Rapid Heart Rate

A 40-year-old man presents to the emergency department with palpitations and shortness of breath that started a few minutes before his arrival. The patient states that he was closing his shop when his heart began to beat rapidly and he had difficulty catching his breath. His symptoms started suddenly and continued when paramedics arrived minutes later. They observed a rapid heart rate on the cardiac monitor and associated rhythm strips (see Images 1-2). He was subsequently given adenosine 0.6 mg en route to the hospital. The patient had a momentary period of asystole, but his rapid heart rate returned.

On his arrival to the emergency department, the patient continued to have the sensation that his heart was racing. He denies having any chest pain, nausea, vomiting, diaphoresis, light-headedness, or recent illness. He felt well before this episode. He denies having any symptoms of infection, such as fever, cough, vomiting, diarrhea, anorexia, or dysuria. He reports increased stress at work and is drinking as many as 4 cups of coffee a day. He reports no notable history of medical conditions except for a similar episode of a rapid heart rate about 4 years ago; for this, he was treated with an unknown drug for 2 years. His family history is significant for a father who died of a myocardial infarction at 45 years of age. The patient takes 1 baby aspirin daily. He denies using any over-the-counter or illicit drugs; however, he smokes 3 packs of cigarettes per week.

On physical examination, the patient is afebrile and has a heart rate of 165 bpm and a blood pressure of 138/79 mm Hg. He appears well and is in no acute distress. Findings on head and neck examination are unremarkable. He has no jugular venous distention. His heart rate is rapid and irregular, with an audible S1 and S2 and no gallops, rubs, or murmurs. His lungs are clear bilaterally. His abdomen is soft, nontender, and without any masses. He has no peripheral edema. Results of his laboratory workup, including a CBC, serum electrolyte and cardiac enzyme measurements, and a coagulation panel, are all normal. His chest radiograph is also normal.

An ECG is obtained (see Image 3). What is the diagnosis?

What is the diagnosis?
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Paramedics gave the patient a single dose of adenosine, which had no lasting effect on the rapid rhythm.