How did nuclear pacemakers work?

Long-life pacemakers powered by plutonium-based thermal decay were implanted beginning in the 1970s, but better battery technology, safety concerns, and regulatory issues made them obsolete within a few decades.

The technology
Q: What did the nuclear pacemaker look like? How is it identified?
A: The unit’s electronics are embedded in epoxy, with the plutonium power cell at the top, as shown in Figures 1 and 2. The hard titanium case is designed to withstand “special events, including gunshots and cremation.

Figure 1. The case of the Cordis RTG pacemaker shows the many groups of critical information that must be “embossed” on the surface, as well as the approximate size. (Image: Los Alamos National Laboratory)

Figure 2. These are examples of other RTG pacemakers from different vendors. (Image: Los Alamos National Laboratory)

Q: How much plutonium was used? What was the radiation risk?
A: The power source has two to four curies of plutonium-238 (it has an 88-year half-life). The whole-body exposure is estimated to be approximately 0.1 rem per year to the patient and approximately 7.5 million per year to the patient’s spouse. To provide some perspective, this is roughly comparable to the exposure from a single chest X-ray at the time (today’s X-rays have lower dosages).

In one of the References, Steven Biegalski, chair of the Nuclear and Radiological Engineering and Medical Physics Program at the Georgia Institute of Technology, said, “Pu-238 decays 100 percent of the time by alpha emissions, which are easily stopped by the housing of the pacemaker and are absolutely no threat outside the pacemaker.”

Q: What was the size of the pacemaker?
A: The RTG pacemaker was relatively large, about 5 × 5 cm. This size was due to the state of the electronics at the time, plus the “capsule” needed for the plutonium power source and its shielding. Since we now have many types of pacemakers for different cardiac issues, a direct comparison with today’s pacemakers is not possible. However, the typical size of a modern unit is about 2 cm in diameter and about 5 to 10 mm thick, which is far smaller than the RTG-powered one.

Q: Where are they now? Are any still in use?
A: Although the expected lifetime of these units was 15 years, many lasted longer than that, and one was still going after 30+ years.

Q: For comparison, how long does the battery last in a modern pacemaker?
A: Industry and medical sources cite between 5 and 10 years, depending on pacemaker type and how often it “jolts” the patient. Replacing the battery is now a relatively modest outpatient procedure in most cases, but sometimes it is an overnight one.

Q: Actually, getting 15 to 20 years or more of continuous use seems like a good “deal,” so why not go with the long-life mini-RTG version?
A: There are several reasons. The most obvious is size, as it can’t be scaled down. Further, in addition to the inherent high cost of the plutonium “subsection” in the pacemaker itself (even when compared to a high-end, long-life battery), there are layers of regulatory issues regarding handling it, disposing of it, and more.

Modern batteries have much higher energy and usage capacity than mercury-zinc batteries. While they can’t run as long as the nuclear-powered ones, doctors began to realize that the longevity of nuclear pacemakers was actually excessive and counterproductive. The reason was that the replacement cycle of lithium-powered pacemakers actually allowed doctors to also replace either the batteries alone or the pacemakers themselves with new and improved technology as it developed in those 10 to 15-year periods. Today’s pacemakers have two-way data links, detailed sensing and performance reporting, and more.

The final part looks at some of the expected and unexpected long-term implications of these pacemakers.