Letter from the Editor

Subtle Differences in IS-1 Pacing Leads from Different Companies Result in a New 
Lead-Device Compatibility Problem

Bradley P. Knight, MD, FACC, FHRS, Editor-in-Chief

Bradley P. Knight, MD, FACC, FHRS, Editor-in-Chief

Standardization of pacemaker and defibrillator leads has allowed for global compatibility across manufacturers of cardiac implantable electrical devices (CIEDs). For example, a physician can implant a pacing lead from one company that has an international standard-1 (IS-1) terminal ring and pin arrangement, and connect it to a pacemaker generator from another manufacturer as long as it has an IS-1 type receptacle in the header. This has permitted the flexibility that is occasionally needed when trying to provide the best system for an individual patient, particularly at the time of a system revision. A recent new standard is the DF-4 lead that allows implantation of any company’s quadripolar in-line defibrillator lead to a different company’s defibrillator generator. 

This standardization makes transvenous leads and headers from different manufacturers compatible, and has many advantages. However, it has resulted in a mindset that there is zero risk to mixing and matching devices and leads. Products of this mindset are single-vendor hospital device contracts created and enforced by some hospital supply chains to reduce supply costs. This can result in patients receiving a different pacemaker brand at the time of generator replacement that is connected to a competitor’s leads. 

In December 2017, one of the pacemaker manufacturers, Boston Scientific (BSCI), issued a Product Advisory1 related to their Minute Ventilation (MV) rate response that raises new concerns that not all IS-1 leads are created equal. BSCI has a proprietary MV method to provide rate response that differs from standard activity-based sensors. It uses transthoracic impedance measurements to track the respiratory rate and accordingly increase the pacing rate. The devices that have this MV feature apparently deliver a subthreshold current waveform approximately every 50 ms between the lead ring electrode and the pacemaker case, and measure the resultant voltage between the ring on the lead tip and the pacemaker case. This current is normally filtered and not detected by the device. However, if there is ever a time when the impedance is very high, the current can be oversensed and inhibit pacing. This could occur if there is an impending lead fracture, under-insertion of the lead in the header, or movement of the ring in the header. Inhibition of pacing is clearly problematic for patients who are pacemaker dependent. The likelihood of occurrence and overall risk to a patient is low, and a software fix is reportedly being developed. However, in the meantime, it seems reasonable for device clinics to identify pacemaker-dependent patients and turn off their MV sensor.

Perhaps the most interesting, but concerning, aspect to BSCI’s MV sensor observations is that this oversensing problem is much more likely to occur in patients who have a BSCI pacemaker connected to a Medtronic or Abbott pacing lead rather than a BSCI lead. The company reported that the probability of patient injury at 5 years is 1 in 33,333 when the pacemaker is attached to one of their own leads, but is 1 in 2,000 for Medtronic or Abbott pacing leads. How can this be if all IS-1 leads are the same? According to BSCI, they detected slight variations in the leads from different companies: “Although all leads evaluated in simulated testing environments comply with appropriate connector standards, we have discovered subtle differences amongst lead manufacturers in the surface finish of the lead terminal ring and amount of axial and radial terminal ring motion within the pacemaker header. These factors may result in intermittent increases in impedance leading to oversensing of the MV sensor signal or changes in daily impedance test measurements.”1

This recent observation by BSCI that subtle differences between companies in pacing lead terminal ring finishes can result in inhibition of pacing in pacemaker-dependent patients using MV sensors forces a reconsideration of pacemaker lead standards and a warning that all leads and devices may not be universally compatible. As new pacemakers and defibrillator features are created, the development process must account for and test for the possibility that their devices might be attached to other manufacturer’s leads. 


  1. AU_AVT Functional Latching. Boston Scientific. Published December 2017. http://www.bostonscientific.com/content/dam/bostonscientific/quality/documents/Recent%20Product%20Advisories/2017Dec_MV_PhsLtr_Final11.pdf. Accessed January 13, 2018.