Why was plutonium-238 used in some pacemakers before high-efficiency batteries were available?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

Plutonium-238 was utilized as a power source for pacemakers in the 1970s because it provided a highly dependable, long-lived energy supply before the development of high-efficiency chemical batteries.1

As an alpha-emitting isotope with a half-life of approximately 87.7 years, plutonium-238 generates heat through radioactive decay at a rate of about 0.57 watts per gram. This thermal energy can be converted into electricity using thermoelectric generators, making it an ideal candidate for applications requiring long-term, reliable power without frequent maintenance. Because it emits negligible gamma rays, it requires minimal shielding compared to other radioactive isotopes, which was a critical factor for its use in implanted medical devices.21