For my husband, it was a feeling of heartburn combined with an elevated heart rate in the middle of the night. Fortunately, my reaction was, "oh my god, get up, we're going to the hospital NOW!"

This was a bit over two weeks after a delayed Transatlantic flight with turbulence bad enough that we barely left our seats.

The main reason I freaked out and took him to the hospital immediately instead of just saying, "that's weird," was that a close friend had had a pulmonary embolism a few weeks after flying back to the US after tearing up her knee skiing (and having to wear a stiff brace on the plane).