Mechanics of lumenless pacing lead strength during extraction procedures based on laboratory bench testing

Medtronic SelectSecure Model 3830 lumenless leads (Medtronic, Inc., Minneapolis, MN) were released in Europe in January 2003 and in the United States in August 2005. These leads were developed to reduce the diameter of pacing leads, improve reliability, and allow precise catheter delivery. Smaller-diameter cardiac leads may provide benefits such as reduced incidence of lead subclavian crush, reduced venous stenosis, reduced tricuspid valve involvement, and improved ease of extraction.^{,  ,  ,}  Initially embraced by the pediatric world, the adoption of the conduction system pacing system has led to a rapid growth in implants of the 3830 lead.^,,

The 3830 pacing lead has a unique lumenless lead design. Due to elimination of the central lumen, which contributes about 40% of the diameter of a standard 7F pacemaker lead, the 3830 lead is only 4.1F diameter. Traditional coaxial, multilumen, and co-radial lead designs use a coiled inner conductor with a central lumen, which allows for insertion of a shapeable stylet to facilitate steering and placement (Figure 1). This inner coiled conductor and lumen has 3 adverse effects. (1) It requires space, necessitating a larger lead diameter. (2) It can be susceptible to tight bending conditions, allowing for conductor kinking with flexion. This kinking can lead to a point of repetitive localized stress causing metal fatigue, potentially resulting in conductor fracture. (3) When tensile stress is applied the coils will elongate, resulting in significant irreversible stretch of the lead, such as occurs in lead extraction. The 3830 lumenless leads contains a flexible, high-tensile-strength, inextensible cable as the inner conductor, which is intended to avoid conductor fatigue issue by reducing stress under high bending conditions. During extraction of traditional coaxial leads (with lumen), locking stylets are placed into the inner lumen and expanded to grip the inner coil surface, thus allowing for traction to the lead tip and reducing lead stretch. Rail strength, or the overall tensile strength of the lead, is the lead’s capability to tolerate the forces of extraction before degradation influences the extraction. Rail strength is needed to apply traction to the lead alone, or during use of extraction tools to deflect the tool away from vascular structures, maintain concentricity with the tool, and for countertraction. With lumenless leads, the central cable itself may be used for traction because the cable does not stretch compared to a coiled conductor and allows for traction directly to the lead tip/screw appropriate for small-diameter lead use conditions. The cable conductor is a 7 × 7 construction (7 bundles of 7 MP35N wires for a total of 49 wires) jacketed with ethylene tetrafluoroethylene. The cable tensile strength is approximately 13 lbf (5.9 kgf). The lead helical screw is not retractable and is designed to begin to straighten with force of about 2 lbf (0.9 kgf). In this study, multiple lead preparation techniques and extraction use conditions were evaluated with the goal of characterizing the extraction rail required to influence successful extraction.

Without the lumen for stylet, the 3830 lead must be implanted using a delivery sheath/catheter system. The steerability of both deflectable and fixed shape delivery systems allows for specific site selection while the small diameter of the lead allows for tissue penetration. Modern conduction system pacing first targeted the bundle of His and now more commonly targets the matrix of the left bundle with lead penetration of the interventricular septum, referred to as left bundle branch area placement.^, As of July 2022, more than 100,000 implants of the model 3830 lead have been performed in the United States, and long-term lead performance has been excellent, with reported Medtronic performance data demonstrating 96% atrial and 97% ventricular lead survival at 10 years.

For any cardiac implantable electronic device lead implanted, implanters must consider long-term management and potential future extraction, and the 3830 lead is no exception. In an extraction, a mechanical or powered sheath is advanced over the lead to free the lead from adhered scar tissue. Accomplishing feasible extraction requires the lead to act as a stable extraction rail for the extraction tool to track. In leads with stylet lumens, that rail is composed of all the components of the lead body and the selected locking stylet that is deployed and locked within the inner conductor lumen (Supplemental Figure 1). The Medtronic 3830 lead has no such lumen; it uses the inner conductor cable and outer lead body components as the extraction rail. Although the potential benefit of 3830 leads in ease of extraction has been reported in the literature, the data are limited.^{,,  ,}  Large extraction studies of long-term 3830 implants are not available.

The small size of the 3830 lead and its lumenless design differ from the standard coaxial lead, and unique leads such as the 3830 require unique extraction techniques. The present study suggests tested lead preparation techniques based on bench test data related to known extraction methods.^, The goal is to provide physicians with an approach for successful extraction of 3830 leads.

