Unipolar LV Pacing with Bipolar Lead Vectors: One Center’s Chance Finding of Presumed Insulation Degradation

Introduction

The advent of automaticity in device diagnostics and evaluation of leads has been a relatively new way of monitoring device patients. Having the ability to reprogram left ventricular (LV) pacing parameters for the best possible clinical outcome using multiple alternative vectors for programming is also a modern clinical tool. We report a case of presumed unipolar LV lead insulation degradation resulting in an atypical impedance pattern and LV capture from non-existing bipolar pacing vectors. 

CLINICAL CASE

An 82-year-old male with non-ischemic cardiomyopathy, chronic systolic heart failure with an ejection fraction of less than 35%, and underlying left bundle branch block presented to our clinic for routine in-office CRT-D device evaluation. 

The initial CRT-D system was implanted in 2003. The system consisted of a Medtronic RV defibrillator lead (model 6947), a Medtronic 5076 pacing lead in the right atrium, a Medtronic 2187 Attain LV pacing lead, and a Medtronic InSync II Marquis^® CRT-D generator. A generator change was performed in 2007, and then again in 2012, at which time a Medtronic Protecta^® XT CRT-D was implanted. 

During a routine in-clinic evaluation, the unipolar LV lead recorded LV tip-coil impedance of 722 ohms. Surprisingly, the LV ring-coil impedance was in range and measured 874 ohms, and LV bipolar impedance was also in range, measuring 285 ohms (Figure 1). Furthermore, LV capture was observed pacing from the LV ring-coil configuration. Pacing threshold was measured at LV tip-coil 3.5V @ 1ms (Figure 2), LV ring-coil 4.25V @ 1ms (Figure 3), and LV true bipolar (tip to ring) non-capture at maximum output (Figure 4). These measurements were confusing, because with a unipolar LV lead, there is no actual ring. So why did this CRT-D give these measurements of this non-existent bipolar vector during the impedance and threshold testing?

HOW CAN THE LV RING TO RV COIL VECTOR BE POSSIBLE IN A UNIPOLAR LEAD? 

All modern device headers are capable of supporting both unipolar and bipolar leads. While many devices also have algorithms that automatically identify the type of lead attached, most still allow manual override of these features. Therefore, the device will allow and attempt bipolar pacing (or in this case, LV ring to RV coil pacing), even if a unipolar lead is attached. Typically, a unipolar lead has a single conductor going from the IS-1 pin to the tip electrode. There still is a ring contact on the lead, as this is part of the IS-1 standard; however, the ring contact is not connected to anything that extends down the lead body.¹ When attempting to pace from LV ring to RV coil through an intact unipolar lead, the pacing impedance should measure >3000 ohms, and no current should flow. To complete the pacing circuit, current has to make its way from the LV ring contact to the RV coil in some manner. The two possible pathways are (1) from the ring contact, out the header, through the body, to the RV coil; and (2) from the ring contact, through the insulation in the header, to the pin connection on the lead. The pin then can conduct the current down to the myocardium. This second circumstance is more likely to support pacing capture. This mechanism could explain our observations; however, current shunting inside the header is rarely described despite millions of implanted bipolar IS-1 leads. Electric current shunting between the ring to tip conductors is more commonly caused by inner insulation degradation in bipolar leads.

ATYPICAL LEAD CONSTRUCTION

Consultations with Medtronic engineers revealed that the inner construction of the 2187 unipolar LV lead is very atypical.² In fact, it is nearly identical to the Medtronic 4024 bipolar pacing lead. Unlike most LV leads used today, the 2187 lead is not an over-the-wire lead, but a stylet-delivered LV lead. An electrically non-functional outer conductor was included in the lead design to provide more support during implant. Therefore, unlike a traditional unipolar lead, where the ring contact is not electrically connected to anything, the ring contact in this lead is connected to a coil conductor that extends the entire length of the lead. Furthermore, unlike a traditional unipolar lead, this lead has an inner insulation layer that separates the inner conductor from the outer conductor. 

Significant drop in the bipolar impedance of the 4024 lead has previously been described.³ We surmised that it is possible that the 2187 lead shares a similar tendency for inner insulation degradation as was seen in the 4024 lead. Therefore, current can shunt from the LV ring (outer) to the LV tip (inner) conductor, explaining both in-range impedances and pacing capture from the non-existing vectors. This provides a more likely mechanism than current shunting directly within the header.

IMPACT TO OUR CLINIC POPULATION

Subsequent to this patient example finding, we reviewed all active Attain LV 2187 leads followed in our clinic. Forty leads implanted from November 2001 to December 2006 are still in use (mean follow-up duration 11.8 years). Impedance measurements were available on all LV pacing configurations for 15 out of the 40 leads. Some older generators did not provide automatic impedance measurements on all available vectors. Of those 15 leads, five (33%) demonstrated similar impedance patterns as in our initial case. Average impedances for the five leads were 775 ± 439 ohms LV tip-coil, 934 ± 744 ohms LV ring-coil, and 414 ± 258 ohms LV bipolar. 

CONCLUSIONS AND CLINICAL IMPLICATION

When our initial patient presented with unusual impedance measurements and elevated LV thresholds, there was concern that a change had occurred with the lead integrity. While examining the bipolar impedances and capture thresholds, the realization that these measurements were coming from a documented unipolar lead created significant confusion. Was there an error in documentation regarding the LV lead type/model? After investigation, it was determined the lead type was documented correctly. This led to discussions with industry representatives in order to troubleshoot. When we found other patients in our database with similar findings, we hypothesized that the 2187 lead appears to be susceptible to insulation degradation, comparable to the 4024 lead with a similar lead construction. At this point, we do not have any evidence of increased lead failure rate as is related to the standard unipolar LV pacing vector. No patient in our series has required lead replacement; however, increased awareness and surveillance of this LV lead may be warranted. Previously explanted or abandoned 2187 leads were not evaluated in this series. To our knowledge, this is the first case study describing this unusual impedance/threshold measurement of the 2187 lead. Lead insulation degradation of this lead type has also not been previously reported, but may be underappreciated. Although this type of lead construction is no longer used in Medtronic LV leads, understanding this phenomenon at replacement time may be important, as older LV leads are connected to CRT generators with diagnostic automaticity in multiple vectors.

Disclosures: The authors have no conflicts of interest to report regarding the content herein. Outside the submitted work, Ms. Brewer reports personal fees as an invited faculty speaker for Medtronic and St. Jude Medical, and Ms. Repoley reports modest honoraria for teaching activities for Medtronic and St. Jude Medical. 

References

1. Furman S, Hayes DL, Holmes DR Jr. A Practice of Cardiac Pacing. Mount Kisco, NY: Futura Publishing Company, Inc., 1986. 
2. Personal communication. Medtronic engineers conference call, November 25, 2015.
3. Johnson WB, Braly A, Cobian K, et al. Effect of insulation material in aging pacing leads: comparison of impedance and other electricals: time-dependent pacemaker insulation changes. Pacing Clin Electrophysiol. 2012;35:51-57.