- Email: [email protected]
Keeping time in the chest
Author’s Accepted Manuscript Keeping time in the chest John K Triedman
www.elsevier.com/locate/buildenv
PII: DOI: Reference:
S1547-5271(16)31007-4S1547-5271(16)30917-1 http://dx.doi.org/10.1016/j.hrthm.2016.11.006 HRTHM6912
To appear in: Heart Rhythm Cite this article as: John K Triedman, Keeping time in the chest, Heart Rhythm, http://dx.doi.org/10.1016/j.hrthm.2016.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Keeping time in the chest Editorial submission to accompany “The Swiss approach for a heartbeat driven leadand batteryless pacemaker” John K Triedman MD Boston Children’s Hospital 300 Longwood Ave Boston, MA 02115 [email protected]
No conflict of interest to disclose.
Pacemaker technology in its current state of evolution is remarkably reliable, and for most patients, minimally intrusive to their lifestyle. Nonetheless, there remain significant shortcomings with our current model of cardiac pacing that ultimately should be overcome as technology continues to evolve. Without a doubt, a principle technical problem that needs to be addressed to further advance the practice of cardiac pacing relates to the standard power source for a pacemaker: its battery. Considerable research has been invested into the paradigm of the electrochemical battery, and many of the design features that determine the utility of a battery type for use in a pacemaker have already been optimized from an engineering perspective, including the maximum achievable energy density, the ability of the battery to deliver sufficient current at useful voltage potential until the end of its life, and the limitation of possible failure modes to those still consistent with safe clinical use. Generally speaking, developments in battery technology have lagged behind broad advances in pacemaker component technology over the last decades. Although they have been very well designed for use in pacemakers, a battery is a battery, and it has a lifespan that is absolutely limited by its total charge capacity. In the majority of pacemakers, the battery accounts for most of the volume of the device, and any desire to extend battery life can only be achieved by building a
larger device. Periodic need for device replacement therefore is a small but predictable risk to which patients are exposed throughout their lives, and although we as clinicians may find this acceptable, other interested parties, such as patients themselves, may see this “feature” of modern pacemakers and an important deficiency and a legitimate target for improvement. Historically and famously, plutonium powered thermoelectric energy cells were once briefly used in pacemakers as a means of safely providing effectively permanent pacing.1,2 Some of those devices were still functioning according to specification and without complications more than 35 years after implant. A more recent example of innovation is pacemaker energy supply is the demonstration of transcutaneous light as a power source for cardiac pacing.3 This attests to continuing interest in this area, and also to the very low power requirements actually necessary for cardiac pacing (see below). In the accompanying paper, Zurbuchen et al present another entirely novel, surprising and extremely creative approach to leadless and batteryless pacing, one that suggests that an entirely new paradigm may be possible for energy management in cardiac pacing.4 They propose to employ the heart itself as the energy source for the pacemaker, by borrowing technology used in self-winding watches invented more than a century ago by Swiss watchmakers. Here they demonstrate the use of prototype device built to illustrate this approach as a proof of concept. They also review some of the energetics of their approach, which explain what at first glance may have some of the appearance of a proverbial “perpetual motion machine” fallacy. We focus in this journal principally on the electrical, but the purpose of the organized electrical activation of heart is merely to initiate and to organize its mechanical function. The heart in the end propels blood under considerable pressure through the circulation, and the transduction of the electrical signal of the QRS into a coordinated muscular contraction is nonlinear “trigger” event, which consumes very little energy. It has long been estimated that the energetic cost of electrical activation of the heart is minute (<1%) compared to that required to Page 1
actually perform the mechanical work of the heart. Given the self-propagating nature of this electrical activity in the heart, the energy necessary to bring that process to threshold for initiation is much lower still. In plain English: pacemakers require only a tiny amount of power (20 µW in total in this implementation) to initiate the much more energetically demanding sequence of muscular contraction and ejection of blood. Raising the pressure of blood in the ventricular chambers and providing it with kinetic energy for ejection accounts for the great majority of myocardial work. The large and continuous metabolic cost of contraction is the energy input into the system which explain why this is not a perpetual motion hoax. A byproduct of this pressure-volume work is the acceleration of the heart muscle itself in contraction, which results in the oscillatory motion of the heart. Although it is a necessary component of cardiac ejection, the movement of the heart muscle itself is a byproduct of the primary function of contraction, analogous to the energy required to accelerate the weight of one’s legs back and forth while running, as opposed to the primary energetic goal of moving from point A to point B. The authors of this paper have discovered and explored that small source of myocardial energy, and devised a means to capture a tiny amount of it. This energy stream is very small compared to the total energy expenditure of the heart - by my calculation, less than 1 part in 10,000. However, they have determined that it is more than sufficient to maintain the very tiny demands associated with cardiac pacing, in this case exceeding the needs of the pacemaker circuit by a factor of 4 or more. The conversion of this energy into useful for through the workings of a Swiss watch is just – elegant! The research presented here is at its very earliest stages, and it forecasts the need for many more years of developmental work. Although the incorporation of a watch mechanism into a device works, it is unlikely to be the most efficient means to transduce an epicardial power source, and several iterations of this technology remain to be invented and tested. Some form of energy storage will be needed (a Page 2
