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Lead Connection Systems and Standards for Cardiac Implantable Electronic Devices
Cardiac implantable electronic devices (CIEDs), first used in 1958, have become highly sophisticated therapeutic tools for the management of bradyarrhythmias, tachyarrhythmias, and more recently, cardiac resynchronization therapy. The implanted system comprises cardiac leads connected to a generator containing a power source and electronic mechanisms. The coupling of the generator to the cardiac leads requires a connector system that allows current to be transmitted with minimal electrical resistance from very low-voltage sensed heart signals, low-voltage pacing stimuli, and very high-voltage implantable cardioverter-defibrillator (ICD) discharges while insulating the conductors from additional conducting elements and corrosive body tissues.
The earliest pacemaker lead connections, created de novo at the time of pacemaker implantation or replacement, were very unreliable. Soon after, unipolar leads were designed with simple connectors that were not interchangeable between manufacturers. With time, more complex bipolar designs evolved, resulting in company-specific lead connectors with large pulse generator header blocks. The desire of clinicians to use a lead from one manufacturer with a pulse generator from another necessitated the development of industry-wide pacemaker connector standards. Similar connector development occurred with the advent of the ICD, although this resulted in a large, cumbersome connector and header block. As a result, a single, standardized, four-terminal plug connector system was developed that also includes a multielectrode standard for left ventricular venous pacing. This chapter focuses on the connector system designs and the evolutionary development of standardized CIED lead connection systems.
Early Years: Clinical Confusion and Need for Standardized Connection Systems
The earliest implantable pacemakers became commercially available during the late 1950s and had no connectors. Leads were permanently attached to the pulse generator during manufacture, and any system failure required replacement of both the lead and the pulse generator. Also, because the lead electrodes were epimyocardial, replacement necessitated an open-chest procedure. It was soon appreciated that the leads needed to be supplied separately from the pulse generator, thus requiring a method of attachment of the lead to the pulse generator. The early designs were very primitive and unreliable ( Fig. 9-1 ). The lead connector plug was typically inserted into a receiving port in the pulse generator and secured with a cutting set screw. These connectors were often designed with insufficient insulation to isolate the lead conductors from body tissues, thus becoming a critical design issue.
Unipolar Pacemaker Connection Systems
By the mid- to late 1960s, most manufacturers had developed unipolar, transvenous pacemaker leads. The early unipolar designs had a coiled wire conductor with a lumen, enabling use of a guiding stylet for more effective lead tip positioning. Thus they became the lead of choice among clinicians. With these designs, the pacing lead connector became part of the manufactured lead, which meant that most pulse generators could be implanted in a subclavicular, prepectoral pocket rather than being tunneled to the abdomen. As a consequence, a proliferation of unipolar connector systems quickly ensued ( Fig. 9-2 ). Many of these leads had high-profile connector plugs that were unique to the manufacturer and limited to certain pulse generator models. As such, during replacement, the unique lead connector design necessitated use of a specific pulse generator, which caused confusion and frustration, especially once the manufacturer had ceased to exist, had switched to a new connector design, or had been purchased by another manufacturer. Despite these drawbacks, it was relatively easy with unipolar systems to sever the old lead connector and replace it by splicing the newer connector onto the remaining lead body. With time, two large-diameter, unipolar connector designs emerged: the Medtronic design (Medtronic, Minneapolis, MN) and the Cordis design (Cordis Corporation, Miami, FL, later Telectronics, and later acquired by St. Jude Medical, St. Paul, MN) ( Fig. 9-3 ). Despite a similar appearance, these two connectors were not interchangeable, due to different lead connector dimensions. The diameters of the lead connector pins were 1.6 mm for the Medtronic design and 2.25 mm for the Cordis design. The diameters of the bulbous silicone rubber connector plugs were 4.75 mm for Medtronic (though referred to by the manufacturer as “5 mm”) and 5.5 mm for Cordis (referred to by the manufacturer as “6 mm”). Cordis connectors, being larger, were designed not to enter the smaller connector cavity of the Medtronic pulse generator header block. In contrast, Medtronic connectors could mechanically fit into the Cordis pulse generators; however, because of a loose fit, they were susceptible to electrical shorting to body fluids in the subcutaneous pocket unless an adaptor sleeve was used. Ultimately, the smaller, “5-mm” Medtronic connector design was adopted by most pacemaker manufacturers, although there was never a formal standard. The pulse generator header receiving port for these lead connectors was simply a connector cavity with or without silicone rubber O-ring seals. The connector mechanism used a single set screw embedded in the pulse generator header block, which tightened onto the terminal pin of the lead connector. This simple set screw approach to fixation, which also provided an electrical connection, ultimately proved to be extremely reliable and is the method of choice for fixation in systems today.
