Puzzle 2004 3 123, One to one, one to two, two to one? What is the rhythm?

Authors: A.A.M. Wilde and H. Tan, Cardiologists, AMC, The Netherlands A 18-year-old boy is suffering from palpitations without specific triggers. It started when he was 12 years old. Sometimes he feels dizzy which usually coincides with very fast palpitations. On other occasions, he feels palpitations (less fast) without dizziness. Also, there are transitions between both sensations. On one such occasion, he presents at the hospital. An ECG is taken of which the six precordial leads are shown here. The calibration is as usual (i.e., 10 mm=1 mV and 400 ms, respectively).

What is your diagnosis?