larger mainspring? additional capacitance? a rechargeable battery??), in the event that changes in cardiac motion result in decreased energy flux. The effects of posture, exercise and evolving illness on ability to continuously collect energy will need to be determined. A means of communicating with the device would be needed. The rate of failure of the mechanism over time would have to be laboriously determined. Minimally invasive techniques for epicardial implantation would need to be developed, and it is certainly conceivable that this approach could be consistent with new, catheter delivered leadless endocardial devices.5, Not mentioned so far in this editorial is its other innovative feature - the leadless aspect of this prototype. This aspect of the device is also aligned with modern catheter delivered pacing devices, and addresses a second major weakness of current pacing systems. However, if the pacing site of the device is the device itself (as currently demonstrated), the potential discrepancy between optimal sites for pacing and for harvesting of energy would also have to be investigated. This is an ambitious list, and the technology could fail at any of these points or at others not proposed here. Even if such a device is able to jump many feasibility hurdles, it still may lack clinical value by virtue of expense of the device, reimbursibility of the procedure, or the inertial willingness of clinicians to simply continue to practice in the manner to which they are accustomed. It is exceedingly difficult to improve on a technology as mature, predictable and widespread as simple cardiac pacing. However, that should not be misunderstood to indicate that such devices cannot still be made better – all that is required is a unbiased assessment of the limitations of current devices, and a highly creative and open mind.
Page 3
References
1.
Smyth NP, Millette ML. The isotopic cardiac pacer: a ten-year experience. Pacing Clin Electrophysiol 1984;7:82-9.
2.
Boulé S, Kouakam C. This is the end. Int J Cardiol. 2016;223:805-806.
3.
Haeberlin A, Zurbuchen A, Schaerer J, Wagner J, Walpen S, Huber C, Haeberlin H, Fuhrer J, Vogel R: Successful pacing using a batteryless sunlight-powered pacemaker. Europace 2014; 230:1534–1539.
4.
Zurbuchen A, Haeberlin A, Bereuter L, Wagner J, Pfenniger A, Omari S, Schaerer J, Jutzi F, Huber C, Fuhrer J, Vogel R. The Swiss approach for a heartbeat driven lead- and batteryless pacemaker. Heart Rhythm 2016; 00:000-000
5.
Reddy VY, Knops RE, Sperzel J, et al. Permanent leadless cardiac pacing: results of the LEADLESS trial. Circulation. 2014;129:1466-71
Page 4
www.elsevier.com/locate/buildenv
PII: DOI: Reference:
S1547-5271(16)31007-4S1547-5271(16)30917-1 http://dx.doi.org/10.1016/j.hrthm.2016.11.006 HRTHM6912
To appear in: Heart Rhythm Cite this article as: John K Triedman, Keeping time in the chest, Heart Rhythm, http://dx.doi.org/10.1016/j.hrthm.2016.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Keeping time in the chest Editorial submission to accompany “The Swiss approach for a heartbeat driven leadand batteryless pacemaker” John K Triedman MD Boston Children’s Hospital 300 Longwood Ave Boston, MA 02115 [email protected]
No conflict of interest to disclose.
Pacemaker technology in its current state of evolution is remarkably reliable, and for most patients, minimally intrusive to their lifestyle. Nonetheless, there remain significant shortcomings with our current model of cardiac pacing that ultimately should be overcome as technology continues to evolve. Without a doubt, a principle technical problem that needs to be addressed to further advance the practice of cardiac pacing relates to the standard power source for a pacemaker: its battery. Considerable research has been invested into the paradigm of the electrochemical battery, and many of the design features that determine the utility of a battery type for use in a pacemaker have already been optimized from an engineering perspective, including the maximum achievable energy density, the ability of the battery to deliver sufficient current at useful voltage potential until the end of its life, and the limitation of possible failure modes to those still consistent with safe clinical use. Generally speaking, developments in battery technology have lagged behind broad advances in pacemaker component technology over the last decades. Although they have been very well designed for use in pacemakers, a battery is a battery, and it has a lifespan that is absolutely limited by its total charge capacity. In the majority of pacemakers, the battery accounts for most of the volume of the device, and any desire to extend battery life can only be achieved by building a
larger device. Periodic need for device replacement therefore is a small but predictable risk to which patients are exposed throughout their lives, and although we as clinicians may find this acceptable, other interested parties, such as patients themselves, may see this “feature” of modern pacemakers and an important deficiency and a legitimate target for improvement. Historically and famously, plutonium powered thermoelectric energy cells were once briefly used in pacemakers as a means of safely providing effectively permanent pacing.1,2 Some of those devices were still functioning according to specification and without complications more than 35 years after implant. A more recent example of innovation is pacemaker energy supply is the demonstration of transcutaneous light as a power source for cardiac pacing.3 This attests to continuing interest in this area, and also to the very low power requirements actually necessary