121. Cardiac Implantable Electronic Device-Related Infective Endocarditis (CIED-IE): Clinical Features and Outcomes of Patients with Definite IE Who Fulfill Both Major Duke Criteria

Background

Cardiac implantable electronic device-related infective endocarditis (CIED-IE) comprises 10–57% of total CIED infections. Patients with definite CIED-IE who fulfill both major modified Duke criteria have not been well characterized.

Methods

Data from the Multicenter Electrophysiologic Device Infection Cohort, a prospective, multinational study of CIED infections were used to describe a subset of patients with CIED-IE who met both major Duke criteria for definite IE (bloodstream infection and intracardiac vegetations [VEG]).

Results

Of 433 patients with CIED infection, 144 (33.3%) had definite CIED-IE. The median age was 68 years and 77.1% were male. Twelve (8.3%) had past CIED infection. Seventy-seven patients (53.5%) had permanent pacemakers, 38 (26.4%) had implantable cardioverter defibrillators, and 29 (20.1%) had combination devices. The median time following the last device procedure was 550 days. CIED-IE was early in 60 patients (41.7%) and late in 84 (58.3%). Most patients presented with fever (77.8%) and sepsis (44.4%) with a median symptom duration of 7 days. On echocardiography, lead VEG was noted in 125 patients (86.8%) and valvular VEG in 54 patients (37.5%) with the tricuspid valve involved in 56.5%. On the basis of VEG location, there were 90 patients (62.5%) with isolated lead-associated IE (LAE), 19 patients (13.2%) with isolated valve-associated IE (VAE), and 35 patients (24.3%) with both (LVAE). All patients had positive blood cultures and 63/119 (52.9%) had positive lead cultures. The predominant organism in blood was Staphylococcus aureus (42.4%), followed by coagulase-negative staphylococci (20.1%). CIED removal occurred in 131 patients (91%). There were 25 deaths during the index hospitalization and 34 total deaths (24.3%) by 6 months. Mortality correlated with age >75 (P = 0.023) and sepsis on presentation (P = 0.052). Infecting organism, site of VEG, and device removal did not impact the risk of death.

Conclusion

Definite CIED-IE is relatively common. The majority of patients tend to have late-onset infection and often present with sepsis. S. aureus is the dominant organism causing definite CIED-IE. Isolated LAE occurs in 63% of patients. Older age and sepsis on admission are associated with higher mortality.

Disclosures

All authors: No reported disclosures.

1085. Enterococcal Cardiac Implantable Electronic Device (CIED) Infections: Clinical Features and Outcomes

Background

Unlike enterococcal native and prosthetic valve infective endocarditis (IE), enterococcal CIED infections are not well described.

Methods

Data from the Multicenter Electrophysiologic Device Infection Collaboration (MEDIC), a prospective, observational, multinational cohort study of CIED infections, were used to provide a descriptive analysis of adult patients with CIED infections due to enterococcal species.

Results

Of 433 patients, 21 (4.8%) were diagnosed with enterococcal CIED infection. Specific data on enterococcal species and antimicrobial susceptibilities were not recorded. The mean age was 70.8 years. No patient had previous CIED infection. Twelve patients (57%) had permanent pacemakers, 5 (24%) had implantable cardioverter defibrillators, and 4 (19%) had biventricular devices. Among the 21 infections, 3 (14%) were categorized as CIED-related bloodstream infections and 18 (86%) as IE; no patient had isolated pocket infection. Of the IE cases, four were valvular IE, eight were lead IE, and six were both. Fourteen cases of IE (78%) were definite by the modified Duke criteria. Median time from last device procedure to infection was 510 days (range 37–2,952 days). The most common presenting symptom was fever (48%); five patients (24%) exhibited local signs of pocket infection. All 21 patients underwent TEE with vegetations demonstrated in 17 (81%). Blood cultures grew enterococci from all patients. The most common antimicrobial regimen was a penicillin plus aminoglycoside (38%); two patients (9.5%) received ampicillin + ceftriaxone. Antibiotics were given for a median of 43 days. Only 14 patients (67%) had complete device removal; the seven patients retaining their device were judged to be at high risk for extraction. There was one death during the index hospital stay with four additional patients dying over the 6 months after therapy (overall mortality 24%); two of the seven patients retaining their CIED died.

