Background

Successive waves of technologic advances have created the modern, sophisticated device comprising a modest-sized, electrically active generator (i.e., active can) implanted pectorally, utilizing defibrillation-efficient biphasic waveforms, one to three transvenously inserted leads, and antibradycardia/antitachycardia and biventricular pacing capability, as well as programmable detection algorithms and defibrillation energy strengths. The success of this technology is mirrored by the estimated 200,000 devices implanted worldwide in 2009. Despite these remarkable achievements, implantable cardioverter-defibrillators (ICDs) still generate controversy, particularly in the United States, where both physicians and the public are questioning ICD reliability and safety. This peaked in 2009 with the withdrawal of the Fidelis lead from the market amid extensive and often critical publicity.^5–11 Further complaints over lead reliability continue, together with concerns over engineering reliability.^6,11

The growing number of lead failures has exposed the Achilles heel of ICDs: the need to attach an electrode directly to the heart via the vascular system. The complicated path that the lead must follow, the hostile environment of the body, and the requirement that leads be supple combine to threaten lead integrity. Moreover, the necessary adherence of the electrode tip to the myocardium and potential attachment of the body of the lead to the veins traversed result in an appreciable risk of injury and death if lead extraction becomes necessary. Reported ICD lead “survival” rates vary from 85% to 98% at 5 years and 60% to 72% at 8 years; Figure 19-1 plots data from 10 studies.¹⁰ Table 19-1 summarizes the reported incidence of complications when lead extraction is judged necessary, with mortality ranging from zero to 2.7%, the latter figure relating to associated overwhelming sepsis.^12–19 These concerns, along with the well-known complications of routine transvenous lead insertion (perforation, tamponade, pneumothorax, hemothorax, sepsis, endocarditis), indicated the need to develop an ICD that would not require transvenous leads.

Figure 19-1 Results of 10 studies of implantable cardioverter-defibrillator (ICD) lead performance selected according to ≥18 months’ follow-up available, publication in a peer-reviewed journal after 1999, and statistical inclusion of patients who died or were lost to follow-up. The size of the data point/circle is proportional to the total number of patients in the study. Data box shows primary author, year of publication, and patient number (n) for the 10 studies.

(Modified from Maisel WH, Kramer DB: Implantable cardioverter-defibrillator lead performance, Circulation 117:2721-2723, 2008.)

 Early Investigation Experience with Subcutaneous Defibrillation

The first efforts to explore subcutaneous defibrillation broke new ground in understanding of energy requirements and the type and location of electrodes. A major corollary was whether a defibrillation threshold of sufficiently low magnitude could be achieved, and if batteries and capacitors then could be small enough to be inserted into a generator of reasonable size. Without these answers, the issue of reliable sensing from an extracardiac location would prove moot.

In August 2001 a collaborative research group explored the index question of how much energy was needed for subcutaneous defibrillation. With appropriate ethics committee approval and patient consent, the first test of subcutaneous defibrillation was performed in humans at Green Lane Hospital in Auckland, New Zealand.²⁰ The original test was conducted in a 56-year-old man undergoing transvenous ICD implantation because of a recent cardiac arrest. A small incision was made over the low lateral chest wall to permit tunneling of an anteriorly and posteriorly situated subcutaneous oval disk electrode of approximately 5 cm² in surface area. A standard transvenous right ventricular lead was used to induce ventricular fibrillation, which was terminated, to the team’s satisfaction, with the first shock, chosen at a strength of 70 joules using a biphasic waveform with a 50% tilt and 100-µF capacitance. This shock resulted in success at an energy level that was much less than was then known possible with transthoracic defibrillation, using two large surface pad electrodes. At that time, the lower limit of average defibrillation threshold efficacy was presumably 100 to 115 J. Clearly, avoiding the resistive effect of the skin was surprisingly beneficial. As discussed later, the energy requirements proved to be even more remarkable.