Lead rail tensile strength of the model 3830 lead was assessed on a mechanical bench test apparatus for various lead preparation methods (Figure 2). Expired clinical product (>2 years past the manufactured date) was used to ensure testing was performed on leads equivalent to products used clinically. The leads were not preconditioned so as to simulate biological degradation over time or due to implantation damage related to torque delivery. Torquing leads may have a wide range of use conditions but, unless excessive, are not thought to have a significant effect on inner cable conductor strength. There were 5 test conditions: 3 lead preparations and 2 use conditions. The 3 lead preparation techniques were (1) IS1 connector removed plus standard Bulldog™ lead extender (BDLE) preparation (Cook Medical, Bloomington, IN); (2) IS1 connector removed plus combination of the BDLE method and the One-Tie® compression coil (OTCC) (Cook Medical); and (3) retained IS1 connector method using an orthopedic suture (FiberWire® [FW], Arthrex, Naples, FL) 2-0 or 0, because of its known tensile strength performance and low profile (Figure 3). The 2 use conditions were (1) simulated mid-lead (tissue) adhesions (MLAs); and (2) use of a distal snare for isolating the lead tip (Table 1).

The first technique used the standard IS1 connector removal preparation described in the Food and Drug Administration–approved instructions for use for the BDLE (Figure 3A).

The second technique for the modified IS1 connector removal lead preparations used both a BDLE and an OTCC for lead extension (Figures 3B–3F). The lead body was cut, removing the IS1 connector and approximately 8 cm of the outer 55D polyurethane tubing, and 6 cm of the outer conductor coil was peeled back and cut, including the silicone tubing over the inner cable. The tubular metal sleeve of the BDLE was removed, and the cable was inserted from the proximal end of the BDLE loop and wrapped 3 times around the loop. The lead was oriented colinear with the BDLE, and the OTCC was positioned with 2 wires bridging the end of the outer coil conductor. The OTCC then was twisted about the end on the lead outer conductor coil end (Figure 3F). Preparation of the OTCC was completed by 2 experienced operators and 1 senior test engineer who assessed the preparation’s joint strength.

The third technique, the retained IS1 connector preparation, used the FW suture applied to the IS-1 proximal connector pin. To avoid slippage of the suture from the proximal connector pin, the proximal seal between the connector pin and ring was removed by shaving off the silicone seal with a scalpel or by an axial incision in the silicone and peeling the seal away. A constrictor knot was used to provide a self-tightening knot (Supplemental Video 1), which eliminated any slippage due to loose knots and a low profile to permit advancement of the extraction sheath.

To measure lead rail tensile strength, the preparation was gripped by the upper grip jaws (capstan) of the tensile force test machine (Model 5565, Instron, Norwood, MA). In preparations 1 and 2, the BDLE was gripped by the Instron. In preparation 3, the FW suture was gripped by the Instron. Distal fixation of the lead was achieved by using an epoxy “knob” in the shape of a cylinder to provide distal tip fixation simulating a robust lead tip adhesion. The epoxy knob was also applied to create an MLA approximating the junction of the innominate and superior vena cava veins. The epoxy knobs were held in place in a “simulated heart fixture” and permitted snaring of the lead above the MLA and between the mid- and distal tip fixation with a snare (Needle’s Eye Snare®, Cook Medical) to simulate femoral snare stabilization techniques (Figure 3). Previous bench tests have produced similar overstress modes of failure as identified in returned products.

Additionally, we evaluated the interaction between the 3830 lead and the 2 rotating mechanical extraction tools—Cook Medical Evolution® RL and Philips TightRail—at various angles of approach to the lead body to assess the potential for lead damage during mechanical activation. Although Philips laser extraction tools are commonly used, interaction with these tools was not evaluated in this study.

Pull force means were compared using a 2-sample t test within Minitab 18. The sample sizes for each condition (Table 2) were used to provide understanding of preparation method performance but was not powered to detect any particular difference.

Techniques 1 and 2 yielded lower mean and greater variability in tensile strength of 5.78 lbf (range 4.55–7.19 lbf) for technique 1 and 8.51 lbf (range 1.66–14.32 lbf) for technique 2 (Table 2). Preparations of technique 2 that included “cinching down” the OTCC onto the junction of the lead end and the BDLE produced a wide range of rail strengths and experienced slipping of the cable from the preparation site in 6 of the 9 samples, with a mean 8.51 lbf (range 1.66–14.32 lbf). The variation in lead preparation strength was observed to be related to the number of wraps and the tightness of the “cinching” of the OTCC and the looping of the coil through the cinching region (Figure 4).

Retaining the connector (technique 3) provided greater mean tensile strength than either of the cut lead preparations (techniques 1 and 2), with an average of 11.42 lbf (range 9.85–12.73 lbf). Although the cut lead method with BDLE can achieve similar rail strengths as the retained connector method, the technique variability and required time and effort make this approach less desirable. The use or addition of sutures on the connector grip sleeve provided similar tensile strength performance as a suture only on the connector pin.