Bipolar Pacemaker Connection Systems
The original bipolar leads were simply created by using a silicone rubber tubing extrusion with two side-by-side lumens for the two conductor coils: one for the cathode electrode's coil conductor and one for the anode electrode's coil conductor. The two conductor coils terminated proximally in a bifurcated lead connector with two separate 5- or 6-mm unipolar lead connector plugs, each with a single connector pin terminal ( Fig. 9-4 ). Thus two connector cavities were required in the pulse generator header block to connect the anode and cathode terminals. To prevent insertion into the wrong connector cavity, which could result in anodal pacing, some manufacturers placed a color-coated band on the lead connector corresponding to the cathode electrode (see Fig. 9-4 ).
By the early 1980s, the large bilumen bipolar pacing leads were being replaced with a smaller-diameter coaxial lead body configuration. In this configuration, there was a central conductor coil that was attached to the cathode electrode and through which the stylet passed, which was then covered by an insulating tube. Around this tube was the outer conductor coil, which attached to the anode ring electrode, and a final outer layer of insulation tubing covered this outer coil. With this construction, it was then possible to create a single, in-line lead connector with two on-board terminals, with one connector pin terminal for the inner cathode conductor coil and one ring terminal for the outer anode conductor coil. An early concern with the in-line bipolar lead connector plug was the ability to electrically isolate the two terminals within the pulse generator header to prevent body fluid ingress that could cause electrical shorting. As such, O-ring seals were located on the lead between the two lead connector terminals or inside the generator's connector cavity. Using this approach, a variety of high-profile and mid-profile lead connector designs emerged on the market. The variety of different O-ring seal designs and their location, either on the lead or in the generator, caused even more confusion than the earlier unipolar connectors. Moreover, it was no longer possible to simply sever the coaxial lead connector at surgery and attach another connector as could be done with the simpler, unipolar lead connectors ( Fig. 9-5 ).
Advent of Adaptors: A Necessary Evil
Although development of unipolar lead connector adaptors had already occurred, the need to convert differing bipolar lead connection systems resulted in numerous new adaptor designs that were much more complex. Most were intended to convert bifurcated, bipolar lead connectors to a single, in-line, bipolar connector plug. Other examples include converting an in-line, bipolar lead connector of one type to an in-line, bipolar lead connector of another type to fit into an older device ( Fig. 9-6A ). Another example was to convert an in-line bipolar lead connector to a 5-mm unipolar connector ( Fig. 9-6B ). Because many of these early adaptors were manufactured by small companies, their designs did not always undergo the same level of preclinical testing as those designed by larger manufacturers. As such, a number of these adaptor designs had reliability issues, particularly when used with ICD systems. Most of the problems were related to poor mechanical fit or improper selection of materials. Although often necessary to preserve function in an already implanted system, essentially all adaptors, even the well-designed models, became infamous for their perceived reliability risks.
Initial Voluntary Connector Standards
With all the chaos and problems of the differing connector systems and the growing confusion and frustration of clinicians, it was clear that a remedy was needed—a formal, standardized connector system design. By the mid-1980s, initial attempts to create a standardized, bipolar lead connector system had begun. Although the need was sounded by both implanters and engineers, the actual initial impetus was from the German Working Group on Cardiac Pacing, which urged manufacturers to develop a low-profile, in-line lead and device header connector standard for both unipolar and bipolar leads. Early progress was slow because of the concern that creating a standard would restrict future innovation in both lead and pulse generator design rather than enhance it.
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