 1 and 6 months after radiofrequency ablation.

PATIENTS

40 consecutive patients with atrioventricular nodal re-entry tachycardia. The anterior approach was used in 23 patients, the posterior approach in 17.

RESULTS

With the anterior approach the PR interval increased significantly and progressively until 48 hours after ablation (maximum 282 (SD 62.2) ms, before ablation 142 (29.5) ms; P < 0.0001). Up to 96 hours no further change was observed, but one month after ablation the PR interval had decreased to a value not significantly different from that 24 hours after the procedure (231 (51.2) ms). In one patient total atrioventricular block developed 24 hours after an uncomplicated procedure and a permanent pacemaker was implanted. With the posterior approach the PR interval increased slightly in the first 24 hours (156 (22.7) ms, before ablation 144 (21.2) ms P = 0.004), but it had returned to preablation values at 1 month. One patient developed second degree atrioventricular block during the first 24 hours after ablation, despite delivery of all radiofrequency pulses posterior to Koch's triangle at sites without His bundle deflection. PR intervals at 6 months did not differ significantly from the values at 1 month.

CONCLUSION

After the anterior approach the progressive delay in atrioventricular conduction up to 48 hours after radiofrequency ablation for atrioventricular nodal re-entry tachycardia warrants continuous in hospital monitoring of patients for at least two days after the procedure.

In vivo ventricular lesion growth in radiofrequency catheter ablation

While radiofrequency catheter ablation has proved highly effective in the treatment of various supraventricular tachyarrhythmias, results in the treatment of ventricular tachycardia invite improvement. Knowledge of lesion growth in vivo might improve understanding of this discrepancy. So far only information from in vitro and in vivo studies using a small 2 mm tip electrode is available. Growth of ventricular radiofrequency lesions created with a 4 mm ablation electrode was studied in 11 closed-chest dogs. Endocardial ablations were performed at 31 left and 15 right ventricular sites at a power setting of 25 Watts and 5, 10, 20, 30 or 60 seconds pulse duration. Macroscopic and histopathologic lesion examination were performed after one week survival. Mean lesion volume increased from 52 mm3 after 5 seconds pulse duration to a maximum 388 mm3 and approximately 7 mm depth after 30 seconds. Lesions were prolate spheroid in form, with a sparing of subendocardial myocardium and maximum lesion diameter at some millimeters depth. Results indicate that catheter positioning at no more than 7 mm from the target is required for successful ablation. Due to lesion geometry, subendocardial targets demand even more exact catheter positioning, while subepicardial substrates may not be ammenable to ablation if ventricular wall thickness exceeds 7 mm at the ablation site. Repeated pulses at adjacent sites may be required for ablation of extended arrhythmogenic areas. Volume at 5 seconds was only approximately 15% of mature lesions. Therefore, the use of a short 'test pulse' after careful mapping may be useful to pinpoint the most appropriate site for ablation in discrete pathways.