Use of a snare distally on the lumenless lead did not significantly affect rail strength, providing 10.06 lbf (range 8.68–10.85 lbf). However, the case of the simulated MLA and snare provided significantly higher rail strength (P <.05), with a mean of 13.41 lbf (range 12.83–13.95 lbf).

Interaction with a mechanical cutting extraction tool demonstrated the potential for damage with the TightRail only at angles to the lead ≥90°, such as may occur with right-sided implant approaches. At tight angles, the small-diameter 3830 lead is no longer coaxial within the TightRail sheath. The size difference allows the lead to potentially be pulled into the advancing internal cutting blades. No damage was observed with the Evolution at any angle because the cutting surface is external (Supplemental Figure 2).

There is limited but growing experience with extraction of 3830 leads with lumenless construction.^{,  ,  ,}  The specific lead construction defines the primary extraction method to be used, and unique lead constructions require unique extraction techniques. In the past, lead extraction techniques were primarily empirically derived and passed on with comparative data regarding which technique can provide greater strength or feasibility. More recently, bench testing has been used for the development, simulation, and introduction of other unique extraction techniques for unique lead constructions. The goal of this study was to use bench testing methodology to understand the unique construction characteristics of the 3830 lead and to utilize known extraction techniques for maintaining rail strength and lead integrity during extraction. The methodology was created based on analysis of returned product and bench testing to simulate extraction methods. These methods were confirmed to produce similar overstress failure mechanisms that had been observed in returned product examinations. Although not an in vivo analysis, this study does offer an understanding of the structural characteristics of the 3830 lead and a recommendation for approaches to lead preparation that can influence lead extraction feasibility.

In an extraction procedure, a sheath (or catheter) is tracked along the lead to remove scar attachments, and the tensile strength of the lead must be greater than the force required to successfully overcome those adhesions. The scar burden is highly variable and determined by implant duration, number of leads, lead characteristics, the patient’s comorbid conditions, as well as undetermined variables. In the case of the 3830 lead, the rail strength is composed of the entire lead body, with the majority of lead strength being derived from the centralized conductor cable. During extraction, securing the 3830 cable is critical to rail integrity and allows the insulation and outer coil to be supported by the cable. Although the goal of extraction is to remove the lead from adherent scar, this study and other investigations have shown that scar adherence to the lead body initially adds to rail strength by more evenly distributing traction force along the length of the lead and is present until that scar is removed. This scar burden supports lead tensile strength when a sheath is advanced, thus protecting distal structures.

There are extraction advantages to use of the 3830 lead system. By replacing the standard conductor coils with a conductor cable, the issues of failure to completely place a locking stylet to the lead tip, the inability of conventional locking stylets to lock at the lead tip stylet, and slippage with traction and coil stretching can be eliminated. In the 3830 lead, the conductor cable within the lumenless lead design spans the entire lead length and is attached directly to the lead screw tip on the distal end and to the proximal connector pin.

Upon market release there was a perception that the 3830 lead had the strength of a locking stylet incorporated within the lead. With the circulation of a video demonstrating the ability to suspend a young child within a doorway, the expectations of the strength of the 3830 may have been inflated. Although wound cables are strong, the 3830 cable has a smaller diameter than a locking stylet or lead locking device. This results in a traction rail of less strength but provides consistent engagement from distal tip to proximal pin. Data from the Philips simulator demonstrate the average extractor pulls at 3–8 lbf while the sheath is advanced. It is important to understand that traction force on the lead is not equal to transmitting that force to the fibrotic attachments in the vein, the other leads, and the heart. When properly applied, the traction force is countered by the advancement force of the extraction sheath, called counterpressure, shielding the heart from direct tugging on the cardiac structures. The lead is used as the rail to break up the fibrosis along the lead and advance the sheath near the endocardial wall. Then with a small force, pluck the lead from the final attachment. This maneuver, called countertraction, usually requires only a few pounds of traction force. Philips and Cook locking stylets tolerate traction forces of 13–16 lbf before failure, and the 3830 (if undamaged) has a cable strength of approximately 13 lbf. During extraction, diligence is required to minimize traction forces, usually much less than 10 lbf; otherwise, the maneuver risks damaging the conductor cable and losing the rail and the ability to remove the entire lead. A device with the ability to monitor applied traction force has been used to a limited degree during clinical extractions, but it is bulky and interferes with the logistics of the procedure. Developing skill in transvenous lead extraction requires experience in limiting the extraction force to the minimum required and limiting its transmission to the cardiac structures, whether the lead is a lumenless lead or another construction. Differing lead constructions will tolerate different levels of traction force.

