On multiple occasions I've experienced the same sequence of events: a popping sensation in my chest (I can't localize what area it's coming from yet) followed by the sensation that I'm about to pass out (syncope) and a suddenly accelerated heart rate (e.g., from 75ish to 115ish). On three occasions this popping sensation has been triggered by straining to pass a bowel movement (i.e., during a valsalva maneuver) and yesterday I experienced it while taking a very deep breath. There isn't any pain accompanying the pop, but it's still highly perceptible. The whole episode lasts anywhere from less than a minute to a couple of minutes before I'm able to bring my pulse back down to ~80 through careful breathing.
The first time it happened sent me into an absolute panic where I was certain I was about to die - this sent me to the ER. A chest x-ray, head CT-scan, blood work, and EKG found nothing. I have an appointment with a cardiologist in a few days. I've certainly been dealing with anxiety since the first incident (something I've never had a problem with before), but I'm increasingly sure that the heart issue is real and the cause of the anxiety.
Any suggestions on what could be causing these episodes?
Background:
Age: 34
Sex: Male
Race: White
Height: 6'-4"
Weight: 150 lbs
Other medical conditions: Premature atrial contractions for which I take no medications.
Medication taken: None