Substernal Space Research Articles

Performance and Safety of the Extravascular Implantable Cardioverter-Defibrillator Through Long-Term Follow-Up: Final Results From the Pivotal Study.

Substernal lead placement of the extravascular implantable cardioverter-defibrillator (EV ICD) permits both defibrillation at thresholds similar to those seen with transvenous ICDs and effective antitachycardia pacing (ATP), while avoiding the vasculature and associated complications. The global Pivotal study has shown the EV ICD system to be safe and effective through 6 months, but long-term experience has yet to be published. We aim to report the performance and safety of the EV ICD system throughout the study. The EV ICD Pivotal study was a prospective, global, single-arm, pre-market clinical study. Individuals with a class I or IIa indication for a single-chamber ICD per guidelines were enrolled. Freedom from major system- or procedure-related complications, as well as appropriate and inappropriate therapy rates, were assessed through 3 years using the Kaplan-Meier method. Anti-tachycardia pacing success was calculated using simple proportions. An implant was attempted in 316 patients [25.3% female, 53.8±13.1 years old, 81.6% primary prevention, LVEF 38.9%±15.4%]. Of 299 patients with a successful implant, 24 experienced 82 spontaneous arrhythmic episodes that were appropriately treated with either ATP only (38, 46.3%), shock only (34, 41.5%), or both (10, 12.2%) for a Kaplan-Meier-estimated rate of first any appropriate therapy of 9.2% at 3 years. Antitachycardia pacing was successful in 77.1% (37/48) of episodes, and ATP usage significantly increased from discharge to last follow-up visit (P<0.0001). Shock therapy was successful in 100% (27/27) of discrete, spontaneous ventricular arrhythmias. The inappropriate shock rates at 1 and 3 years were 9.8% and 17.5%, respectively, with P-wave oversensing the predominant cause. No major intraprocedural complications were reported and the estimated freedom from system- or procedure-related major complications was 91.9% at 1 year and 89.0% at 3 years. The most common major complications were lead dislodgement (10 events; n=9 patients, 2.8%), postoperative wound or device pocket infection (n=8, 2.5%), and device inappropriate shock delivery (n=4, 1.3%). Twenty-four system revisions were performed as a result of major complications related to the EV ICD system or procedure. From implant to study completion, the EV ICD Pivotal study demonstrated that a single integrated system with an extravascular lead placed in the substernal space maintains high ATP success, effective defibrillation, and a consistent safety profile.

Comprehensive analysis of substernal lead removal: experience from EV ICD Pilot, Pivotal, and Continued Access Studies.

The extravascular implantable cardioverter-defibrillator (EV ICD) has been shown to be safe and effective for patients at risk of sudden cardiac death, but little is known about EV ICD lead removal in humans. This analysis aimed to characterize the EV ICD lead removal experience thus far. This was a retrospective analysis of lead removals from the EV ICD Pilot, Pivotal, and Continued Access Studies. Patients with a successful EV ICD implant who underwent lead removal were included. The main objective was lead removal success. Ancillary objectives included characterizing technique used, procedure complications, and reimplantation status. An EV ICD system was successfully implanted in 347 patients across the 3 studies (25.9% female; 53.4 ± 13.3 years; left ventricular ejection fraction: 39.7 ± 15.9). Of these patients, 29 (8.4%) underwent lead removal with a mean lead dwell time of 12.6 ± 14.3 months (0.2-58.4). The main reason for lead removal was lead dislodgement (n = 9, 31.0%). Lead removal was successful in 27/29 (93.1%) cases [100% (19/19) success rate <1 year and 80% (8/10) success rate >1 year post-implant]. Simple traction was used in 22/26 (84.6%) and extraction tools in 4/26 (15.4%) successful cases where technique was known. No complications were reported for any of the removal procedures. All 11 EV ICD reimplant attempts were successful. Complete removal of the EV ICD lead was successful in 93.1% of cases, and simple traction was sufficient in most instances. Based on these results, lead removal from the substernal space was safe and achievable up to 3 years post-implant.

Substernal Extravascular Implantable Cardioverter-Defibrillator System Infections in Large Animals.