Conclusion

Enterococci caused 4.8% of all CIED infections in our cohort. Most infections appeared to be hematogenous in origin with late onset. IE was the most common infectious syndrome. A penicillin plus aminoglycoside, given for 6 weeks, was the most frequent therapy. Only 67% of patients underwent device removal. At 6 months follow-up, no relapses had occurred but overall mortality was 24%.

Disclosures

J. E. Peacock Jr., Pfizer, Inc.: Shareholder, Owns common stock in Pfizer which was inherited and held in a trust. M. R. Sohail, TyRx Inc.: Investigator, Research support. Medtronic Inc.: Investigator, Research support. Medtronic Inc.: Consultant, Speaker honorarium. Spectranetics: Consultant, Speaker honorarium. Boston Scientific Corp: Consultant, Speaker honorarium. L. M. Baddour, UpToDate: Collaborator, Royalty payment. J. M. Miro, Abbvie: Consultant and Grant Investigator, Consulting honoraria and Research grant. Bristol-Myers Squibb: Consultant and Grant Investigator, Consulting honoraria and Research grant. Genentech: Consultant and Grant Investigator, Consulting honoraria and Research grant. Medtronic: Consultant and Grant Investigator, Consulting honoraria and Research grant. Novartis: Consultant and Grant Investigator, Consulting honoraria and Research grant. Gilead Sciences: Consultant and Grant Investigator, Consulting honoraria and Research grant. Pfizer: Consultant and Grant Investigator, Consulting honoraria and Research grant. ViiV Healthcare: Consultant and Grant Investigator, Consulting honoraria and Research grant. A. J. Greenspon, Medtronic: Consultant, Speaker honorarium. Boston Scientific: Consultant, Speaker honorarium. St. Jude: Consultant, Speaker honorarium. R. G. Carrillo, St. Jude Medical Group: Speaker’s Bureau, Research support. Spectranetics: Consultant, Speaker honorarium. Sorin Group: Speaker’s Bureau, None. Boston Scientific Corp: Speaker’s Bureau, None. D. Z. Uslan, Medtronic: Investigator, Research support. Boston Scientific: Consultant, Speaker honorarium.

Attempted salvage of infected cardiovascular implantable electronic devices: Are there clinical factors that predict success?

BACKGROUND

Published guidelines mandate complete device removal in cases of cardiovascular implantable electronic device (CIED) infection. Clinical predictors of successful salvage of infected CIEDs have not been defined.

METHODS

Data from the Multicenter Electrophysiologic Device Infection Collaboration, a prospective, observational, multinational cohort study of CIED infection, were used to investigate whether clinical predictors of successful salvage of infected devices could be identified.

RESULTS

Of 433 adult patients with CIED infections, 306 (71%) underwent immediate device explantation. Medical management with device retention and antimicrobial therapy was initially attempted in 127 patients (29%). "Early failure" of attempted salvage occurred in 74 patients (58%) who subsequently underwent device explantation during the index hospitalization. The remaining 53 patients (42%) in the attempted salvage group retained their CIED. Twenty-six (49%) had resolution of CIED infection (successful salvage group) whereas 27 patients (51%) experienced "late" salvage failure. Upon comparing the salvage failure group, early and late (N = 101), to the group experiencing successful salvage of an infected CIED (N = 26), no clinical or laboratory predictors of successful salvage were identified. However, by univariate analysis, coagulase-negative staphylococci as infecting pathogens (P = 0.0439) and the presence of a lead vegetation (P = 0.024) were associated with overall failed salvage.

CONCLUSIONS

In patients with definite CIED infections, clinical and laboratory variables cannot predict successful device salvage. Until new data are forthcoming, device explantation should remain a mandatory and early management intervention in patients with CIED infection in keeping with existing expert guidelines unless medical contraindications exist or patients refuse device removal.

Clinical presentation of CIED infection following initial implant versus reoperation for generator change or lead addition

Objective

To explore differences in clinical manifestations and outcomes in those patients who develop infection after undergoing initial implantation versus reoperation.

Methods

We compared cases of cardiac implantable electronic device (CIED) infection based on initial implantation versus reoperation from 11 centres.

