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RADIOISOTOPE POWERED CARDIAC PACEMAKERS
Fred N. Huffman, Ph.D., Joseph J. Migliore, M.D., William J. Robinson, B.S.
and John C. Norman, M.D.
Over 100,000 implantable cardiac pacemakers are currently being used
to rehabilitate patients with heart block. The chemical batteries powering these pacemakers usually fail within a few years. A variety of nuclear
batteries with the potential of providing long-lived (10-20 years) pacemaker power, are under development. This paper reviews the status of this
development. Nuclear powered pacemakers have reached the stage of
clinical evaluation. Their cost, although initially high, is not prohibitive.
In our institution, third party insurance carriers have assumed their cost.
The primary concern of widespread application is the maintenance of fuel
capsule integrity under all credible conditions.
PACEMAKER ENERGY SOURCES
A wide variety of chemical, biologic and nuclear energy sources have
been explored to power cardiac pacemakers. Almost all currently implanted pacemakers utilize primary batteries. Formerly, mercury-zinc
cells and electrodes produced an average pacemaker longevity of under
two years. Contemporary electrodes require significantly lower energy
input to produce consistent pacing. As a result of reduced battery drain
and improved circuits, commercially available pacemakers using primary
batteries are expected to average three to five years of longevity.' Since
the average patient is 72 years at implantation, primary batteries are
likely to be adequate for the majority of clinical situations. Solid state
lithium-halide batteries2'3 offer potential longevities significantly greater
than zinc-mercury cells. Although the first clinical application of an implantable cardiac pacemaker4 used a nickel-cadmium battery, there has
been only limited clinical use of rechargeable systems due to operational
complexity, patient anxiety and lack of cells that would operate at body
temperature. Recently, Fishell et al.5 addressed the latter problem and
developed a rechargeable pacemaker with an estimated lifespan of 20 years
(Pacesetter Systems Inc.). Other investigators have sought to develop
biogalvanic systems7'8 using body fluids as well as biological fuel cells.9
While biological fuel cells should provide adequate power for pacing, no
such unit has reached prototype evaluation.
In addition to anxiety and expense, pacemaker replacement constitutes
a small but definite risk of infection and prolonged hospitalization. As a
result, batteries using the energy of radioisotope decay to provide longlived (10-20 year) pacemaker power are being developed internationally.
A genealogy of the more highly developed nuclear-to-electric conversion
techniques is included in Fig. 1. These conversion techniques can be
grouped into two types: thermal and non-thermal. The thermal converters
(whose output power is a function of a temperature differential) include
thermoelectric and thermionic generators. The non-thermal converters
From the Cardiovascular Surgical Research Laboratories, Texas Heart Institute, of St. Luke's
Episcopal and Texas Children's Hospitals, Texas Medical Center, Houston, Texas; The Thermo
Electron Research and Development Center, Waltham, Massachusetts, and the Cardiovascular
Division, Sears Surgical Research Laboratories, Harvard Medical School, Boston, Massachusetts.
52
(whose output power is not a function of a temperature difference) extract a fraction of the incident energy as it is being degraded into heat
rather than using thermal energy to run electrons in a cycle.
< ATOMIC ENERGY RESEARCH ESTABLISHMENT
Figure 1. Genealogy of Cardiac Pacemaker Power Sources
53
THERMAL CONVERTERS
Both thermionic and thermoelectric converters have been evaluated
for pacemaker application. A thermionic converter,10 (Fig. 2), consists of a
hot electrode which thermionically emits electrons over a space charge
barrier to a cooler electrode, producing a useful power output. Cesium
vapor is used to optimize the electrode work functions and provide an
ion supply (by surface contact ionization) to neutralize the electron space
charge. A thermoelectric converter" connects thermocouples in series.
Each thermocouple (Fig. 3) is formed by the junction of two dissimilar
materials, one of which is heated and the other cooled. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier
density and charge can be adjusted in semiconductor materials such as
bismuth telluride and silicon germanium to achieve much higher conversion efficiencies. A block diagram for thermoelectric and thermionic systems is shown in Fig. 4.
Effective thermal insulation is essential to minimize heat loss with
attendant reductions in fuel inventory, radiation exposure and expense.