The extravascular implantable cardioverter-defibrillator (EV ICD) with lead implantation in the substernal space may provide clinical advantages over transvenous and subcutaneous systems. This is the first reported examination of substernal infection in large animals implanted with the EV ICD system.The system was implanted in 13 large animals (canine, porcine, and ovine). The porcine were co-implanted with a transvenous cardiac resynchronization therapy with defibrillator (CRT-D) system. Infection was promoted through a cadence of immunosuppressive monitors and study interventions. The animals were monitored for clinical presentation of infection over 12-18 weeks, and cultures were collected to confirm infection. Treatment was bifurcated: 1) some infections were treated only with antibiotics ( "antibiotics only" ), whereas 2) some infections were treated with system removal and antibiotics ( "antibiotics + explant" ). Histopathology was conducted at the study closure.Five infections were confirmed over the course of the study, four of which involved infection of the EV ICD system and one infection of only the concomitantly implanted transvenous CRT-D system without EV ICD-related infection. Among the four EV ICD infections, two of two infections treated with antibiotics only did not resolve whereas two of two infections treated with antibiotics + explant resolved, as shown by histology. The transvenous CRT-D system infection progressed to septicemia and endocarditis, requiring early study discontinuation. No EV ICD-related infection progressed to blood stream infection, and the sternal bone did not become infected when infection was present in the substernal tissues.The study findings suggest that EV ICD-related infections are treatable with system removal and antibiotic therapy.

Characterization of the substernal space with computed tomography imaging in patients with and without median sternotomy: Assessing a novel implant location for extravascular defibrillation lead placement.

Implantable cardioverter-defibrillators (ICDs) provide clinically significant therapy for the prevention of sudden cardiac death. This study aimed to characterize the substernal space using computed tomography (CT) in patients with and without prior midline sternotomy to investigate the feasibility of substernal ICD lead implantation in post-sternotomy patients. High-quality electrocardiogram-gated CT images from 100 patients (71% male, average body mass index 23.5±2.9) were retrospectively collected, including 50 patients with prior midline sternotomy (S-group) and 50 patients with no prior sternotomy (NS-group). Distances were measured from the retrosternal surface to the epicardial surface of the heart and segmented into four regions from the xiphoid tip and superiorly along the sternum. Results generally showed a measurable but narrower average sternum-to-heart distance in the prior sternotomy group compared to the non-sternotomy group in all four regions (p<.05). In the S-group, the sternum-to-heart distances across all regions ranged from 0 to 32.0mm, while in the NS-group, the distances ranged from 0 to 39.9mm. Small but measurable separations between the heart and sternum were observed in patients with prior sternotomy, particularly near the xiphoid region, indicating the potential viability of extravascular substernal ICD lead implantation in post-sternotomy patients.

Three-year extraction experience of a novel substernal extravascular defibrillation lead in sheep.

BackgroundThe extravascular implantable cardioverter‐defibrillator (EV ICD) with lead implantation in the substernal space may provide an alternative to transvenous and subcutaneous systems. This is the first‐reported chronic extraction experience for EV ICD leads. The aim of the study is to evaluate the chronic encapsulation and extractability of EV ICD leads.MethodsTwo EV ICD leads and one transvenous lead were implanted in each of 24 mature sheep. A subset of animals was evaluated yearly for histology and lead extractability. Extractions were performed using simple traction or extraction tools. Histology evaluated the encapsulating tissue.ResultsAt 1 year, extraction was performed successfully for two of five EV ICD leads with traction alone using ≤3.1 kg‐force (kgf) and the remainder extracted successfully with extraction tools; no transvenous leads were removed with traction alone. At 2 years, no EV ICD or transvenous leads were extracted with traction alone, while at 3 years, one of eight EV ICD leads and two of four transvenous leads were extracted with traction (0.8 and ≤2.3 kgf, respectively). There was one observation of hemopericardium resulting in tamponade with EV ICD extraction but without injury to cardiovascular structures and related to the unique implant tract. Among transvenous leads, inversion of the ventricle with loss of cardiac output resulted in abandonment of traction for two animals.ConclusionsChronic extraction of EV ICD leads from the substernal space was successfully performed using traction and simple tools through 3 years in sheep with one observation of hemopericardium that did not originate from cardiovascular injury.

