I'm a little selfish myself, Jim, baiting you to feed me some more useful information. I'll jot down a few things that might give you something to think about, but maybe not.

My heart is serviceable with a-fib or PAC my main concern, the result I think of ventricular hyperplasia from 20 years of hypertension, which continues today despite three medications (Amiloride, Carvedilol, and Lisinopril). Without those, my BP would be about 170/120. I think the hypertension triggered the a-fib/PAC, but sped up my heart from 50bpm to 70 because of the new short-stroke in the atria.

Bradycardia! My nephrologist noted that my calcium channel blocker was leaving water in the tissues of lower legs, so she switched me to a beta blocker (Carvedilol) that is more kidney-friendly than Metoprolol or Atenolol. Literature warned of possible complications of taking beta blockers when a-fib is on-site, so I wangled advice from my cardiologist whose curb-stone opining is suspect. He said with a-fib there is no escaping diastolic dysfunction, so don't worry about a beta blocker causing added problems (as a Mayo study had suggested). As you noted, beta blockers slow the heart rate, which could be a problem for me whose rate was 50bpm most of my life -- from running not hyperplasia -- until a-fib; in fact, a-fib made a beta blocker possible for me for the first time, because it raised my rate just enough.

Your sodium levels confuse me. My HMO says 137 is in the 135-145 normal range. So they wouldn't call 137 hypo. I've been steady between 141 and 146 for the last three years. Sodium is one of my main problems, trying to drag off potassium and leaving me hypokalemic. So my diuretic is potassium-sparing Amiloride; newer ones in this class worked better but offered various side effect risks I couldn't take (how's breast cancer?).

In the end, your alert about hyponatremia will be very good advice for some of our new friends -- and hopefully for you as well.
Martin