Approach to pacing in patients with various septal defects

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Introduction

Cardiac septal defects are the most common type of congenital heart disease (CHD), with ventricular and atrial septal defects occurring in 4 and 2 per 1000 live births, respectively.^1,2 Whether they occur isolated or part of a syndrome, studies have shown that septal defects are associated with conduction abnormalities through several different mechanisms, including geometric and electric remodeling, conduction system displacement, postsurgical-related complications, or as part of a genetic disorder.³ Conductional abnormalities may also occur irrespective of CHD in patients with incidental septal defects.

With 200,000 pacemakers implanted annually in the United States,⁴ electrophysiologists encounter patients with cardiac septal defects requiring pacemaker implantation. Patients with septal defects may constitute a challenge, as they may have atypical cardiac anatomy and complications related to structural anomalies. The ideal type of pacing may also largely depend on the patient’s septal defect, individualized risk factors, chance of progression to high-grade conduction abnormalities, and associated structural variances. Therefore, an adequate understanding of septal defects, their association with conduction abnormalities, and implications of pacemaker placement are key in providing patients with optimal care. In this paper, we present a patient with a septal defect undergoing pacemaker implantation and provide a review of the latest literature regarding the topic.

Case report

A 76-year-old African American woman, who had a history of diabetes mellitus, large patent foramen ovale (PFO), severe obstructive sleep apnea (apnea-hypopnea index 89), severe pulmonary hypertension (pulmonary artery systolic pressure 85 mm Hg by echocardiogram), obesity (body mass index 34 kg/m²), and history of breast cancer, presented with weakness and was found to have symptomatic intermittent complete heart block, as shown in Figure 1. The patient had a first-degree atrioventricular (AV) block, as shown in Figure 2. Her complete heart block was likely secondary to advanced age in the setting of first-degree AV block and PFO.

Figure 1.

Electrocardiogram showing complete heart block with junctional escape rhythm.

Figure 2.

Electrocardiogram showing normal sinus rhythm with first-degree atrioventricular block and premature ventricular contraction.

Permanent transvenous pacing leads are well known to be associated with thrombus formation. This is particularly important in patients with patent septal defects. Given the patient has a large PFO, the decision was made to choose a leadless pacemaker over a transvenous pacemaker.

A leadless pacemaker (Micra AV, MC1AVR1; Medtronic, Dublin, Ireland) was implanted under general anesthesia emergently using traditional fluoroscopic views to confirm that the PFO was not crossed during the implantation. In addition, bedside transthoracic echocardiography using Vivid^TM IQ (GE Vivid IQ, Milwaukee, WI) using views from the parasternal long axis with an inclination toward the right ventricular inflow tract view confirmed position, as shown in Figure 3, showing the delivery system crossing the tricuspid valve with avoidance of the septal defect.

Figure 3.

Echocardiographic view of the Micra AV pacemaker (Medtronic) during deployment. PFO = patent foramen ovale.

The delivery system and the tether with the device were visualized using traditional fluoroscopic views, as shown in Figure 4. The initial attempt at implantation in the distal part of the right ventricular septum yielded excellent results. The parameters, including pacing threshold, impedance, and sensitivity, on the day of the implant and the subsequent day were excellent.

Figure 4.

Fluoroscopic image of the final Micra Transcatheter Pacing System (Medtronic) position after tether cord was cut and removed.

The device was programmed initially with a lower rate of 50 beats/min and an upper rate of 115 beats/min, right ventricle amplitude of 2.5 V, pulse width of 0.24 ms, and sensitivity of 2.0 mV, and with the following parameters: lead impedance was 710 ohms, pacing threshold was 0.5 V at 0.24 ms, sensing threshold was 6.9 mV. Ten weeks postimplant device interrogation showed the following parameters: lead impedance was 490 ohms, pacing threshold was 0.38 V at 0.24 ms, and sensing threshold was 19 mV. A sensing vector was changed from 1+3 to 1+2 device for better P-wave sensing and A3 window end changed from 800 ms to 700 ms. Electrocardiogram showed ventricular paced rhythm (Figure 5). Chest radiograph showing the implanted leadless pacemaker (Figure 6).

Figure 5.

Electrocardiogram showing ventricular paced rhythm.

Figure 6.

Chest radiograph showing the implanted Micra system (Medtronic).