Although defibrillation with our initial configuration was successful, the posterior space proved too difficult to access and therefore was subsequently abandoned. Attempting to access a posterior subcutaneous space from an anterior approach with a supine patient clearly was impractical, regardless of how low the defibrillation energy would prove to be. Four more patients were tested using various anterior/anterolateral configurations in the latter half of 2001, and defibrillation was successful in all. Each of these pilot explorations led to a more definitive and sequential approach to vector and electrode selection. Our original intention had been to develop a curved generator, resembling a flattened banana, which would mold to the natural contour of the intercostal space (Fig. 19-2). In fact, early prototypes with this shape were tested, but unfortunately it proved too difficult to accommodate the necessary components of the generator to this design and was reluctantly abandoned. Subsequent studies undertaken with additional help (Johannes Sperzel, Jorg Neuzner, and Stefan Spritzer in Germany; Andrew Grace at Papworth Hospital in the United Kingdom; Andrei Ardashev at Burdenko Hospital in Moscow, as well as at Green Lane; Jon Hunt of Cameron Health) allowed rapid progress in identifying a practical lead system.

Figure 19-2 Torso model showing original curved electrode designed to mold to the lateral intercostal space.

 Optimal Defibrillation Configuration

Having demonstrated proof of concept, the research aim was then to evaluate systematically which of many electrode configurations would be most effective, as well as most practical from a surgical perspective. Although multiple systems were examined and abandoned, four viable configurations were eventually chosen for detailed study. In this next phase of development, these four defibrillation systems compared the utility of retaining the conventional pectoral generator site versus a novel, left lateral placement of the generator. In addition, a variety of electrode shapes, lengths, and positions were examined. The specific four combinations chosen were as follows (Fig. 19-3):

Figure 19-3 Four lead systems acutely tested for the “optimal” system.

1, Left lateral pulse generator with 8-cm coil electrode at left parasternal margin, designated LGen-S8. 2, Left pectoral pulse generator with left parasternal 4-cm coil electrode positioned at the inferior sternum, designated PGen-S4. 3, Left pectoral pulse generator with 8-cm coil electrode curving from left inferior parasternal line across to inferior margin of left sixth rib, designated PGen-C8. 4, Left lateral pulse generator with left parasternal 5-cm² oval disk, designated LGen-S5 (disk).

(Modified from Bardy GH, Smith WM, Hood MA, et al: An entirely subcutaneous implantable cardioverter-defibrillator, N Engl J Med 363:36-44, 2010.)

Defibrillation testing of these temporary lead configurations was completed immediately before permanent transvenous ICD implantation in a total of 78 patients in a number of centers, most at Green Lane and Papworth Hospitals. These data have been published previously.²¹ A Latin Square design was used for testing with analysis of variance (ANOVA). A step-up/step-down protocol was used to determine defibrillation thresholds (DFTs) using a 50%-tilt biphasic waveform. An initial test shock of 40 J was delivered after 10 seconds of induced VF, incrementing to a maximum of 80 J or decrementing to a minimum of 10 J. Bardy et al.²¹ fully describe the DFT testing protocol. Unsuccessful shocks were followed by prompt, transthoracic rescue defibrillation.

For the four configurations just described, the mean DFT (±SD) was as follows, respectively (Fig 19-4):

Figure 19-4 Delivered defibrillation threshold energies.

Defibrillation threshold data (mean ± SD) for four practical lead/electrode configurations (see Fig. 19-3) in acute human trials (total of 78 patients).

(Modified from Bardy GH, Smith WM, Hood MA, et al: An entirely subcutaneous implantable cardioverter-defibrillator, N Engl J Med 363:36-44, 2010.)

These data suggested the optimal configuration was a left lateral canister placement and an 8-cm parasternal electrode with both a low DFT and a relatively narrow standard deviation range (±SD; CI, confidence interval; ANOVA P = .07). Despite being a novel location, the left lateral position proved to be “user friendly,” with easy tissue planes to dissect and good visibility for electrode and generator placement. With meticulous attention to hemostasis, no pocket hematomas were encountered, and the incisions healed soundly, even during these early years and with relatively limited procedural experience.

The next phase of the research involved a comparison between DFTs with the “optimal” subcutaneous system just identified and a standard transvenous system. The purpose was not only to examine relative efficacy in terminating VF in the same patients, but also to ensure that there would be a comparatively parallel degree of energy reserve in any design of a subcutaneous-only ICD (S-ICD).