Results

There were 432 patients with CIED infection, 178 occurring after initial device placement and 254 after repeat reoperation. No differences were seen in age, sex or device type. Those with infection after initial implant had a higher Charlson Comorbidity Score (median 3 (IQR 2–6) vs 2 (IQR 1–4), p<0.001), shorter time since last procedure (median 8.9 months (IQR 0.9–33.3) vs 19.5 months (IQR 1.1–62.9), p<0.0001) and fewer leads (2.0±0.6vs 2.5±0.9, p<0.001). Pocket infections were more likely to occur after a reoperation (70.1%vs48.9%, p<0.001) and coagulase negative staphylococci (CoNS) was the most frequently isolated organism in this group (p=0.029). In contrast, initial implant infections were more likely to present with higher white cell count (10.5±5.1 g/dL vs 9.5±5.4 g/dL, p=0.025), metastatic foci of infection (16.9%vs8.7%, p=0.016) and sepsis (30.9%vs19.3%, p=0.006). There were no differences in in-hospital (7.9%vs5.2%, p=0.31) or 6-month mortality (21.9%vs14.0%, p=0.056).

Conclusions

CIED infections after initial device implant occur earlier, more aggressively, and often due to Staphylococcus aureus. In contrast, CIED infections after reoperation occur later, are due to CoNS, and have more indolent manifestations with primary localisation to the pocket.

Impact of Abandoned Leads on Cardiovascular Implantable Electronic Device Infections: A Propensity Matched Analysis of MEDIC (Multicenter Electrophysiologic Device Infection Cohort)

OBJECTIVES

This study sought to evaluate the impact of abandoned cardiovascular implantable electronic device (CIED) leads on the presentation and management of device-related infections.

BACKGROUND

Device infection is a serious consequence of CIEDs and necessitates removal of all hardware for attempted cure. The merits of extracting or retaining presumed sterile but nonfunctioning leads is a subject of ongoing debate.

METHODS

The MEDIC (Multicenter Electrophysiologic Device Infection Cohort) prospectively enrolled patients with CIED infections at 10 institutions in the United States and abroad between January 1, 2009, and December 31, 2012. Within a propensity-matched cohort, relevant clinical information was compared between patients who had 1 or more abandoned leads at the time of infection and those who had none.

RESULTS

Matching produced a cohort of 264 patients, including 176 with no abandoned leads and 88 with abandoned leads. The groups were balanced with respect to Charlson comorbidity index, oldest lead age, device type, sex, and race. At the time of admission, those with abandoned leads were less likely to demonstrate systemic signs of infection, including leukocytosis (p = 0.023) and positive blood cultures (p = 0.005). Conversely, patients with abandoned leads were more likely to demonstrate local signs of infections, including skin erosion (p = 0.031) and positive pocket cultures (p = 0.015). In addition, patients with abandoned leads were more likely to require laser extraction (p = 0.010).

CONCLUSIONS

The results of a large prospective registry of CIED infections demonstrated that patients with abandoned leads may present with different signs, symptoms, and microbiological findings and require laser extraction more than those without abandoned leads.

Reimplantation and Repeat Infection After Cardiac-Implantable Electronic Device Infections: Experience From the MEDIC (Multicenter Electrophysiologic Device Infection Cohort) Database

BACKGROUND

Infection is a serious complication of cardiovascular-implantable electronic device implantation and necessitates removal of all hardware for optimal treatment. Strategies for reimplanting hardware after infection vary widely and have not previously been analyzed using a large, multicenter study.

METHODS AND RESULTS

The MEDIC (Multicenter Electrophysiologic Device Infection Cohort) prospectively enrolled subjects with cardiovascular-implantable electronic device infections at multiple institutions in the United States and abroad between 2009 and 2012. Reimplantation strategies were evaluated overall, and every patient who relapsed within 6 months was individually examined for clinical information that could help explain the negative outcome. Overall, 434 patients with cardiovascular-implantable electronic device infections were prospectively enrolled at participating centers. During the initial course of therapy, complete device removal was done in 381 patients (87.8%), and 220 of them (57.7%) were ultimately reimplanted with new devices. Overall, the median time between removal and reimplantation was 10 days, with an interquartile range of 6 to 19 days. Eleven of the 434 patients had another infection within 6 months, but only 4 of them were managed with cardiovascular-implantable electronic device removal and reimplantation during the initial infection. Thus, the repeat infection rate was low (1.8%) in those who were reimplanted. Patients who retained original hardware had a 11.3% repeat infection rate.

