MSFinals-0071

A 70-year-old known cardiopath is brought to hospital by ambulance, complaining of chest pain and shortness of breath. He looks pale and is very sweaty. Examination reveals a blood pressure of 80/55 mmHg, pulse of 135 bpm, SpO2 of 93% and bibasal wet crackles in the chest, as well as peripheral oedema. Peripheral pulses are palpable. A previous median sternotomy is noted. An electrocardiogram (ECG) reveals regular tachycardia, with QRS complexes of uniform amplitude, a QRS width of 164 ms and a rate of 135 bpm.
What is the most important step in management?