By Chu-Pak Lau
Limitations of current pacemakers
Pacing has changed little over the last 50 years. Pacemaker implantation requires a surgical procedure to create a pocket together with venous access to deliver one or more transvenous leads. Patients are concerned about device failure and longevity, cosmetic effect of the pocket, environmental interference and compromised life style and mobility.
Long-term clinical problems following implantation include pocket and lead infection, and lead interaction with cardiac structures. Lead fracture is a major concern (Table 1). Finally, the optimal site(s) of pacing to preserve or enhance heart function remains unclear or cannot be achieved. The complexity is increased with multiple pacing sites such as in cardiac resynchronisation therapy (CRT). In addition, increase in patient longevity will raise the incidence of lead-related problems (Poole et al; Circulation. 2010;122:1553-61). Amongst all components, the pacing and defibrillation leads are the “weakness link” between patients and cardiac implantable electronic devices (CIEDs). Apart from lead complications, CRT is currently delivered epicardially through the coronary sinus. The coronary sinus anatomy can make implantation difficult, and the benefit unpredictable since the optimal pacing site may not be always achieved.
Endocardial pacing in the left ventricle that obviates this limitation may provide an alternative approach in the 30% of patients who failed to respond to coronary sinus pacing (Derval et al; J Am Coll Cardiol 2010;55:566-575).
Consequently, the concept of a “leadless” pacemaker has been developed using either a totally self-contained intracardiac pacemaker or pacing from an external energy source.
Table 1. The limitation of current pacemakers and cardiac resynchronisation therapy (CRT)
|Surgical pocket||▪ Surgical morbidities|
|▪ Vascular access complications|
|Transvenous leads||▪ Difficulty in achieving acceptable pacing and sensing at desired sites (especially for CRT)|
|▪ Lead dislodgement|
|Device casing/ Connectors /Leads/ Antennae/ Battery||▪ Interference|
|▪ Failure of each component especially the leads|
|▪ Finite battery longevity requiring device replacement|
|▪ Device failure and longevity|
|▪ Life-style limitations|
|▪ Environmental interference|
|Long term clinical problems|
|▪ Pocket and lead infection|
|▪ Lead interactions with cardiac structures (tricuspid valve, venous thrombosis) and fracture|
|▪ Best pacing site(s) to preserve or enhance heart function|
Totally self-contained intracardiac pacemaker
The concept of a totally self-contained leadless intracardiac pacemaker was reported in 1970 by Spickler et al (J Electrocardiol. 1970;3:325-31) who delivered a 18 x 8mm totally implanted pacemaker through a transvenous sheath.
With improved battery energy and endocardial fixation and delivery system, this concept has now been revisited by several manufacturers. While stable implants in animals have been achieved, there remains concerns on the stability and durability of pacing and sensing function, extraction and replacement, and size that is at present too large to be used in cardiac chambers other than the right ventricle.
Externally powered device
Smaller electrodes can be used via either an endocardial or epicardial approach if powered externally. Potential energy sources include intrinsic cardiac mechanical energy, ultrasound, magnetic induction and cardiac electrical energy.
Ultrasound in the range of 330kHz is an energy source that can power a remotely positioned electrode for cardiac stimulation without any histological evidence of myocardial damage (Echt et al; HeartRhythm. 2006; 3:1202-1206). The feasibility of this technique was further demonstrated in subsequent acute human studies (Lee et al; J Am Coll Cardiol 2007;50, 877-83 and Lee et al; Heart Rhythm. 2009;6:742-8) (Figure 1). In the first-in-man study (Lee et al; J Am Coll Cardiol 2007;50, 877-83), ultrasound-mediated pacing was successfully achieved during acute electrophysiological study at all endocardial sites. A maximum of 2.16±1.10 V was induced by ultrasound at a transmitter to receiver distance of 11.3±3.2cm, adequate for reliable pacing without mechanical or thermal cardiac injury. This concept has been instrumented as a co-implant in humans…
There are many concerns on leadless technology: interference from ubiquitous environmental sources, the sensing circuit, and a reliable, easy to use and low risk delivery system. Nevertheless, with the increase recognition of lead related problems and advantages of endocardial pacing, leadless technology will be the next frontier of cardiac pacing.
Figure 1. Mechanism: Investigational equipment for ultrasound mediated leadless pacing
(A). Ultrasound transducer. (B). Electrophysiology catheter. (C). Enlarged view of the receiver-electrodes incorporated in the distal end of the catheter between the bipoles used for sensing and electrical pacing. (D). A recording of an external acoustic energy induced electrical waveform was collected to determine the achieved pacing energy (oscilloscope). (Reproduced with permission Lee KL, Lau CP, Tse HF, Echt DS, Heaven D, Smith W, Hood M. First human demonstration of cardiac stimulation with transcutaneous ultrasound energy delivery: Implications for wireless pacing with implantable devices. J Am Coll Cardiol 2007;50, 877-83)
Chu-Pak Lau, Cardiology Division, Department of Medicine, Queen Mary Hospital, the University of Hong Kong, Hong Kong SAR, China
See the expanded version of this article in the printed issue 20 of Cardiac Rhythm News.