CONCLUSIONS

Our study findings confirm that a broad range of reimplant strategies are used in clinical practice. They suggest that it is safe to reimplant cardiac devices after extraction of previously infected hardware and that the risk of a second infection is low, regardless of reimplant timing.

Variability in clinical features of early versus late cardiovascular implantable electronic device pocket infections

BACKGROUND

Cardiovascular implantable electronic device (CIED) pocket infections are often related to recent CIED placement or manipulation, but these infections are not well characterized. The clinical presentation of CIED pocket infection, based on temporal onset related to last CIED procedure, deserves further study.

METHODS

The MEDIC (Multicenter Electrophysiologic Device Infection Cohort) prospectively enrolled subjects with CIED infection. Subjects were stratified into those whose infection occurred <12 months (early) or ≥ 12 months (late) since their last CIED-related procedure.

RESULTS

There were 132 subjects in the early group and 106 in the late group. There were more females (P = 0.009) and anticoagulation use (P = 0.039) in the early group. Subjects with early infections were more likely to have had a generator change or lead addition as their last procedure (P = 0.03) and had more prior CIED procedures (P = 0.023). Early infections were more likely to present with pocket erythema (P < 0.001), swelling (P < 0.001), and pain (P = 0.007). Late infections were more likely to have pocket erosion (P = 0.005) and valvular vegetations (P = 0.009). In bacteremic subjects, early infections were more likely healthcare-associated (P < 0.001). In-hospital and 6-month mortality were equivalent.

CONCLUSION

A total of 45% of patients with CIED pocket infection presented >12 months following their last CIED-related procedure. Patients with early infection were more likely to be female, on anticoagulation, and present with localized inflammation, whereas those with late infection were more likely to have CIED erosion or valvular endocarditis.

Management of refractory atrial fibrillation post surgical ablation

Over the past two decades, invasive techniques to treat atrial fibrillation (AF) including catheter-based and surgical procedures have evolved along with our understanding of the pathophysiology of this arrhythmia. Surgical treatment of AF may be performed on patients undergoing cardiac surgery for other reasons (concomitant surgical ablation) or as a stand-alone procedure. Advances in technology and technique have made surgical intervention for AF more widespread. Despite improvements in outcome of both catheter-based and surgical treatment for AF, recurrence of atrial arrhythmias following initial invasive therapy may occur.Atrial arrhythmias may occur early or late in the post-operative course after surgical ablation. Early arrhythmias are generally treated with prompt electrical cardioversion with or without antiarrhythmic therapy and do not necessarily represent treatment failure. The mechanism of persistent or late occurring atrial arrhythmias is complex, and these arrhythmias may be resistant to antiarrhythmic drug therapy. The characterization and management of recurrent atrial arrhythmias following surgical ablation of AF are discussed below.

Navigated DENSE strain imaging for post-radiofrequency ablation lesion assessment in the swine left atria

AIMS

Prior work has demonstrated that magnetic resonance imaging (MRI) strain can separate necrotic/stunned myocardium from healthy myocardium in the left ventricle (LV). We surmised that high-resolution MRI strain, using navigator-echo-triggered DENSE, could differentiate radiofrequency ablated tissue around the pulmonary vein (PV) from tissue that had not been damaged by radiofrequency energy, similarly to navigated 3D myocardial delayed enhancement (3D-MDE).

METHODS AND RESULTS

A respiratory-navigated 2D-DENSE sequence was developed, providing strain encoding in two spatial directions with 1.2 × 1.0 × 4 mm(3) resolution. It was tested in the LV of infarcted sheep. In four swine, incomplete circumferential lesions were created around the right superior pulmonary vein (RSPV) using ablation catheters, recorded with electro-anatomic mapping, and imaged 1 h later using atrial-diastolic DENSE and 3D-MDE at the left atrium/RSPV junction. DENSE detected ablation gaps (regions with >12% strain) in similar positions to 3D-MDE (2D cross-correlation 0.89 ± 0.05). Low-strain (<8%) areas were, on average, 33% larger than equivalent MDE regions, so they include both injured and necrotic regions. Optimal DENSE orientation was perpendicular to the PV trunk, with high shear strain in adjacent viable tissue appearing as a sensitive marker of ablation lesions.

