What Caused Patient’s Palpitations?

A 52-year-old man developed acute-onset palpitations, shortness of breath, and lightheadedness while sitting at his desk at work. He noticed his heart rate was rapid and asked a coworker to take his pulse for confirmation. He did not experience chest pain, syncope, or near syncope, but if he stood up and tried to walk, he very quickly became fatigued. His coworker tried to call 911; however, the patient asked to be driven to the urgent care center six blocks from their office instead. The patient’s heart rate and symptoms did not change en route. There is no previous history of heart disease. Although the patient works in an office, he is very active. He played hockey in high school and college and continues to play in an amateur league as well as coaching a youth group at the local ice rink. He is also an active member of a local bicycling club and recently completed a 150-mile recreational ride. He has no history of hypertension, diabetes, or pulmonary disease. Surgical history is remarkable for a medial meniscus repair of his right knee and a laparoscopic cholecystectomy, both performed more than 10 years ago. He works as a certified public accountant, does not smoke, and drinks one or two glasses of wine in the evening with meals. He is married and has two adult children. He denies using recreational drugs or herbal medicines. The only medication he uses is ibuprofen as needed for musculoskeletal aches and pains associated with his active lifestyle. He has no known drug allergies, and his immunizations are current. The review of systems is positive for a recent viral upper respiratory illness. He reports having vague, nonspecific substernal chest discomfort, but no pain, at the time of his illness. Symptoms have resolved. There are no other complaints. On arrival, the patient appears anxious and in mild distress, but without pain. Vital signs include a heart rate of 160 beats/min; blood pressure, 100/64 mm Hg; respiratory rate, 18 breaths/min-1; and temperature, 98.4°F. The HEENT exam is unremarkable except for corrective lenses. The chest is clear in all lung fields. There is no jugular venous distention, and carotid upstrokes are brisk. The cardiac exam reveals a regular rhythm at a rate of 150 beats/min with no murmurs or gallops; however, a rub is noted. The abdomen is soft and nontender with no organomegaly. Well-healed scars from his laparoscopic ports are present. The lower extremities show no evidence of edema. Peripheral pulses are strong and equal in both upper and lower extremities, and the neurologic exam is normal. Laboratory studies including a metabolic panel, complete blood count, and cardiac enzymes all yield normal results. An ECG reveals the following: a ventricular rate of 155 beats/min; PR interval, not measured; QRS duration, 78 ms; QT/QTc interval, 272/437 ms; P axis, unmeasurable; R axis, 34°; and T axis, –50°. What is your interpretation of this ECG?