Discussion

Septal defects and conduction abnormalities

Cardiac septal defects are one of the common forms of CHD that can present either in isolation or as part of a syndrome, as shown in Figure 7. Ventricular septal defects are reported in as many as 4 per 1000 live births, while atrial septal defects (ASDs) comprise 6%–10% of congenital heart defects, with an incidence of 2 per 1000.^1,2 ASDs can present in different anatomical forms: ostium secundum defects, ostium primum defects, sinus venosus defects (inferior and superior), and coronary sinus defects.⁵ The most common presentation is the ostium secundum defects, comprising 75% of ASDs. AV canal defects are the least common form of septal defects, occurring in approximately 4–5 cases per 10,000 live births.¹ Intra-atrial communication may also occur in patients with PFO, which has an incidence as high as 30% of the general population.⁶

Figure 7.

Prevalence of the different types of septal defects and highlights of the main mechanisms underlying the associated conduction abnormalities. ASD = atrial septal defect; CSD = coronary sinus defect; PD = ostium primum defect; SD = ostium secundum defect; SVD = sinus venosus defect; VSD = ventricular septal defect.

The relationship between congenital septal defects and arrhythmias has been studied, as multiple risk factors for conduction system defects and tachyarrhythmias exist in this population.³ Sinus node dysfunction and varying degrees of AV nodal dysfunction have been reported in the literature, with the prevalence, however, remaining uncertain.7, 8, 9 Retrospective studies of patients with secundum defects report sinoatrial node dysfunction and varying AV blocks in 60% and 30%–40% of patients, respectively.⁸ It is important to highlight that the number of patients in these studies was small and may not be representative of the larger population of patients with septal defects.

Several mechanisms have been suggested regarding the association between septal defects and the resulting conduction abnormalities. Septal defects are typically associated with left-to-right shunt, which can lead to myocardial stretch under chronic volume overload and electrophysiologic remodeling, which may precipitate arrhythmias.¹⁰ While some defects are isolated from the specialized conduction structures, other defects can occur in the path of the congenital conduction system, causing disruption. Some examples include the inferoposterior displacement of the AV node and bundle of His, which is often associated with ostium premium defects and frequently results in high-grade AV node blocks.¹¹ Septal defects and conduction blocks may also occur simultaneously in patients with genetic disorders such as Down syndrome and Holt-Oram syndrome.⁷ Corrective surgical intervention for septal defects can induce inflammation, edema, and potential damage to the conduction system. Postoperative conduction disturbances are generally transient and recover within 7–10 days in half of the patients.¹² The incidence of postsurgical complete AV blocks requiring pacemaker implantation after congenital heart surgery is estimated to be around 1%.¹³ Indications for postoperative pacemaker implantation include those with persistent AV block >7 days postoperatively or presence of concerning permanent fascicular block.³

Pacemaker implantation in patients with septal defects

The number of studies assessing the implications of pacemaker implantation in patients with septal defects is limited, as these patients comprise only a small proportion of all pacemaker implantations. Electrophysiologists may face several challenges during pacemaker implantation in patients with a septal defect, given structural variations and conduction system anatomy. Adequate knowledge about preoperative investigations, procedural techniques, anatomical variations, different pacemaker settings, and unusual complications is vital to improve outcomes and prevent adverse events.

Preoperative assessment in patients with known septal defects should include a 12-lead electrocardiogram and imaging modalities such as a chest radiography, transthoracic echocardiography, or transesophageal echocardiography. Chest computed tomography, cardiac magnetic resonance imaging, and cardiac catheterization might also be necessary to accurately identify the patient’s cardiovascular anatomy. This is important, as septal defects can be part of a more complex CHD (such as tetralogy of Fallot, dextrocardia, transposition of great arteries, etc) rather than an isolated finding, and imaging is essential to determine the anatomy.

The choice between transvenous vs epicardial pacing depends on multiple factors, including patient profile, cardiac anatomy, and patency of the shunt and ease of access to the desired cardiac chamber, especially when surgical interventions have been performed, as shown in Table 1. Generally, epicardial pacing is more invasive, requiring thoracotomy compared to transvenous pacing.¹⁴ Epicardial leads are also associated with higher thresholds and a less reliable long-term lead durability.¹⁵ However, transvenous pacing has several limitations in patients with septal defects and CHD. Complex cardiovascular and venous anatomy can be seen in patients with CHD, hindering the placement of transvenous leads if the desired chamber cannot be accessed easily or if there is significant scarring of the chamber, leading to poor capture thresholds.¹⁴ This can especially be seen in patients who underwent septal device closure, where the resulting fibrosis may lead to difficulties in the achievement of desired lead parameters in optimal locations.

Table 1.

