Trigeminal Rhythm during Ventricular Pacing in a Patient with Ventricular Preexcitation: What Is the Mechanism?

Abstract

Case PresentationA 52-year-old woman presented to the outpatient clinic complaining of recurrent brief episodes of chest pain without other symptoms. Her previous medical history was unremarkable and physical examination was normal. Her resting electrocardiogram (ECG) showed sinus rhythm with ventricular preexcitation that proved to be persistent during a 24-hour Holter monitoring and on a treadmill stress test (maximum heart rate 155 beats/min). The echocardiogram ruled out structural heart disease.Given atypical symptoms, manifested preexcitation that did not disappear with exercise and accessible pathway location; an electrophysiology study was performed. The baseline ECG (Fig. 1) showed ventricular preexcitation consistent with a left-sided accessory pathway (AP). During ventricular pacing at a cycle length of 550 ms a persistent trigeminal rhythm was observed (Fig. 2). What's the mechanism? DiscussionThe ECG depicted in Figure 2 shows two paced ventricular beats followed by an anticipated wide QRS complex, with the third pacing stimulus falling in the ventricular refractory period (RP). It is also apparent that the first two paced ventricular beats are followed by P waves, with the first one falling in the peak of the T wave (compare with the T wave of the second beat) and the second one after the end of the T wave. The P waves have different morphologic features, suggesting activation of the atrium along two different retrograde routes. In fact, the first P wave (with a short V-P interval) is negative in lead I (compared to the second paced beat), indicating a left to right atrial activation, while the second P wave (with a longer V-P interval) is isoelectric in this lead and deeply negative in leads II and III, compatible with direct caudocraneal atrial activation. Thus, it is plausible that the first paced ventricular impulse proceeds retrogradely along the left-sided AP, while the second paced ventricular beat is blocked in the AP and slowly conducted to the atria along the atrioventricular (AV) node. Then, it may invade the AP in anterograde direction, giving rise to an antidromic reentrant beat, which shows a higher degree of ventricular preexcitation than the observed during sinus rhythm.The diagnosis of antidromic echoes induced in a trigeminal pattern is confirmed by intracardiac recordings (Fig. 3), which show that the first two of each group of three stimuli capture the ventricles and propagate to the atria. The retrograde ventriculoatrial conduction after the first paced beat occurs rapidly via the left-sided AP and is eccentric. The second paced ventricular beat finds the AP refractory and proceeds slowly along the AV node (note concentric atrial activation) to depolarize the atria and then the AP anterogradely causing a fully preexcited QRS complex.The anterograde RP of the AP was 270 ms and AV reentry tachycardia could not be induced, however, atrial programmed stimulation induced a self-limited episode of AF accompanied by retrosternal discomfort and with some preexcited R-R intervals ࣘ230 ms...