Abstract 13966: Chronic ICD Lead Removal From the Substernal Space is Feasible Without Complication

Introduction: The transvenous ICD is the device standard for treatment of ventricular arrhythmias, but ICD lead extraction from the heart and vasculature has morbidity and mortality concerns. The extravascular ICD (EV ICD) with lead placement in the anterior mediastinum (substernal space) is in development as an alternative to transvenous ICD. Described is the first reported substernal lead removal experience from humans. Hypothesis: The anterior mediastinum, a soft tissue space absent major vascular elements, provides a suitable environment for chronic lead removal without complication. Methods: The first-in-human EV ICD pilot study was a prospective study conducted at 4 sites in Australia and New Zealand. Subjects with a class I or IIa indication for single-chamber ICD were implanted via minimally invasive technique, with the distal end of the lead placed substernally via an incision near the xiphoid, and the proximal end tunneled subcutaneously to a left midaxillary device. Results: A total of 21 patients underwent the EV ICD procedure, and 17 entered chronic follow-up after successful implant and testing. Among these 17 patients, 3 were electively explanted at 85, 114 and 310 days post-implant. Respectively, the first patient experienced a lead tip displacement resulting in P-wave oversensing and inappropriate shock; the second underwent elective chronic defibrillation testing 3 months post-implant, where a 10-Joule safety margin was not established; and the third experienced recurrent VT and intolerance to ATP due to high pacing threshold. Lead removals were conducted per HRS guidelines. The proximal end of the lead was removed from the device pocket to the xiphoid incision, with minor resistance noted in pulling the lead connector through the subcutaneous tunnel. Light traction was sufficient to remove the distal end of the lead from the substernal space, with minor tissue adhesions observed near the lead incision site. All leads were removed intact, and there were no adverse events associated with system removal. Conclusions: ICD lead removal from the substernal space is feasible through 310 days without adverse events. Further understanding from a larger patient population with greater implant duration will help verify these early results.

The extravascular implantable cardioverter-defibrillator: characterization of anatomical parameters impacting substernal implantation and defibrillation efficacy.

AimsThe aim of this study is to provide a thorough, quantified assessment of the substernal space as the site of extravascular implantable cardioverter-defibrillator (ICD) lead placement using computed tomography (CT) scans and summarizing adverse events and defibrillation efficacy across anatomical findings. Subcutaneous ICDs are an alternative to transvenous defibrillators but have limitations related to ICD lead distance from the heart. An alternative extravascular system with substernal lead placement has the potential to provide defibrillation at lower energy and pacing therapies from a single device.Methods and resultsA multi-centre, non-randomized, retrospective analysis of 45 patient CT scans quantitatively and qualitatively assessing bony, cardiac, vascular, and other organ structures from two human clinical studies with substernal lead placement. Univariate logistic regression was used to evaluate 15 anatomical parameters for impact on defibrillation outcome and adjusted for multiple comparisons. Adverse events were summarized. Substernal implantation was attempted or completed in 45 patients. Defibrillation testing was successful in 37 of 41 subjects (90%) using ≥10 J safety margin. There were two intra-procedural adverse events in one patient, including reaction to anaesthesia and an episode of transient atrial fibrillation during ventricular fibrillation induction. Anatomical factors associated with defibrillation failure included large rib cage width, myocardium extending very posteriorly, and a low heart position in the chest (P-values <0.05), though not significant adjusting for multiple comparisons.ConclusionRetrospective analysis demonstrates the ability to implant within the substernal space with low intra-procedural adverse events and high defibrillation efficacy despite a wide range of anatomical variability.

B-PO01-043 THE EXTRAVASCULAR ICD: CHARACTERIZATION OF ANATOMICAL PARAMETERS IMPACTING SUBSTERNAL IMPLANTATION AND DEFIBRILLATION EFFICACY

