1. ECG.
http://www.bloodpressureuk.org/BloodPressureandyou/Medicaltests/ECG
2. Standard range of intervals, E MEDICINE.
http://emedicine.medscape.com/article/2172196-overview
3. PQRST INTERVAL and AD8232 guide, SPARK FUN.
https://learn.sparkfun.com/tutorials/ad8232-heart-rate-monitor-hookup-guide
4. Conduction System, QCG.
http://www.qcg.com.au/conduction_system.html
5. Types of heart arrhythmia, MEMORIAL HERMANN.
http://heart.memorialhermann.org/types-of-heart-arrhythmia/