CONCLUSIONS

Magnetic resonance imaging strain may be a non-contrast alternative to 3D-MDE in intra-procedural monitoring of atrial ablation lesions.

Catheter ablation of scar-related atypical atrial flutter

AIMS

The aim of the study was to assess the impact of isthmus location of atypical atrial flutters/atrial tachycardias (ATs) on outcomes of catheter ablation. Atrial tachycardias are clinically challenging arrhythmias that can occur in the presence of atrial scar--often due to either cardiac surgery or prior ablation for atrial fibrillation. We previously demonstrated a catheter ablation approach employing rapid multielectrode activation mapping with targeted entrainment manoeuvrs. However, the role that AT isthmus location plays in acute and long-term success of ablation remains uncertain.

METHODS AND RESULTS

Retrospective multicenter analysis of 91 consecutive AT patients undergoing ablation using a systematic four-step approach: (i) high-density activation mapping; (ii) analysis of atrial activation to identify wavefronts of electrical propagation; (iii) targeted entrainment of putative channels; and (iv) irrigated radiofrequency ablation of constrained regions of the circuit. Clinical outcomes, procedural details, and clinical profiles were determined. A total of 171 ATs (1.9 ± 1.0 per patient, 26% septal ATs) were targeted for ablation. The acute success rates were 97 and 77% for patients with either non-septal ATs or septal ATs, respectively (P = 0.0023). Similarly, the long-term success rates were 82 and 67% for patients with either no septal ATs or at least one septal AT, respectively (P = 0.1057). The long-term success rates were 75, 88, and 57% for patients with ATs associated with prior catheter ablation, cardiac surgery or MAZE, and idiopathic atrial scar, respectively.

CONCLUSION

Catheter ablation of AT can be successfully performed employing a strategy of combined high-density activation and entrainment mapping. Septal ATs are associated with higher rates of acute and long-term recurrences.

Visually-Guided Balloon Catheter Ablation of Atrial Fibrillation

Background— Electric isolation of the pulmonary veins (PVs) can successfully treat patients with paroxysmal atrial fibrillation. However, it remains technically challenging to identify the left atrial–PV junction and sequentially position the ablation catheter in a point-by-point contiguous fashion to isolate the PVs. In this study, a novel endoscopic ablation system was used to directly visualize and ablate tissue at the left atrial–PV junction with laser energy.

Methods and Results— This study consisted of 2 phases: a short-term (n=9) and long-term (n=11) canine experimental validation phase and a multicenter clinical feasibility phase (n=30 paroxysmal atrial fibrillation patients). After transseptal puncture, the balloon-based endoscopic ablation system was advanced to each PV ostium, and arcs of laser energy (90° to 360°) were projected onto the target left atrial–PV junction. Electric PV isolation was defined with a circular multielectrode catheter. In the short-term preclinical experimental phase, 15 of 17 targeted PVs (88%) were successfully isolated. Pathological examination revealed well-demarcated circumferential lesions with minimal endothelial disruption. In the long-term experiments, 9 of 10 targeted veins (90%) remained persistently isolated (at 4 to 8 weeks). In the clinical phase, 105 of 116 PVs (91%) were successfully isolated. After a single procedure, the 12-month drug-free rate of freedom from atrial fibrillation was 60% (18 of 30 patients). There were no significant PV stenoses, but adverse events included 1 episode of cardiac tamponade, 1 stroke without residual defect, and 1 asymptomatic phrenic nerve palsy.

Conclusion— This study establishes the feasibility of a novel paradigm for AF ablation: direct visualization to guide catheter ablation of the left atrial–PV junction.