Subcutaneous ICDs are an alternative to transvenous defibrillators but have limitations related to ICD lead distance from the heart. An alternative extravascular system with substernal lead placement has the potential to provide defibrillation at lower energy and pacing therapies from a single device. To provide a thorough, quantified assessment of the substernal space as the site of extravascular implantable cardioverter-defibrillator (ICD) lead placement using computed tomography (CT) scans and summarizing adverse events and defibrillation efficacy across anatomical findings. A multi-center, non-randomized, retrospective analysis of 45 patient CT scans quantitatively and qualitatively assessing bony, cardiac, vascular, and other organ structures from two human clinical studies with substernal lead placement. Univariate logistic regression was used to evaluate 15 anatomical parameters for impact on defibrillation outcome and adjusted using the Holm procedure. Adverse events were summarized. Substernal implantation was attempted or completed in 45 patients. Defibrillation testing was successful in 37 of 41 subjects (90%) using ≥ 10-Joule safety margin. There were 2 intraprocedural adverse events in 1 patient, including reaction to anesthesia and an episode of transient atrial fibrillation during ventricular fibrillation induction. Anatomical factors associated with defibrillation failure included large rib cage width, myocardium extending very posteriorly and a low heart position in the chest (p-values <0.05), though not significant adjusting for multiple comparisons. Retrospective analysis demonstrates the ability to implant within the substernal space with low intraprocedural adverse events and high defibrillation efficacy despite a wide range of anatomical variability.

B-PO02-063 ELECTROGRAM R-WAVE AMPLITUDE FOR DIFFERENT SENSING VECTORS ACROSS DIFFERENT POSTURES FOR PATIENTS WITH AN EXTRAVASCULAR IMPLANTABLE CARDIOVERTER DEFIBRILLATOR

The extravascular (EV) ICD lead is in the substernal space. The amplitude of the sensed R-wave is less than that of transvenous ICDs. Because the lead does not move with the heart, R-wave amplitude may vary with posture. To characterize postural and temporal R-wave amplitude variability from near-field (Ring1-Ring2) and far-field (Ring1-Can) sensing vectors using data from the EV ICD clinical, pilot study. We studied 17 patients with scheduled follow-up at pre-hospital discharge, 2, 6, and 12 weeks. Unfiltered sinus rhythm R waves for Ring1-Ring2 and Ring1-Can vectors were recorded with each patient in 7 postures: supine, lying on right and left sides, prone, sitting, bending over and standing. R-wave amplitude was measured across time, posture and vector. The mean and standard deviation across patients was calculated for each posture, follow-up time, and vector. The Figure plots R-wave amplitude. The averaged R-wave amplitude was 1.86±0.93mV for Ring1-Ring2, 2.22±1.33mV for Ring1-Can. It was marginally greater for Ring1-Can than Ring1-Ring2 (p=0.076). Neither changed significantly over time (p=0.33). Posture-related changes were significant for sitting versus standing on Ring1-Can (p=0.002) and for bending over versus lying on left for Ring1-Ring2 (p=0.031). In clinical follow-up of 354 patient-months, decreases in R-wave amplitude caused transient oversensing but no inappropriate therapies. For both EV ICD sensing vectors, R-wave amplitude is stable over the first 12 weeks. Despite minor postural changes, R-wave amplitude variability did not cause inappropriate therapy.

Design and Preliminary Results of Sensing and Detection for an Extravascular Implantable Cardioverter-Defibrillator.

This study reports the sensing and arrhythmia detection performance of a novel extravascular (EV) implantable cardioverter-defibrillator (ICD) in a first-in-human pilot study. The EV ICD lead is implanted in the substernal space, resulting in novel sensing and detection challenges. It uses a programmable sensing profile with new or modified discrimination of oversensing and of ventricular tachycardia (VT) from supraventricular tachycardia (SVT). Electrograms were post-processed from induced ventricular fibrillation (VF) at implant to determine virtual detection times for each programmable sensitivity and the least-sensitive safe sensitivity setting. In ambulatory patients, programmed sensitivity provided at least a twofold safety margin for detecting induced VF. Noise discrimination was stress tested, and the effects of source, posture, and lead maturation were determined on electrogram amplitude. Telemetry Holter monitors were used to quantify undersensing and oversensing. In 20 patients at implant, the least-sensitive safe sensitivity for VF detection ranged from 0.1 to 0.6mV. Seventeen patients were followed up for a total of 16.6 patient-years. Electrogram amplitudes were stable over time, but there were significant differences among postures and sensing vectors. For the primary sensing vector, the weighted oversensing and undersensing rates were 1.03% and 0.40% respectively, on a beat-to-beat basis. Oversensing did not cause inappropriate therapy in patients with in situ leads. Oversensing discriminators withheld VF detection in 4 of 5 spontaneous, sustained oversensed episodes. SVT-VT discriminators correctly classified 93% of 128 sustained SVTs in monitor zones. In the EV ICD pilot study, oversensing did not cause inappropriate therapy during ambulatory follow-up of stable leads.