Both fiber and vacuum foil insulations are being used. Vacuum foil insulation, consisting of multiple layers of thin foils separated in a vacuum by
oxide particles, provides lower thermal losses in a more compact unit
than does fiber insulation.
The dc-to-dc power conditioner is optional depending on the output potential of the converter. In principle, the voltage required (typically, about
five volts) can be obtained by connecting in series adequate numbers of
thermocouples or thermionic diodes. In practice, fabrication is often simplified by designing the converter for a low voltage output (approximately
0.5 volts) and subsequently employing a dc-to-dc circuit to transform the
converter output voltage to the level required by the pacemaker electronics.
The subsystem combinations are evident from Fig. 1. The Isomite thermionic converter'2 developed by McDonnell Douglas has been discontinued for pacemaker application. ARCO Nuclear'3 uses 528 metal (CupronTophel) thermocouples in conjunction with vacuum foil insulation to obtain a high output voltage that can power a pacemaker without a dc-to-dc
converter. However, the low efficiency of the metal thermocouple results
in a relatively high fuel loading. The development of this pacemaker has
been sponsored by the USAEC. CIT-Alcatel14 in France, the Atomic Energy Research Establishment15 in Great Britain, and the Gulf Energy and
Environmental Systems Company in the United States all use low voltage
bismuth telluride modules in combination with fiber insulations and dcto-dc power conditioner units which decrease the relatively high thermoelectric conversion efficiencies. Nuclear Battery Corporation'6 uses a similar system, with the exception that vacuum foil insulation is substituted
for fiber insulation. Their Atomcell has the lowest fuel loading (0.120 gram)
of the thermal conversion systems. Siemans AG17 in West Germany vapor
deposit 1420 bismuth telluride couples on a polyimide substrate to obtain
a high potential thermopile. The thermocouples are connected electrically
in two series strings. Syncal Corporation18 is working on a high potential
silicone germanium module in which all thermocouples are connected in
series-parallel.
54
COOL
COLLECTOR
HOT
EMITTIER
kCOLLECTOR
I
EMITTER9
CESIUM
/ AI/
VAPOR
EL
VL
_-VL_
F,L////-//
Figure 2.
HOT
JUNCTION
Thermionic Converter with Potential Diagram.
COLD
JUNCTION
HOT
JUNCTION
COLD
JUNCTION
rfl_
HEAT
IN
EL.
Ipu
EL.
HEAT
OUT
Figure 3.
Thermocouoe with Potential Diagran.
_ HEAT PATH
TISSUE
--ELECTRICAL PATH
THERMAL
Figure 4.
RADIOISOTOPE POWERED
PACEMAKER
Block Diagram of Radioisotope-Fueled Thermoelectric and Thermionic Systems.
55
NON-THERMAL CONVERTERS
Non-thermal converters extract a fraction of the nuclear energy as it is
being degraded into heat. Their outputs are not functions of temperature
differences as are thermoelectric and thermionic converters.
Non-thermal generators can be grouped into three classes. The primary
generators consists of a condenser19 which is charged by the current of
charged particles from a radioactive layer deposited on one of the electrodes. Spacing can be either vacuum or dielectric. Negatively charged
beta particles or positively charged alphas, positrons or fission fragments
may be utilized. Although this form of nuclear-electric generator dates
back to 1913, few applications have been found for the extremely low
currents and inconveniently high voltages provided by direct charging
generators (Fig. 5).
The betavoltaic cell is the most successful of several generators that rely
on secondary electronic excitation. One of the oldest is the ionic or contact potential battery which dates back to 1924 (Fig. 6). Two electrodes
of dissimilar work function enclose a gas. Any source of ionizing radiation
can generate ion pairs in the gas, which are collected by the electric field
set up by the contact potential owing to the dissimilar work function
electrodes. The currents of oppositely charged particles flowing in opposite
directions add and may be passed through an external load to deliver an
electrical output. The U. S. Army sponsored work on this concept in the
early fifties at Ohmart Corporation and Tracerlab. Its drawbacks were
poor efficiency, instability and low power density.