In this study, we found preservation of the proximal connector end of lumenless leads provided the most consistent and best ability to tolerate extraction loading forces compared to cutting of the connector. For 3830 leads, retention of the terminal pin has 2 major advantages: (1) incorporation of insulation into the extraction rail allowing load bearing by the insulation; and (2) a consistent engagement with the conductor cable with optimum load-bearing capabilities. A well-prepped lead has equal load-bearing characteristics as a lead with retention of the terminal pin; however, in this study, a wide variability in load-bearing characteristics was seen in a range of prepped leads. In addition, a poorly prepped lead can result in cable damage and subsequent breakage. The simple and easily reproducible technique of using a strong suture tied on a retained terminal pin yielded the most consistent and best rail for load-bearing characteristics.

With retention of the terminal pin with removal of the silicone rubber material on the terminal pin, the extraction sheath requires upsizing to a 14F Philips laser, 11F Cook Evolution, or 11F Philips TightRail. If the silicone rubber material is retained, the size increases further to a 16F laser, 13F Evolution RL, or 13F TightRail. Alternatively, removal of the connector sleeve from the terminal pin and underlying insulation decreases the terminal lead diameter to 8F, allowing utilization of a Philips 12F Laser, 9F Cook Evolution, or 9F Philips TightRail (Figure 5). However, fibrosis increases the diameter of the lead body, the friction of passing the extraction sheath over the lead, and the stress of the tensile properties of the lead. Using a larger sheath may increase the stiffness but also may ease the passing of the sheath over the lead body.

In cases of rail failure of the conductor cable, the outer insulation and the outer conductor coil still offer a rail. However, the outer insulation and outer conductor coil will stretch significantly, resulting in elongation, insulation break, and exposure of conductor components. Figure 6 shows that after stretching more than twice the lead original length, a rail strength of 3–10 lbf still can be achieved. An example of an extracted 3830 lead returned for product analysis and a lead test specimen from our evaluation are shown in Supplemental Figure 3.

Snaring did not decrease the rail strength, thus allowing use of this technique during the extraction procedure. Snaring can be of assistance in the 3830 extraction procedure in gaining access through occluded vessels, preserving leads that have lead-on-lead binding with the 3830 lead, and protecting vulnerable structures, for example, when the 3830 lead is attached to the membranous septum or fossa ovalis. In the case of rail failure, lead stretching can be reduced by femoral snaring of the lead body proximal to the fracture point.

When extracting a 3830 leas, remove leads planned for extraction that are of greater tensile strength before removing the 3830 lead. This can reduce adherent scar burden on the 3830 lead. Retain the 3830 terminal pin and remove silicone seals to decrease lead diameter, which allows use of a small-diameter extraction sheath. Further size reduction can be accomplished by removing the silicone sleeve as discussed earlier (Figure 5). Tying a strong suture (orthopedic suture FW) behind the terminal pin and feeding the suture through the extraction sheath creates an extraction rail. Limit traction force to just enough to provide a rail for advancement of a powered sheath and keep traction force below 10 lbf (4.5 kgf).

Because the 3830 4.1F lead body diameter is significantly undersized compared to that of the extraction sheath, it is important to use tensile rail strength to keep the lead body coaxial when advancing the extraction sheaths. With lead angulations of 90°, the recommendation is to alter the geometry with snaring or to use sheaths without a cutting mechanism, such as the TightRail. Observations of lead buckling did not significantly reduce lead rail strength but may be associated with stalled progression from the superior approach and be an indicator of nearing the point of conductor fracture.

Use of a strong but thin suture and low-profile knot added substantially to the tensile properties of the lead preparation and can change the dynamics of the tensile properties of the lead during sheath advancement.

This study was not designed to provide specific information on extraction of conduction system pacing. Although limited as a bench study and not being performed in humans, this study does provide a framework for future human studies. Small sample sizes were studied to provide directional data, and with manufacturing consistency similar findings would be expected with large sample sizes. The leads were not preconditioned to simulate long-term implant degradation due to the wide variation in use conditions.

The 3830 lead has a unique construction and requires unique preparation techniques when it is being extracted. Creating a proper rail and maintaining lead integrity are critical when extracting the 3830 lead. This is best accomplished by retaining the terminal pin, using a strong suture attached to the terminal pin, and keeping traction pull force below 10 lbf. It is suggested that the Philips TightRail be avoided when the lead angle course is ≥90°. Femoral snaring did not decrease rail strength and can be used as needed in the extraction procedure. If the internal cable fails, the outer coil undergoes considerable stretching but can provide a rail for advancement of an extraction sheath. Ultimately, by characterizing appropriate lead preparation techniques and understanding lead tensile strength in the simulated extraction setting, in vivo extraction may be feasible based on the available evidence, but a more definitive conclusion requires additional study.

The authors thank Amy Molan, PhD (employee of Medtronic, Inc.) for help with manuscript formatting; and Taylor Noble (employee of Medtronic, Inc.) for mechanical bench testing and analysis.