Patient comfort factors associated with the chronically implanted extravascular ICD (EV ICD): retrospective analysis of the first-in-human EV ICD trial

Abstract Background The extravascular ICD (EV ICD) utilizes a lead positioned within the substernal space and a device placed along the left midaxillary line to provide pacing and defibrillation therapies. Since substernal implantation positions the lead close to the heart, a lower energy for defibrillation is needed for the EV ICD relative to an ICD with subcutaneous lead placement. With lower energy requirements, the EV ICD system requires only a 33 cubic centimeter (cc) device, whereas the commercially available subcutaneous ICD utilizes a ∼60 cc device. The smaller form factor for EV ICD may require a smaller device incision site with smaller pocket, potentially resulting in less post-operative pain. Figure 1 illustrates the comparative sizes of the EV ICD and a subcutaneous device. While issues related to patient comfort have been published for the subcutaneous ICD, patient comfort factors associated with the EV ICD system have not been described previously. Purpose To characterize post-operative factors related to patient comfort associated with the EV ICD system. Methods The EV ICD first-in-human study was conducted at four sites in Australia and New Zealand. Data from chronically implanted patients were retrospectively analyzed for changes in analgesics in the post-operative window between implant and the two-week follow-up. Adverse events were collected between implant and six months related to patient-reported pain/discomfort, need for system revision based on discomfort, impaired wound healing and seroma formation. Results In total, 21 patients underwent implantation of the EV ICD system. Three patients had initiated pain control medications prior to enrollment, including one patient with fractured ribs. Overall, 12 patients (57.1%) received no anesthetic or analgesic medications besides acetaminophen beyond the day of implant. For the remainder of patients, post-operative medications included fentanyl (1), discontinued after two days; morphine (4), discontinued the day of implant (1) or after ≤2 days (3); or oxycodone (10), discontinued after ≤1 day (7) or ≤1 week (3). There were no intraprocedural complications. There were four non-serious, early-onset (≤1 week post-implant) adverse events identified, which included swelling (1), impaired healing at the xiphoid incision site (1), and two instances of inspiratory pain ≤1 day post-implant, all of which resolved without sequelae. There were no system revisions or removals associated with incision sites or pocket discomfort, and there were no reports of seroma formation through six months of follow-up. Conclusions Analysis of factors related to patient comfort indicate that the EV ICD system has generally been well tolerated across patients. A larger data set is needed to more fully characterize whether the smaller device size of the EV ICD relative to the subcutaneous defibrillator results in improved patient comfort. Sizes of extravascular devices Funding Acknowledgement Type of funding source: Private company. Main funding source(s): Medtronic plc

Chronic follow-up from the pilot study of a novel substernal extravascular implantable cardioverter-defibrillator

Abstract Background The investigational Extravascular Implantable Cardioverter Defibrillator (EV ICD) is an implantable defibrillator with defibrillation and pacing therapies, including the potential for antitachycardia pacing (ATP). Purpose To characterize chronic EV ICD system performance through 12 months of follow-up. Methods The first-in-human EV ICD Pilot study was prospectively conducted at four centers in Australia and New Zealand. Patients with Class I or IIa indications for ICD were enrolled between July and December of 2018. A defibrillation lead was inserted into the substernal space under radiologic guidance and a device was positioned close to the left midaxillary line. Figure 1 illustrates the implanted EV ICD. Defibrillation testing was conducted at implant and chronically in 5 patients. Follow-ups were conducted at pre-hospital discharge, 2 weeks, 4–6 weeks and 3 months post-implant, and every 6 months thereafter. Results A total of 26 patients were enrolled and 21 underwent the EV ICD implant procedure with no intraprocedural complications. Median energy for defibrillation at implant was 15 J; mean R-wave amplitude was 3.4±2.0 mV; mean pacing capture threshold was 5.1±2.0 volts. Seventeen of 21 implanted patients entered into chronic follow-up after successful system placement and implant testing, and 14 patients continue to be followed, representing &amp;gt;180 patient-months of total follow-up. Multiple episodes of spontaneous ventricular tachycardia (VT) were detected in one patient and either self-terminated or successfully cardioverted following unsuccessful ATP. One patient received an inappropriate shock secondary to lead tip displacement with subsequent P-wave oversensing. Elective chronic defibrillation testing was performed at physician discretion and converted 5 of 5 patients with ≤40 J (maximum energy of device). Three patients had the system electively removed due to 1) inappropriate shock after lead displacement, 2) recurrent VT without ATP termination, and 3) inadequate chronic safety margin for defibrillation. Conclusions The EV ICD is a novel platform for delivering high voltage therapy. Chronic system performance has generally remained stable during 12 months of follow-up and the device has demonstrated the ability to detect and treat VT. Fluoro images of implanted system Funding Acknowledgement Type of funding source: Private company. Main funding source(s): Medtronic plc

