A 51-year-old man presents to the emergency department (ED) after “feeling terrible” for the past 24 hours. His symptoms began with a single episode of chest tightness and pressure, which lasted for approximately 30 minutes. This occurred at rest and resolved spontaneously. Following this episode, he developed nausea, fatigue, dizziness, shortness of breath, and palpitations, which persisted until his ED visit.
The patient’s history is remarkable for one episode of atrial fibrillation that was treated with cardioversion five years ago. He is otherwise healthy; he works as a park ranger and is very active outdoors. He has no family history of cardiac disease, does not use alcohol, and does not smoke.
Symptom review reveals no episodes of syncope, presyncope, vision changes, vomiting, focal weakness, or loss of sensation. His current medications include aspirin (325 mg/d) and a multivitamin. He has no known drug allergies but is allergic to shellfish.
The physical exam reveals a thin, healthy-appearing man who is pale, diaphoretic, and in mild distress. His blood pressure is 126/80 mm Hg; pulse, 145 beats/min and irregular; respiratory rate, 24 breaths/min; and oxygen saturation, 99% on room air. He is afebrile. Pertinent findings from the cardiovascular exam include an irregular rate without murmurs, rubs, or gallops, and jugular venous distention to approximately 9 cm. There are no carotid bruits and no peripheral edema.
During the course of the exam, the patient’s heart rate abruptly decreases from 145 to 90 beats/min, and the patient becomes much less symptomatic. An ECG shows the following: a ventricular rate of 89 beats/min; PR interval, not measurable; QRS duration, 98 ms; QT/QTc interval, 400/486 ms; P axis, 83°; R axis, 76°; and T axis, 68°.
What is your interpretation of this ECG, and how do you account for the differences in heart rate?