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Had a follow-up with the cardiologist about the echocardiogram. Turns out it didn't rule out heart failure - what he now suspects is something called "Heart Failure with Preserved Ejection Fraction," or HFpEF for short (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3056204/). It's a tricky little deal where the heart builds up fluid, but also gets stiffer.
More tests are on order, including a nuclear chemical stress test. Until then, it's baby aspirin and watch the salt.
@leejackson At least they are doing the chemical version! It’s just a couple of minutes of discomfort and then more readings. The hardest part is the waiting.