Análisis con componentes principales y estimación de parámetros genéticos para medidas zoométricas en caballo pura raza española de México

En el Caballo de Pura Raza Española (CPRE), los objetivos de selección son las disciplinas ecuestres en las que se desenvuelve, y los criterios de selección son las medidas zoométricas (MZ). El objetivo de este trabajo fue caracterizar el CPRE con análisis de componentes principales (ACP); estimar parámetros genéticos (PAG) para MZ, y los componentes generados con el ACP. Las MZ analizadas en 1806 individuos (n) fueron: perímetro torácico (PT), perímetro de rodilla, perímetro de caña, longitud escápulo – isquial (LEI), anchura de pecho (AP), altura del hueco sub – esternal (AHS), altura de pecho (ALP), altura a la cruz. El ACP se realizó con la matriz de correlaciones y el software SAS. Para los PAG se realizaron dos análisis multivariados: con todas las MZ, y otro a partir del ACP. Con el software MTDFREML se estimó la heredabilidad (h²) para cada variable y las correlaciones genéticas (rg) entre variables. Del ACP, cuatro componentes se utilizaron para PAG. Del ACP, las variaciones en tamaño se atribuyen a PT y ALP, para conformación media a AHS; AP y LEI, definen caballos pequeños. Las h2 para MZ oscilaron de 0.12 a 0.55 y un promedio de 0.31; todas las rg fueron positivas en el intervalo de 0.12 a 0.98 (promedio de 0.65). Las h² en los componentes fluctuaron de 0.29 a 0.47 (promedio de 0.38); las rg en su mayoría fueron positivas de 0.12 a 0.65, y una negativa (-0.13). En las MZ la respuesta a la selección directa estará en función de h², y la posible respuesta correlacionada será por la magnitud y el signo de las rg. El ACP caracterizó al CPRE a través de tres estratos, y pueden ser utilizados en los criterios de selección.

Therapy From a Novel Substernal Lead

The ASD2 (Acute Extravascular Defibrillation, Pacing, and Electrogram) study evaluated the ability to adequately sense, pace, and defibrillate patients with a novel implantable cardioverter-defibrillator (ICD) lead implanted in the substernal space.Subcutaneous ICDs are an alternative to a transvenous defibrillator system when transvenous implantation is not possible or desired. An alternative extravascular system placing a lead under the sternum has the potential to reduce defibrillation energy and the ability to deliver pacing therapies.An investigational lead was inserted into the substernal space via a minimally invasive subxiphoid access, and a cutaneous defibrillation patch or subcutaneous active can emulator was placed on the left mid-axillary line. Pacing thresholds and extracardiac stimulation were evaluated. Up to 2 episodes of ventricular fibrillation were induced to test defibrillation efficacy.The substernal lead was implanted in 79 patients, with a median implantation time of 12.0 ± 9.0 min. Ventricular pacing was successful in at least 1 vector in 76 of 78 patients (97.4%), and 72 of 78 (92.3%) patients had capture in ≥1 vector with no extracardiac stimulation. A 30-J shock successfully terminated 104 of 128 episodes (81.3%) of ventricular fibrillation in 69 patients. There were 7 adverse events in 6 patients causally (n = 5) or possibly (n = 2) related to the ASD2 procedure.The ASD2 study demonstrated the ability to pace, sense, and defibrillate using a lead designed specifically for the substernal space.