The irradiated P-N junction20 is a solid state analog of the ionic battery
(Fig. 7). Incident radiation (e.g. light, X-rays, alpha or beta particles)
ionize hole-electron pairs. Minority carriers near the junction are collected
and constitute a current through an external load. Compared to the ionic
battery, efficiency is high, conversion volume is small and output is stable.
Beta radiation of the P-N junction is the most practical. RCA was responsible for most of the early work on this betavoltaic generator. More recently, McDonnell Douglas2' has significantly improved the efficiency and
power output. Their betavoltaic unit is termed the "Betacel".
COLLECTOR
CHARGED PARTICLE
F.L.
DECAY ISOTOPE
/
EMITTER
COLLECTOR
VL
BETA
EMITTER
F L.
VACUUM
OR
DIELECTRIC
+
V
-
L
Figure 5. Direct Charging Generator with Potential Diagram
56
IONIZING
RADIATION
3
LOW WORK
HIGH WORK
FUNCTION
ELECTRODE
t
o
A_
|
FUNCTION
ELECTRODE
HIGH#
E)---w
ARGON
o_@
F.L.
4
YL
F-
-
VL
Figure 6.
Work Function Battery with Potential Diagram.
..p..
I
"N".
opts
,
CZ
I
I
I
I
IONIZING
RADIATION
" N"
CONDUCTION
I
I
177 7 7 7EL
F.L.7p
vm
VALENCE
BAND
VL
Figure 7.
Irradiated P-N Junction Battery with Potential Diagram.
IONIZING
RADIATION
|
PHOTOVOLTAIC
CELL
Figure 8. Radioisotope-PhosphorliPhotovoltaic-Generator
57
The phosphor-photovoltaic generator22 is an example of a dual conversion nuclear battery (Fig. 8). First, the radioactive energy is converted
into light by a scintillating phosphor. Second, the photovoltaic cell converts the light into an electrical output. Theoretically, the phosphor-photovoltaic generator has the potential of higher power density than the betavoltaic generator. In practice, phosphor degradation has limited its application.
RADIOISOTOPE FUELS
Plutonium-238 (Pu-238) is the unanimous choice of thermoelectric pacemaker developers while most betavoltaic converters have utilized Promethium-147 (Pm-147). Plutonium-238 has a long half-life (89 years), reasonable power density (3.5 watts/cm3), low radiation output, established
containment technology and low cost. Since Pu-238 is primarily an alpha
emitter, its decay builds up a helium pressure inside the sealed fuel capsule.
Most thermoelectric power sources (see Table 1) use a plutonium oxide
fuel form. This refractory material (melting point of 4050°F) is chemically
and biologically inert. To minimize the probability of particulate inhalation if the encapsulation is compromised, the fuel pellet is pressed and sintered. The dose rate can be further reduced by using high purity "medicalgrade"' Pu-238 oxide23 enriched with Oxygen-16.
The high energy and mass of the Pu-238 alpha particles would rapidly
degrade a P-N junction. Consequently, a low energy and mass beta particle
is used for radiating a P-N junction to obtain an electrical output. Most
betavoltaic investigations have used the inexpensive Promethium-147
(Pm-147) beta emitter which has a half-life of only 2.62 years. Unfortunately, there does not appear to be a radioisotope with the requisite combination of beta energy, half-life and low dose rate that can be produced
economically to take full advantage of this conversion technique. Although
tritium24 and Nickel-63 have been considered, the beta range of the former
is quite short (resulting in excessive source absorption), while the latter
is too expensive. Both give a drastically reduced electrical power density
relative to the Pm-147 fueled betavoltaic converters. McDonnell-Douglas
projects a useful lifetime of 7 to 10 years for their Pm-147 fueled Betacel
units. Betavoltaic converters do not appear to have the potential for extended life (up to 20 years) of the Pu-238 thermoelectric generators. In
the case of either the Pu-238 or Pm-147 units, the integrity of the fuel containment under all credible conditions is the primary design consideration.
NUCLEAR BATTERY COMPARISON
Of the many nuclear conversion methods investigated for powering
pacemakers, only thermoelectric and betavoltaic systems have been applied clinically and are commercially available. The subsystem combinations
of these power sources have been identified in Fig. 1. With a single exception, all thermoelectric generators use semiconductor thermocouples to
achieve a higher conversion efficiency than metal thermocouples. A summary of the fuel, pacemaker electronics, cost and evaluation status is given
in Table 1.