Comparison between the different methods of pacing and highlights of their main advantages and disadvantages

	Pacemaker type
	Transvenous pacemakers	Epicardial pacemakers	Leadless pacemakers
Invasiveness	Minimally invasive	More invasive via thoracotomy (at times mini-thoracotomy	Minimally invasive
Long-term stability	Stable long-term pacing	Less reliable long-term performance	Comparable long-term performance to transvenous pacemakers
Feasibility with complex congenital anatomy	Difficult placement in complex congenital anatomy and associated venous malformation	Easier to implant in complex congenital anatomy than transvenous pacemakers	Can be easier to implant in complex congenital anatomy. May be complicated if venous malformations are present. Easier to implant if there are issues with subclavian vein patency from prior transvenous devices.
Risk of thromboembolic events with septal defects	Increased risk than those without CHD (×2-fold)	No increased risk	Unknown risk
Other	- Risk of inadvertent lead placement into the systemic circulation - Risk of valvular regurgitation - Difficult to extract leads in CHD	-Easier to perform with concomitant correctional surgery for CHD -Decreased risk of infection compared to transvenous system	- Minimal risk of lead-related complications - No lead tunneling or pocket required -Decreased risk of infection -No risk of subclavian stenosis -Lower risk of need for device extraction -Limited data available - Requires advanced operator experience -Difficult to extract, if needed

CHD = congenital heart disease.

Permanent transvenous pacing leads are well known to be associated with thrombus formation.¹⁶ This is particularly important in patients with patent septal defects, where inadvertent lead placement in the systemic circulation may lead to embolic strokes, as was highlighted in a retrospective study of 202 patients with intracardiac shunts. In this study, transvenous pacing was associated with a >2-fold increase in the risk of thromboembolic events, including acute strokes.¹⁷ Interestingly, 3 of the patients who suffered from thromboembolic events were receiving warfarin therapy, with an international normalized ratio >2. This is in line with previous reports of thromboembolic events in patients with inadvertent lead placement in the systemic ventricular circulation despite antiplatelet therapy, raising questions regarding the efficacy of antithrombotic therapy in this patient population.¹⁸ Therefore, transvenous leads should be avoided in patients with large, particularly right-to-left shunts, unless shunt closure has been performed.

Newly emerging leadless pacing can be a valuable alternative to transvenous and epicardial pacing. The implantation of a leadless pacemaker is performed via a femoral vein transcatheter approach, compared to the more invasive approach required in epicardial pacing.¹⁹ The lead-related risks seen in transvenous pacing, including inadvertent left ventricular lead placement, are avoided with a leadless pacemaker with the help of transthoracic echocardiogram and fluoroscopic images, as highlighted in our case. Additionally, unlike conventional pacing, leadless pacemaker implantation does not require creating a pocket or lead tunneling, which can be one of the major challenges in patients with complex CHD. The need for lead extraction with aging leads in younger patients with congenital heart disease can also be avoided with leadless pacemaker implantation, although there may be limitations on how many devices can be implanted over a lifetime for younger patients. However, if there are issues with subclavian patency or need for extraction in patients that are high risk, a leadless pacemaker implantation may avoid high-risk interventions. The endothelialization of leadless pacemakers with maturity of the system may also be attractive in patients with shunting from systemic circulation, which can increase the risk of thromboembolic events.

The studies regarding the safety and efficacy of leadless pacemaker implantation are promising. The nonrandomized LEADLESS, LEADLESS II, and Micra Transcatheter Pacing studies involved 33, 526, and 725 patients, respectively, and reported rates of successful implantation ranging from 97% to 99.2%, with complications occurring in only 4%–6% of enrolled patients.19, 20, 21 However, randomized controlled studies are still unavailable, with very limited data regarding the utility of leadless pacemakers in patients with CHD.

The dimensions of the available leadless pacemakers, Nanostim and Micra, are 42 × 5.9 mm and 25.9 × 6.7 mm, respectively.²² Septal defect diameters can exceed 12 mm and grow to more than 20 mm with age, and PFO maximum diameter can reach 19 mm, with a mean size of 4.9 mm.^6,23 Therefore, the risk of inadvertent placement of the device through a defect or a PFO is rare but should be considered, as it was reported as a complication in 1 patient in the LEADLESS trial.¹⁹

In summary, patients with septal defects have a higher incidence of conduction abnormalities when compared to the general population. When indicated, pacemaker implantation requires careful assessment of the structural anomalies to avoid adverse outcomes. The risks and benefits of the different pacing modalities should be considered for each individual case. Leadless pacemakers may be a promising modality in patients with CHD, offering the ability to assess the implantation site on transthoracic echocardiogram as well as fluoroscopy at the time of the implant. Further studies are required to assess the efficacy and safety of leadless pacemakers in patients with CHD.