Question: What is the treatment for this patient?

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Answer: Potassium supplementation (and magnesium as well)! The patient had severe hypokalemia (and also hypomagnesemia).

The key clue here is the profoundly prolonged “QT interval.”  This is difficult to observe due to ventricular bigeminy, but two consecutive sinus beats are present near the very end of the strip.  The QT interval appears to be impossibly prolonged with a biphasic T-wave. In reality, this is almost certainly a T-wave fused with a U-wave to create a bifid/complex-appearing T-wave.  QT prolongation with a bifid “double-hump” morphology suggests hypokalemia, certain medications, or LQTS type II (see differential diagnosis lists for QT prolongation based on morphology are here).

The ideal rate of potassium infusion for severe hypokalemia is difficult to know.  For patients with recurrent malignant arrhythmia, faster rates of potassium infusion may be beneficial (as compared to standard treatments for routine potassium repletion).  This is discussed here.

A final pearl is that the initial key treatment is aggressive IV magnesium. IV magnesium is safe to give at fast rates. So it's often possible to quickly improve the magnesium level, which rapidly removes the patient from immediate danger.  For this patient, aggressive magnesium repletion eliminated the bigeminy and improved the ECG long before the potassium could be fully repleted.