Prevention of ventricular arrhythmias with sarcoplasmic reticulum Ca2+ ATPase pump overexpression in a porcine model of ischemia reperfusion

BACKGROUND

Ventricular arrhythmias are life-threatening complications of heart failure and myocardial ischemia. Increased diastolic Ca2+ overload occurring in ischemia leads to afterdepolarizations and aftercontractions that are responsible for cellular electric instability. We inquired whether sarcoplasmic reticulum Ca2+ ATPase pump (SERCA2a) overexpression could reduce ischemic ventricular arrhythmias by modulating Ca2+ overload.

METHODS AND RESULTS

SERCA2a overexpression in pig hearts was achieved by intracoronary gene delivery of adenovirus in the 3 main coronary arteries. Homogeneous distribution of the gene was obtained through the left ventricle. After gene delivery, the left anterior descending coronary artery was occluded for 30 minutes to induce myocardial ischemia followed by reperfusion. We compared this model with a model of permanent coronary artery occlusion. Twenty-four-hour ECG Holter recordings showed that SERCA2a overexpression significantly reduced the number of episodes of ventricular tachycardia after reperfusion, whereas no significant difference was found in the occurrence of sustained or nonsustained ventricular tachycardia and ventricular fibrillation in pigs undergoing permanent occlusion.

CONCLUSIONS

We show that Ca2+ cycling modulation using SERCA2a overexpression reduces ventricular arrhythmias after ischemia-reperfusion. Strategies that modulate postischemic Ca2+ overload may have clinical promise for the treatment of ventricular arrhythmias.

Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts

Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.

Analysis of the left atrial appendage by magnetic resonance angiography in patients with atrial fibrillation

BACKGROUND

Recent interest has focused on the left atrial appendage (LAA) in the setting of atrial fibrillation as a potential source of thromboembolism and stroke, which may be amenable to permanent occlusion by a variety of investigational catheter-delivered devices. Precise anatomic characterization of the LAA is necessary to determine the suitability of a patient for device placement and for device selection and sizing.

OBJECTIVES

The purpose of this study was to perform detailed three-dimensional characterization of LAA size and geometry by magnetic resonance angiography.

METHODS

Fifty patients with chronic atrial fibrillation undergoing cardiac magnetic resonance angiography in preparation for catheter ablation of atrial fibrillation were analyzed for LAA volume, neck size, depth, and overall geometry.

RESULTS

The average LAA volume was 17.3 +/- 6.7 mL, with a depth of 26.6 +/- 4.9 mm and a "neck" diameter of 20.0 +/- 5.3 mm x 14.1 +/- 4.7 mm. The average number of LAA lobes was 1.4 +/- 0.7 (range 1-4). Substantial interpatient variability was present in the relative dimensions and morphology of the LAA. There was a significant correlation between left atrial size and LAA neck dimensions.

CONCLUSION

There is significant heterogeneity in LAA size and dimensions among patients with atrial fibrillation. Device occlusion of the LAA may require devices that are available in multiple sizes/shapes or that can adapt to this heterogeneity.

Focal gap junction uncoupling and spontaneous ventricular ectopy

Genetic studies in the mouse have demonstrated that conditional cardiac-restricted loss of connexin43 (Cx43), the major ventricular gap junction protein, is highly arrhythmogenic. However, whether more focal gap junction remodeling, as is commonly seen in acquired cardiomyopathies, influences the propensity for arrhythmogenesis is not known. We examined electrophysiological properties and the frequency of spontaneous and inducible arrhythmias in genetically engineered chimeric mice derived from injection of Cx43-deficient embryonic stem cells into wild-type recipient blastocysts. Chimeric mice had numerous well-circumscribed microscopic Cx43-negative foci in their hearts, comprising approximately 15% of the total surface area as determined by immunohistochemical analysis. Systolic function in the chimeric mice was significantly depressed as measured echocardiographically (19.0% decline in fractional shortening compared with controls, P < 0.05) and by invasive hemodynamics (17.6% reduction in change of pressure over time, P < 0.01). Chimeras had significantly more spontaneous arrhythmic events than controls (P < 0.01), including frequent runs of nonsustained ventricular tachycardia in some of the chimeric mice. However, in contrast to mice with conditional cardiac-resricted loss of Cx43 in the heart, no sustained ventricular tachyarrhythmias were observed. We conclude that focal areas of uncoupling in the myocardium increase the likelihood of arrhythmic triggers, but more widespread uncoupling is required to support sustained arrhythmias.

