2. Abnormal Heart Rhythms -- Information from the Merck Manual
Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart. The most common cause of arrhythmias is heart disease, particularly...
http://www.merck.com/mmhe/sec03/ch027/ch027a.html#sec03-ch027-ch027a-461

3. Palpitations -- EMedicineHealth Consumer Information
This condition may be simply a sudden awareness of a normal heartbeat, but it may also be a potentially dangerous abnormal heart rhythm (arrhythmia).. Palpitations are noticeable heartbeats that a person senses without intending to, usually...
http://www.emedicinehealth.com/articles/11139-1.asp

4. palpitations -- University of Michigan Health System
You may have an ultrasound test of the heart called an echocardiogram or an exercise stress test to see if heart problems are causing the palpitations. You may feel that your heart is beating harder or faster than usual or that it is skipping a...
http://www.med.umich.edu/1libr/aha/aha_hrtpalps_car.htm