<<Disclaimer: Verify this information before applying it to your situation.>>

I had trouble looking this up because it's spelled with an "H" in it.
I have had heart rhythm problems off and on all my life, which I don't
pay much attention to.  But I decided to research it a bit, since your
question made me curious.

I see that my own dysrhythmias are caused by a combination of prescribed
medications and various problems like hardened arteries and my two damaged
heart valves.  However, they are not a problem unless I either get very
anxious or else drink something that tips the balance of sodium/potassium
too far one way or the other.  Diet sodas with sodium- or potassium-based
artificial sweeteners can do that.
I am printing out what I found at the web site:

http://www.heartcenteronline.com/myheartdr/common/articles.cfm?
Artid=25&startpage=3

There are links on that page to articles about diagnosis, treatment,
etc., of the condition.
===========================================
Risk factors and causes for arrhythmias
To understand what causes abnormal heart rhythms, one must first have a
basic understanding of what causes a normal heart rhythm.

Each of the heart’s contractions, or heartbeats, is triggered by
electrical impulses. These electrical impulses are sent from the
sinoatrial node (the heart’s natural pacemaker), which is located at
the top of the upper-right chamber of the heart or right atrium. From
there, the electrical impulses travel through the upper chambers of the
heart (atria) and to the atrioventricular (AV) node, where they are
transmitted to the lower chambers of the heart ventricles via the
"bundle branches. Thus, the electrical impulses travel from the
sinoatrial node to the ventricles, to trigger and regulate the
heartbeat.

In general, the abnormality that results in an arrhythmia is one of the
following: Abnormal function of the cells in the sinoatrial node (sick
sinus syndrome), such that these cells are either not firing or
transmitting impulses properly. Delayed or improperly produced/conducted
impulses through the
atrioventricular junction (which could cause heart block) or the
ventricles (which could cause bundle branch block).

     * An extra pathway in the conduction system, causing additional
heartbeats.

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