A 68-year-old man presents with 30 minutes of rapid palpitations and light-headedness. Three years ago he had a heart attack treated with a coronary artery stent, and he has never had palpitations before this episode. He is alert, his blood pressure is 104/70 mmHg and his chest is clear. The ECG shows a regular tachycardia at 170/min with a QRS duration of 0.16 seconds and the same QRS shape in every beat; no P waves can be identified. Which one of the following is the most likely diagnosis?
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