Not quite glowing in the dark

kw: autobiographical entries, medicine, heart disease, nuclear materials

A few weeks ago I was seeing a doctor on a follow-up about blood pressure, and mentioned I had had a brief dizzy spell with mild nausea. I was standing, felt off balance, then a bit nauseated, and sat down until it passed. In total, about a minute. A day or two later I had a milder moment of dizziness while sitting down. He said I needed to have my heart checked. I guess he thought it might be atrial fibrillation or a precursor to a heart attack.

A few days later I had an "ordinary" stress test. This is a pretty quick test, 8-9 minutes on a treadmill that definitely makes a guy sweat and breathe hard! All while hooked up to ten leads of a fancy EKG machine. Then they did a heart ultrasound and attached a Holter monitor to me, which I wore for two days. Fortunately I was allowed to unhook it for showers.

I got a phone call that the stress test was "slightly abnormal" and that they were ordering a stress test with a "tracer". I was to consult with a cardiologist the next day. He wrote the order and explained a little. He also said that, though my heart rate is slow, it is not alarming (yet). I told him of my father, whose heart slowed down a lot when he was my age, and that he has worn a pacemaker ever since. The doctor said I show no signs of "blockage" in the cardiac nerves, so my natural pacemaker is doing fine.

I studied what I could about the tracer material. The tracer is trademarked "Cardiolite®". It is a solution containing the synthetic element Technetium, the metastable isotope Tc99m. This is an amazing, mildly radioactive nuclide with a half life of 6 hours. It emits only a weak gamma ray, with an energy of 140,000 eV (conventional X-ray machines emit a spectrum of X-rays that typically range from 30,000 to 100,000 eV). After emission a Tc99m atom becomes Tc99, which is a beta emitter having a half-life of 211,000 years. It emits no gamma rays, just the beta particle (energetic electron) and converts to Ruthenium 99, which is stable. The beta particle's energy is 292,000 eV.

All this is good for several reasons. The gamma photon is low energy so it does less damage than the ones emitted by more familiar elements such as Uranium or Radium, which are 4-5 million eV. The beta particle is also in the same energy range. After a day, only about 1/16 of the Tc99m dose remains, and after two days, only a quarter of a percent. It is soon undetectable. The Tc99 that remains has very low activity. Its half life is about 300 million times as long, which means the number of particles released per second is one-300-millionth as much. I just didn't know what the dose would be.

First thing this morning I had the test. I asked the technician, a nice young man (and very skilled putting in an IV), named Mark, "how many milli- or micro-Curies?" He said, "For you, 7.4 milli-Curies." Now I had a number I could conjure with later.

Once Mark put in the IV he infused the tracer material. He said it was manufactured to order in Philadelphia, made a few percent stronger so it would be the right strength when it arrived, which takes no more than one hour. I sat for half an hour to give the stuff a chance to circulate everywhere. Then I was put in a scanner, which is a little like a MRI machine, but a special scanner swings up alongside and above the chest. It scans for eleven minutes, gathering "counts" of the gamma rays it detects. The detectors can tell the direction of each gamma particle.

Right after that I went to the next room where the EKG was hooked up and the treadmill test began. I lasted a little longer than I had a week earlier, but was still quite exhausted after 9 minutes. Partway through the test Mark put some more tracer in. My heart rate got to 144 (they needed it at 129 or above), and blood pressure rose to 180/100 during the test. My usual blood pressure is about 135/75, now that I take a medication for it. It was 20 points higher before that. When the nurses unhooked the EKG they left four electrodes in place, that Mark would use.

After five minutes of cool-down it was back in the scanner for another eleven minutes, this time hooked to a less elaborate EKG. Then I was free to go home. I guess I'll get a phone call in a few days with the analysis. I come from a family that doesn't get heart attacks, so it is a puzzle what might be happening.

Now, what can I do with the number I got, 7.4 milli-Curies (mCi)? A Curie is a large unit, a standard of radioactivity, equal to 3.7x10¹⁰ "breakdowns" per second. That is 37 billion. It is the number seen with one gram of pure Radium. The SI unit of radioactive decay is the Becquerel, which is one per second, so the Curie is 37 billion Becquerels. 7.4 mCi is just under 274 million per second. Once the tracer is injected and spreads around, that is 274 million for the whole body. I weigh about 100 kg. My heart probably weighs 1/3 kg, or 330 kg more or less. So it gets 1/300th of the dose, which would emit about 900,000 counts per second. I don't know how many of these the scanner captured. I glanced at the screen as I left the room and saw a vague heart shape composed of a few hundred white dots, and a sort of oval shape that looked similar. Probably front and side views of my heart in gamma vision!

Who knew a moment of dizziness could lead to all this?