Atrial arrhythmia is when the top two chambers of the
heart do not
beat properly. If the arrhythmia is so fast that the atrial chambers do not pump properly, it is called atrial fibrillation. Pulse rates can top 220 beats per minute, which makes you feel very light-headed.
Folks who get atrial fibrillation take an anti-coagulant (clots can form in the heart and travel to the brain). The heart can be treated with medicine or with electric shock therapy (called cardioverting).
If the arrhythmia occurs in the atrial chambers such that it contracts the chambers before they are set to pump, it is called PAC, or premature atrial contraction. If it occurs in the lower two chambers (ventricals), it is called PVC, or premature ventrical contractions.
Everyone has one or two PAC/PVC's a day. When you have them every third beat, the feeling is similar to a myocardial infarction. Your heart seems to pause, then pump doubly hard. It sorta feels like a mule kick in the chest, and you feel like you can't inhale for a second. Very disconcerting feeling.
Usually caused by an electrical signal malfunction, related to body chemistry or actual damage to heart muscle tissue.
I've had these medical problems off and on, and it's one reason I was medically retired from the US Military. At least they pay for my college now :)