But as I would discover later, my heart had snapped back into regular rhythm.
With a quick study of the .gpx file from the run on Trainingpeaks, I think I can guess to within a minute when I converted back to Normal Sinus Rhythm by looking at how efficiently I run. The metric of "Efficiency Factor" (EF) is computed by dividing running speed (normalized for grade) in yards per minute by average heart rate. Note the overall EF of 1.43 in the first frame:
Frame 2 highlights a roughly 2 minute segment 12-13 minutes into the run, with EF of 1.28. This is typical of what I've seen during my week of Afib, and is far from the EF of 1.5-1.6 that I usually see. A 10-20% loss of high end cardiac output, and therefore efficiency, is what is expected for Afib. Not a literal killer, but certainly a killer of any chance of qualifying for the Ironman World Championships, which has been a perennial goal of mine for the last 3 years.
And in the final frame, the EF is suddenly back to normal values:
|GE. We bring good things to life. Like NSR.|
P-waves! Glorious P-waves! And they've remained there since. With the exception of a couple of skipped beats that scared the holy hell out of me, they've remained.
The next morning, I felt as if someone had slipped me an (extra) quad espresso. I was walking on air, and my swim felt supercharged.
Unfortunately, speaking of espresso, I'm still going to pursue the prudent course and keep ratcheting down caffeine. I'm grieving. I love me some stimulating beverage. It's basically my only remaining vice.
The leading theory for how it happened to me remains the altitude tent, or at the very least, the simulated height to which I pushed it and how soon I did it (increasing 500-1,000 "feet"/week, sleeping nightly (and often poorly) at 10,000 "feet." This article (Thanks to Ted Rasoumoff) would suggest that pulmonary artery pressures go up substantially, even in asymptomatic residents of high altitude. Between this and three 20 hour weeks of training, plus work, plus family time and duties...I know, but there are just so many awesome things in my life that it's easy to want to do it all...there was just too much stress and not enough recovery going on. But I'll say it one last time: As someone who grew up at 7300 feet, I would've thought this was going to be pure gravy.
I had felt some sense of strain and for lack of a better word, bogginess, in my chest for the week or two before recognizing the irregular heartbeats and Afib, and it was starting to feel a little difficult and uncomfortable to lay flat and sleep in the tent. I wrote some of this off to the small enclosure itself (it's just a hood that covers the head of the bed, and can be mildly uncomfortable), but clearly there was more to it than that. It makes mechanistic sense that increased right-sided heart pressures led to stretching of the right atrium and the corresponding cardiac conduction pathway, which is how you get arrhythmias like atrial fibrillation. It also follows that quitting the tent would allow pulmonary pressures, then right-sided heart pressures, to decrease. Then the atrial conduction pathways would shrink back to normal length (as this was not yet permanent cardiac remodeling). Then the aberrant conduction would go away when the normal course of electrical impulses through the atrium overrode it and took back over.
Responses of this sort to an altitude tent have not been reported with any great frequency. But obviously, that's irrelevant to me. The altitude tent experiment is certainly over. Look for it on Ebay.
Since I'm feeling like a cool million, I'm going to slowly and cautiously start ramping up the mileage again in preparation for Ironman Arizona. Assuming my working hypothesis is right, this shouldn't be a problem. But I guess we'll see. Say a prayer to Our Lady of Regular Heartbeats for me.