for cardiac pacing (see below). In the accompanying paper, Zurbuchen et al present another entirely novel, surprising and extremely creative approach to leadless and batteryless pacing, one that suggests that an entirely new paradigm may be possible for energy management in cardiac pacing.4 They propose to employ the heart itself as the energy source for the pacemaker, by borrowing technology used in self-winding watches invented more than a century ago by Swiss watchmakers. Here they demonstrate the use of prototype device built to illustrate this approach as a proof of concept. They also review some of the energetics of their approach, which explain what at first glance may have some of the appearance of a proverbial “perpetual motion machine” fallacy. We focus in this journal principally on the electrical, but the purpose of the organized electrical activation of heart is merely to initiate and to organize its mechanical function. The heart in the end propels blood under considerable pressure through the circulation, and the transduction of the electrical signal of the QRS into a coordinated muscular contraction is nonlinear “trigger” event, which consumes very little energy. It has long been estimated that the energetic cost of electrical activation of the heart is minute (<1%) compared to that required to Page 1
actually perform the mechanical work of the heart. Given the self-propagating nature of this electrical activity in the heart, the energy necessary to bring that process to threshold for initiation is much lower still. In plain English: pacemakers require only a tiny amount of power (20 µW in total in this implementation) to initiate the much more energetically demanding sequence of muscular contraction and ejection of blood. Raising the pressure of blood in the ventricular chambers and providing it with kinetic energy for ejection accounts for the great majority of myocardial work. The large and continuous metabolic cost of contraction is the energy input into the system which explain why this is not a perpetual motion hoax. A byproduct of this pressure-volume work is the acceleration of the heart muscle itself in contraction, which results in the oscillatory motion of the heart. Although it is a necessary component of cardiac ejection, the movement of the heart muscle itself is a byproduct of the primary function of contraction, analogous to the energy required to accelerate the weight of one’s legs back and forth while running, as opposed to the primary energetic goal of moving from point A to point B. The authors of this paper have discovered and explored that small source of myocardial energy, and devised a means to capture a tiny amount of it. This energy stream is very small compared to the total energy expenditure of the heart - by my calculation, less than 1 part in 10,000. However, they have determined that it is more than sufficient to maintain the very tiny demands associated with cardiac pacing, in this case exceeding the needs of the pacemaker circuit by a factor of 4 or more. The conversion of this energy into useful for through the workings of a Swiss watch is just – elegant! The research presented here is at its very earliest stages, and it forecasts the need for many more years of developmental work. Although the incorporation of a watch mechanism into a device works, it is unlikely to be the most efficient means to transduce an epicardial power source, and several iterations of this technology remain to be invented and tested. Some form of energy storage will be needed (a Page 2
larger mainspring? additional capacitance? a rechargeable battery??), in the event that changes in cardiac motion result in decreased energy flux. The effects of posture, exercise and evolving illness on ability to continuously collect energy will need to be determined. A means of communicating with the device would be needed. The rate of failure of the mechanism over time would have to be laboriously determined. Minimally invasive techniques for epicardial implantation would need to be developed, and it is certainly conceivable that this approach could be consistent with new, catheter delivered leadless endocardial devices.5, Not mentioned so far in this editorial is its other innovative feature - the leadless aspect of this prototype. This aspect of the device is also aligned with modern catheter delivered pacing devices, and addresses a second major weakness of current pacing systems. However, if the pacing site of the device is the device itself (as currently demonstrated), the potential discrepancy between optimal sites for pacing and for harvesting of energy would also have to be investigated. This is an ambitious list, and the technology could fail at any of these points or at others not proposed here. Even if such a device is able to jump many feasibility hurdles, it still may lack clinical value by virtue of expense of the device, reimbursibility of the procedure, or the inertial willingness of clinicians to simply continue to practice in the manner to which they are accustomed. It is exceedingly difficult to improve on a technology as mature, predictable and widespread as simple cardiac pacing. However, that should not be misunderstood to indicate that such devices cannot still be made better – all that is required is a unbiased assessment of the limitations of current devices, and a highly creative and open mind.
Page 3
References
1.
Smyth NP, Millette ML. The isotopic cardiac pacer: a ten-year experience. Pacing Clin Electrophysiol 1984;7:82-9.
2.
Boulé S, Kouakam C. This is the end. Int J Cardiol. 2016;223:805-806.
3.
Haeberlin A, Zurbuchen A, Schaerer J, Wagner J, Walpen S, Huber C, Haeberlin H, Fuhrer J, Vogel R: Successful pacing using a batteryless sunlight-powered pacemaker. Europace 2014; 230:1534–1539.
4.
Zurbuchen A, Haeberlin A, Bereuter L, Wagner J, Pfenniger A, Omari S, Schaerer J, Jutzi F, Huber C, Fuhrer J, Vogel R. The Swiss approach for a heartbeat driven lead- and batteryless pacemaker. Heart Rhythm 2016; 00:000-000
5.
Reddy VY, Knops RE, Sperzel J, et al. Permanent leadless cardiac pacing: results of the LEADLESS trial. Circulation. 2014;129:1466-71
Page 4