Now, all kinds of engineers are rethinking this equation, too. As are doctors.
Since Medtronic had their recent thin Sprint Fidelis lead (6.6Fr) recalled due to an elevated rate of fractures, new scrutiny is being placed on St. Jude's even thinner (6.3Fr) Riata® ST defibrillator lead. But the concern in this case is less about fractures, but rather heart perforations.
This morning the Wall Street Journal reports that St. Jude's Riata ST lead is being evaluated for an increased cardiac perforation rate compared to conventional thicker defibrillator leads. Cardiac perforations are not new to defibrillator or pacemaker implantations. An example of this can be found on a dramatic case recently published in Circulation (reference below) where a pacemaker lead had punctured a patient's heart and had eroded through his 7th rib. But with the recent FDA hypersensitivity and the concerns about thin defibrillator leads, it is not surprising that such concerns have been raised.
You see in cross section, the thinner a lead becomes, the more force from the lead pressing against heart muscle is concentrated over a smaller area. And unfortunately, as the radius of leads are downsized, the force applied over that area increases geometrically (radius squared, remember?). An example of the difference that surface area makes on pressure to tissues can be felt when you apply the same amount of pressure from a pencil's erasure over your hand versus its sharpened tip.
So when a thin lead has a narrowed diameter, it might be more likely to perforate. And given the concerns aired in the Wall Street Journal article, I bet implanting electrophysiologists will be slow to rush these leads until the dust settles a bit more. After all, we've become a bit sensitized to these issues.
Wall Street Journal.
Singhal S, Cooper JM, Cheung AT, Acker MA. "Rib Perforation From a Right Ventricular Pacemaker Lead." Circulation; 115: e391-392.
Update 21:49 CST - The PACE articles with the studies regarding the Riata lead issues are now up. Dr Steven V. Vlay of Stony Brook University, New York in his accompanying editorial states:
"What is particularly troublesome about this lead is not only the perforation but its late presentation. Sometimes the lead perforation does not come to clinical attention until it is well beyond the right ventricular free wall. Furthermore with the Riata, the movement continues until it protrudes out several cm into adjacent tissue. This factor seems to distinguish it from a small perforation in which only the tip of the lead penetrates the right ventricular wall. It could result in ventricular asystole if a patient is pacemaker dependent or in failure to successfully defibrillate, both resulting in sudden death. If perforation does not occur in the first three months, can we stop worrying? We do not yet know the answer to this question.
Here's the references from this issue of PACE:
1. Krivan L, oz M, Viasinova J, Sepšsi M. Right ventricular perforation with an ICD defibrillation lead managed by surgical revision and epicardial leads.
2. Fisher JD, Fox M, Kim SG, Goldstein D, Haramati LB. Asymptomatic anterior perforation of an ICD lead into subcutaneous tissues.
3. Satpathy R, Hee T, Esterbroooks D, Mohiuddin S. Delayed defibrillator lead perforation: An increasing phenomenon.