Reduced intercellular coupling leads to paradoxical propagation across the Purkinje-ventricular junction and aberrant myocardial activation

Ventricular tachycardia is a common heart rhythm disorder and a frequent cause of sudden cardiac death. Aberrant cell-cell coupling through gap junction channels, a process termed gap junction remodeling, is observed in many of the major forms of human heart disease and is associated with increased arrhythmic risk in both humans and in animal models. Genetically engineered mice with cardiac-restricted knockout of Connexin43, the major cardiac gap junctional protein, uniformly develop sudden cardiac death, although a detailed electrophysiological understanding of their profound arrhythmic propensity is unclear. Using voltage-sensitive dyes and high resolution optical mapping techniques, we found that uncoupling of the ventricular myocardium results in ectopic sites of ventricular activation. Our data indicate that this behavior reflects alterations in source-sink relationships and paradoxical conduction across normally quiescent Purkinje-ventricular muscle junctions. The aberrant activation profiles are associated with wavefront collisions, which in the setting of slow conduction may account for the highly arrhythmogenic behavior of Connexin43-deficient hearts. Thus, the extent of gap junction remodeling in diseased myocardium is a critical determinant of cardiac excitation patterns and arrhythmia susceptibility.

Modulation of cardiac gap junction expression and arrhythmic susceptibility

Connexin43 (Cx43), the predominant ventricular gap junction protein, is critical for maintaining normal cardiac electrical conduction, and its absence in the mouse heart results in sudden arrhythmic death. The mechanisms linking reduced Cx43 abundance in the heart and inducibility of malignant ventricular arrhythmias have yet to be established. In this report, we investigate arrhythmic susceptibility in a murine model genetically engineered to express progressively decreasing levels of Cx43. Progressively older cardiac-restricted Cx43 conditional knockout (CKO) mice were selectively bred to produce a heart-specific Cx43-deficient subline ("O-CKO" mice) in which the loss of Cx43 in the heart occurs more gradually. O-CKO mice lived significantly longer than the initial series of CKO mice but still died suddenly and prematurely. At 25 days of age, cardiac Cx43 protein levels decreased to 59% of control values (P<0.01), but conduction velocity was not significantly decreased and no O-CKO mice were inducible into sustained ventricular tachyarrhythmias. By 45 days of age, cardiac Cx43 abundance had decreased in a heterogeneous fashion to 18% of control levels, conduction velocity had slowed to half of that observed in control hearts, and 80% of O-CKO mice were inducible into lethal tachyarrhythmias. Enhanced susceptibility to induced arrhythmias was not associated with altered invasive hemodynamic measurements or changes in ventricular effective refractory period. Thus, moderately severe reductions in Cx43 abundance are associated with slowing of impulse propagation and a dramatic increase in the susceptibility to inducible ventricular arrhythmias.

Subdiaphragmatic murine electrophysiological studies: sequential determination of ventricular refractoriness and arrhythmia induction

Programmed electrical stimulation (PES) is a crucial aspect of the evaluation of the risk of arrhythmias in cardiac patients and provides a powerful tool for understanding the mechanisms of arrhythmia in experimental models. Whereas PES in the mouse is well characterized, the procedures allowing for follow-up studies in the same animal have not been developed. In this report, we describe a novel subdiaphragmatic approach that allows for repeat electrophysiological studies in the mouse. Under inhaled anesthesia, PES was performed in 36 wild-type mice via a stimulating electrode introduced through an epigastric incision and placed directly into the diaphragmatic surface of the heart. The procedure was repeated 7 days later. Ventricular effective refractory periods (VERP) did not change significantly between the initial and follow-up trials. Chronic treatment with amiodarone, however, was associated with a 70% prolongation in VERP from initial to follow-up studies (P < or = 0.001). In addition, PES of a genetically modified strain with sudden cardiac death, the connexin43 conditional knockout mouse consistently induced lethal polymorphic ventricular tachycardia. Thus sequential PES in mice is feasible with the use of a subdiaphragmatic approach, yields reproducible VERP values, and can be used to follow pharmacologically induced changes in VERP and identify mice at risk of lethal ventricular arrhythmias.