The tabulated nuclear power sources are comparable in size and weight
58
to chemical batteries. The fuel loadings of the semiconductor thermoelectric
generators range from 0.120 to 0.200 gram of Pu-238; The ARCO Nuclear
unit uses lower efficiency metallic thermocouples and requires a higher
fuel inventory. The Nuclear Battery Corporation Atomcell is the most
efficient of the batteries, since it uses vacuum foil insulation as well as
bismuth telluride thermocouples.
The radiation dose rates from all the nuclear batteries are less than
20 mrem/hr at contact and less than 0.5 mrem/hr at 5 cm. Based on continuing studies in our laboratories, the somatic effects to the patient from
these dose rates should not be a deterrent to clinical use. Effects on
canines25 implanted with 16- and 24-watt Pu-238 sources (two orders of
magnitude larger than those contained in nuclear-powered cardiac pacemakers) have been benign except for minor pathological alterations in
tissues within 6 mm of these sources. Radiation equivalent sources simulating the dose rate fields of 50 gram Pu-238 sources, but with negligible
thermal outputs, have been implanted in dogs26 for periods up to four
years with similar results.
The whole body, time averaged dose rate from a typical nuclear pacemaker is approximately 0.5 rem/year compared to the whole body occupational exposure limit of 5 rem/year recommended by the National Committee on Radiation Protection.
To date most nuclear pacemakers have been implanted in younger patients. At our institution, two ARCO Nuclear and four Medtronic
(Laurens-Alcatel battery) nuclear pacemakers are being clinically evaluated. There have been no unusual results from these studies.
In summary, nuclear-powered cardiac pacemakers have reached the
stage of clinical evaluation. Such pacemakers have the potential of long
life. Their cost, although initially high, is not prohibitive. Animal studies
indicate that the somatic and genetic radiation effects from a sealed source
should not preclude clinical use.The primary concern of widespread application is the maintenance of the fuel capsule integrity under all credible
conditions.
TABLE I
SUMMARY OF PRINCIPAL NUCLEAR PACEMAKERS
Fuel
Radioisotope
Loading Pacemaker
Ev,aluation
Status
Facility
Nation
ARCO Nuclear
U. S.
Pu-238 Oxide
"Atomcell"
(,Nuclmear
Battery Corp.)
U. S.
Pu-238 Oxide
0.120
(0- 16 Enriched)
-
$ 2500/
Batte ry
Animal Evaluation
ost efficient of available
n-clear bvatteri,s
Atomic Energy
Research Estab.
(AERE)
Great
Britain
Pu-238 Nitride 0. 180
-
_
-80 Clinical Cases
Based on
"Btacel 400"
(McDonnell Douglas
U. S.
Pm- 147 Oxide
0. 105
Gulf Energy and
Environmental Systemls
U. S.
Pu-238 Oxide
0. 180
CIT-Alcatel
France
Pu-238/
Sc Alloy
0. 160
Siemens AC.
West
Pu-238 Oxide
0.200
Pu-238 Oxide
-
Fuel
(GM)
Electronics
Cost
$ 5000/
0.410 ARCO
(0-16 Enriched)
C.mn,, nts
42 Clinical Cases
Pacemaker
Biotronik
IF;P- S1;
I
USAEC
sS
Z:0oo/
Animal
B3attery
5000/
Medtronics
$
Model 9000
Psac-mak-
-
-
Fsaluati.n
U. S.
Clinical
C-
First pa-a
17k,r to
e
used
linically
e1
ap-)at. d tEin-filn,
-
rototype
under t, st
59
that
Basi-Ily if 1ential to AFb
th
-
ha
>a tt,- ry
400
Germany
Sync,al
RIPPLE techn7,l7gy
( 2375/
60 Clinical Ca.sc
1'otential life 1
I
P'a emas.k" r)f P'l- Z 38 unit,
-
(0- 16 Enriched)
Developnunt sponsored by
th-n-pil-
rmopile
Direct highi -ol.tage -ith
siIico:n-gern,.niu., th--r,).pil,-
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VISIT www.prutchi.com for more information and pictures on implantable devices