A 55-year-old man presents with a four-day history of shortness of breath, fatigue, and dyspnea with minimal exertion. He is concerned that his pacemaker isn’t working properly. Medical history is remarkable for coronary artery disease, hypertension, tachycardia-bradycardia syndrome, and type 2 diabetes.

Coronary stents were placed in the mid-right coronary artery and in the first obtuse marginal arteries in 2005. Later the same year, he underwent implantation of a dual-chamber pacemaker for recurrent near-syncope from tachycardia-bradycardia syndrome.

The patient associates his current symptoms with those he experienced prior to pacemaker implantation. He has had no chest pain or symptoms to suggest is­chemia. He is taking atenolol, clopidogrel, lisinopril, simvastatin, and metformin. 

The physical exam reveals a blood pressure of 100/70 mm Hg, with a regular pulse of 75 beats/min. He is afebrile. The respiratory rate is 14 breaths/min and unlabored. The cardiac exam reveals a regular rate and rhythm without murmurs, rubs, or gallops. The lungs are clear in all fields. The abdominal exam is benign, and the extremities are warm with strong pulses throughout. There is no peripheral edema.

Interrogation of the pacemaker documents a dual-chamber St. Jude Identity^® pacemaker system programmed DDD with a lower rate of 50 beats/min, an upper sensor rate of 120 beats/min, an upper tracking rate of 150 beats/min, and mode switch programmed on at a rate of 70 beats/min.

An ECG reveals the following: a ventricular rate of 73 beats/min; PR interval, not measured; QRS duration, 182 ms; QT/QTc interval, 476/524 ms; R axis, –82°; and T axis, 78°. What is your interpretation of this ECG, and why is the patient symptomatic?

ANSWER

The ECG documents atrial fibrillation (A-fib) with ventricular pacing in a patient with a permanent pacemaker. A-fib is evident by the absence of P waves and a rapid, chaotic atrial rate. Ventricular pacing is evident by widening of all QRS complexes without R-wave progression in the precordial leads. Careful inspection reveals pacing spikes immediately before each QRS complex.

The ventricular rate is regular due to a programmable pacemaker function called mode switch. This is a safety mechanism designed to avoid ventricular pacing at the high atrial rates seen in A-fib. The pacemaker is functioning normally. The patient’s symptoms are due to atrioventricular dyssynchrony due to the A-fib.