There are five parts to this answer and summary

Questions & Answers

1. What is the mechanism of the fast rhythm? Supraventricular tachycardia
2. Why do you think she has it? Not sure by the history and ECG alone (see follow-up below)

Click for clincal follow-up on the answers

56 year old woman undergoing chemotherapy for bronchio-alveolar carcinoma. Two years previous underwent emergency pericardiocentesis for cardiac tamponade (malignant effusion). Complaining of rapid palpitations.

Vent. rate 134 bpm
PR interval 258ms

QSR duration 104ms
QT/QTc 302/450ms

P-R-T axes -10 31 -35