A cardiac arrhythmia is a medical condition characterised by an irregular heart beat. The heart of people affected by arrhythmia beats either too slow, too fast or irregular at times. It is not a single, defined disease but rather a term used for a number of conditions with the same characteristic but with different underlying causes. The effects can vary between being just mildly annoying for the patient over dizziness to life threatening because of cardiac arrest.
Howard has mentioned twice that he has "transient idiopathic arrhythmia" (The Middle-Earth Paradigm, The Cushion Saturation). Idiopathic is a medical term for "occuring spontaneously". His condition therefore basically means that he spontaneously develops an irregular heart beat frequency which disappears again. So far the condition did not seem to affect him.