ECG Challenge: A 67-year-old man with a known dilated cardiomyopathy and a left ventricular ejection fraction of 25% presents to the emergency department with worsening shortness of breath and lightheadedness. His heart rate is 30 bpm and his blood pressure is 90/50 mm Hg. His ECG shows complete heart block with an escape junctional rhythm. As a result of his heart failure and low left ventricular ejection fraction, it is decided to implant a pacing implantable cardioverter defibrillator. After insertion, an ECG is obtained (ECG A). The patient returns for a follow-up device check 1 week later and an ECG is repeated (ECG B).
ECG A shows a regular rhythm with a rate of 110 bpm. There is a P wave before each QRS complex (+) with a stable PR interval (0.16 s). The P wave is positive in leads I, II, aVF, and V4 through V6. Hence, this is a sinus tachycardia. There is a pacemaker stimulus preceding each QRS complex (^; ie, there is a ventricular pacemaker). The mode of the pacemaker is A-sensed V-paced or P-wave synchronous ventricular pacing. The atrioventricular delay is 0.16 s. The QRS complex duration is prolonged (0.14 s) and there is a QS morphology in lead I (↑) with an indeterminate axis between –90° and ±180° (negative QRS complex in leads I and aVF). This is not the morphology seen with a right ventricular pacemaker, which is associated with a left bundle-branch block morphology and a broad R wave in lead I as the impulse direction is from right to left. The presence of a QS complex in lead I means that the impulse conduction is going from left to right and is a result of a biventricular pacemaker, because the initial impulse comes from the left ventricle. Further support for a biventricular pacemaker is a tall R wave in lead V1 (←). The QT/QTc intervals are prolonged (380/515 ms) but are normal when the prolonged QRS complex is considered (340/460 ms).
ECG B shows a regular rhythm at a rate of 90 bpm. There is a P wave before each QRS complex (+) with a constant PR interval (0.16 s). The P wave is positive in leads I, II, aVF, and V4 through V6. Hence, this is a normal sinus rhythm. The P wave and PR interval are the same as seen in ECG B. There is a pacemaker stimulus seen before each QRS complex (↑) and, hence, the pacing mode is A-sensed V-paced or P-wave synchronous ventricular pacing, similar to the pacing mode seen in ECG A. The QRS complex duration is increased (0.12 s) and the morphology is a right bundle-branch block with an RSR′ in V1 (→) and a terminal broad S wave in leads I and V5 through V6 (←). The QT/QTc intervals are normal (320/440 and 300/415 ms when the prolonged QRS complex duration is considered). The QRS complex morphology is not typical for either a right ventricular pacemaker (which should have a left bundle-branch block morphology) or a biventricular pacemaker (with a Q wave or QS complex in lead I as seen in ECG A). Therefore, the typical right bundle-branch block morphology is likely the morphology of the native QRS complex, and, hence, the ventricular stimulus is not capturing the ventricle. This represents pseudofusion. This occurs if the programmed atrioventricular delay of the pacemaker is the same as the intrinsic PR interval. In this situation, there will be impulse conduction through the normal atrioventricular node His-Purkinje system that will fuse with the impulse simultaneously produced by the ventricular lead. The third QRS complex (*) is premature, with a different morphology and without a preceding P wave. This is a ventricular premature complex. It is sensed by the pacemaker because there is pause following this complex.
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- © 2015 American Heart Association, Inc.