FLEXIBLE INTERCOSTAL GENERATOR FOR PACING AND DEFIBRILLATION

Extravascular (EV) implantable cardioverter defibrillators (ICD) are surgically implanted into the substernal space to provide effective defibrillation and circumvent problems of vascular access in patients with severe left ventricular dysfunction for primary or secondary prevention. Since these

Feasibility of extravascular pacing with a novel substernal electrode configuration: The Substernal Pacing Acute Clinical Evaluation study

Subcutaneous implantable cardioverter-defibrillators provide an alternative to transvenous defibrillation but require higher shock outputs and offer no antitachycardia pacing. The Substernal Pacing Acute Clinical Evaluation (SPACE) study evaluated the feasibility of pacing from an extravascular substernal location. The primary purpose of the SPACE study was to characterize pacing from the substernal space. Secondary objectives included evaluating extracardiac stimulation and recording electrograms. The SPACE study prospectively evaluated the feasibility of pacing with a commercially available electrophysiology catheter acutely implanted in the substernal space via minimally invasive subxiphoid access. Pacing data were collected in ≥7 vectors using constant current stimulation up to 20 mA and pulse width up to 10 ms. Catheter placement was successful in all 26 patients whounderwent the procedure, with a mean placement time of 11.7 ± 10.1 minutes. Eighteen patients (69%) had successful ventricular capture in ≥1 tested vector. The mean pacing threshold at apulse width of 10 ms was 7.3 ± 4.2 mA across all vectors (5.8 ± 4.4 V). Failed capture was generally associated with suboptimal catheter placement or presumed air ingression. A low level of extracardiac stimulation was observed in 1 patient. The mean R-wave amplitude ranged from 2.98 to 4.11 mV in the unipolar configuration and from 0.83 to 3.95 mV in the bipolar configuration. The data from the SPACE study demonstrate that pacing is feasible from the extravascular substernal location. A substernal electrode configuration has the potential to provide pacing in a future extravascular device without need for intracardiac hardware placement.

Assessment of the Extravascular Implantable Defibrillator: Feasibility of Substernal Ventricular Pacing.

The objective of this study was to assess feasibility of ventricular pacing and thresholds from within the substernal space to examine a new extravascular ICD configuration with pacing capabilities. In patients undergoing midline sternotomy, a duodecapolar diagnostic pacing catheter was positioned in the substernal space anterior to the pericardium, and a cutaneous patch in left lateral position. Different unipolar and bipolar pacing configurations were assessed. Strength-duration curves were performed to identify the optimal output, starting at 25 mA with a pulse width of 10 milliseconds. Eight patients with mean age 69 ± 9 years were included. In 5, ventricular capture was achieved in ≥1 configuration. The mean bipolar pacing thresholds at PW 10, 5, 3, 1 milliseconds were 12.4 ± 3.7 mA (5 patients), 13.3 ± 5.8 mA (3 patients), 18.3 ± 5.7 mA (3 patients), and 25 ± 0 mA (2 patients), respectively. The 60-mm electrode spacing was the most successful bipolar configuration. Unipolar pacing was successful in 3 out of 4 patients with mean thresholds of 10 ± 0 mA at 10 milliseconds (3 patients), 15 ± 0 mA at 5 milliseconds (3 patients), 16.7 ± 2.9 mA at 3 milliseconds (3 patients), and 20 ± 7.1 mA at 1 milliseconds (2 patients). Ventricular pacing from the substernal space in patients with midline sternotomy is feasible. Closed sternum studies are needed to determine pacing thresholds more accurately.

Abstract 13083: Anatomy Characterization Informs Implant Procedure Design for a Novel Extravascular ICD System

Introduction: Extravascular approaches to ICD therapy in some clinical situations require unique implant methods. Current systems require high energy and don’t provide anti-tachycardia pacing. A new extravascular approach involves placement of a defibrillation lead between the heart and sternum. Due to the unfamiliarity most ICD implanters have with this space, quantification of the anatomy there is crucial for development of a safe and effective procedure. Methods: CT scans were segmented and analyzed to characterize relative locations of structures in or near the substernal space including heart, lungs, sternum, and mammary arteries. Segmentation resulted in faceted 3D surface models. A lead was virtually implanted behind the sternum in the intended location. Proximity distributions from the lead to various structures in the space were characterized at multiple levels of the sternum (up to the 3 rd intercostal space (ICS)). Additionally, the location along the right ventricular (RV) long axis where the lead was closest to the heart was measured. Results: Analysis was completed in 68 patients. The myocardium was closest on average to the virtual lead at the level of the xiphisternal junction (Fig. 1)(5 th - 95 th percentile: 1.0 - 9.9 mm) and furthest away at the 3 rd ICS (4.2 - 47.5 mm). It was closest to lung tissue at the 3 rd ICS (1.2 -14.7 mm) and furthest away at the tip of the xiphoid process (20.7 - 73.9 mm). The mammary arteries were found to be no closer than 11.5 mm to either margin of the sternum at any level. At the xiphisternal junction, the lead sat at ~60% along the RV long axis (0%=RV apex, 100%=tricuspid valve), suggesting close proximity to the central portion of the RV. Conclusions: Meaningful variation exists in the geometric relationships between many of the anatomical structures of the substernal space. Despite this variation, these results support that a lead can be placed in the substernal space close to myocardium while avoiding other organs.

