How can anxiety cause heart arrhythmias?

Anxiety: Not cause but possible effect of irregular heart beats.

Sinus arrhythimia: This is the kind of heart arrhythmia that anxiety can cause and is a normal heart beat that happens to be irregular in timing. The high rate is similar to when you exercise. Since heart is pumping normally during sinus arrhythmia, it is harmless (other than causing more anxiety). Also, they tend to be short and self-limited, especially if anxiety is relieved.