The Minimally Invasive Repair of Pectus Excavatum Using a Subxiphoid Incision.

Purpose Several surgeons have documented outcomes following the Nuss operation. Most reports have described the use of thoracoscopy to avoid cardiac injury. Since 1999, our group has utilized a subxiphoid incision, allowing insertion of the surgeon's finger into the substernal space to help guide the bar across the mediastinum. Our initial experience has been reported and we are now reporting our entire experience to date. Methods A retrospective review was conducted on all patients who underwent pectus excavatum repair using a subxiphoid incision from December, 1999 to September, 2015. Results During the study period, 554 repairs were performed. A total of 80% of the patients were male. The mean age was 14.3 years ± 3.1, the mean operating time was 52 minutes ± 17.4, the mean length of stay was 4.2 days ± 1.1, and the mean time to bar removal was 2.7 years ± 0.7. A total of 20 patients (3.6%) received two bars. No patients sustained cardiac injury or evidence of pericarditis. Postoperatively, 22 patients (4%) developed an infection, either cellulitis or a local abscess requiring incision and drainage and/or antibiotics. In four of these 22 patients, the wound infection developed after the bar had been removed. Only one patient required bar removal before 2 years due to an infection. A total of 12 patients required either repositioning of the bar due to rotation (4) or removal of a stabilizer due to chronic discomfort (8), 2 required early bar removal for chronic pain, and 1 patient developed a tension pneumothorax in the operating room. A recurrence has developed in two patients but neither patient has desired correction. Conclusion In this relatively large series of patients, the addition of a subxiphoid incision to the technique has allowed for safe passage of the bar across the mediastinum to avoid cardiac injury during the Nuss operation.

59-02: Ventricular Pacing from the Substernal Space in Humans

Introduction: Transvenous lead failure remains an important drawback of transvenous implantable cardioverter-defibrillators (ICD). The subcutaneous ICD overcomes this issue, but lacks pacing capabilities. The objective of this study was to assess feasibility of ventricular pacing and thresholds from within the substernal space to examine a new extravascular ICD configuration with pacing capabilities. Methods: Patients with midline sternotomy for coronary arterial bypass grafting (CABG) or aortic valve replacement (AVR) were enrolled. After the sternum was opened, a duodecapolar diagnostic pacing catheter was positioned in the substernal space anterior to the pericardium, and a cutaneous patch in the left midaxillary line, after which the chest was closed. Different pacing configurations were assessed: bipolar between the catheter electrodes (inter electrode spacing 2mm, 30mm, or 60mm) and unipolar between the catheter electrodes and the patch. A downwards stepwise pacing protocol was used to identify the best configuration, starting at 25mA with a pulse width (PW) of 10ms. Results: Eight patients, (6 males), with mean age 69 ± 9 years, of whom 7 underwent AVR and one CABG, were included. In five out of eight patients ventricular capture was achieved in ≥1 configuration. The mean bipolar pacing thresholds at PW 10ms, 5ms, 3ms, 1ms were 12.4 ± 3.7mA (5 pts), 13.3 ± 5.8mA (3 pts), 18.3 ± 5.7mA (3 pts) and 25 ± 0mA (2 pts) respectively. The 60mm electrode spacing was the most successful bipolar configuration. Unipolar pacing was attempted in 4 pts and successful in 3 pts with mean thresholds of 10 ± 0mA at 10ms (3 pts), 15 ± 0mA at 5ms (3 pts), 16.7 ± 2.9mA at 3ms (3 pts) and 20 ± 7.1mA at 1ms (2 pts). The optimal unipolar vector was from the catheter electrodes over the right ventricle to the patch.