br Materials and methods br Results Shortening
Materials and methods
Results Shortening of the QRS duration in 7 and lengthening of the QRS duration in 3 of the 24 patients were observed during RV pacing at a mean pacing corticotropin releasing factor length of 346±37ms and 258.3±14.4ms (Fig. 1; Tables 1 and 2). The mean shortening of the QRS duration was −12±3ms, and the mean lengthening of the QRS duration was 13±3ms (Tables 1 and 2). Patient 1 demonstrated shortening in QRS duration by 20ms at a pacing CL of 350ms and a diastolic interval of 123ms (Fig. 1). Fig. 2 shows a tracing from a patient in whom conducted programmed stimulation was performed, demonstrating shortening of the QRS duration by 20ms, and shortening of the stimulus to left ventricular bipolar electrogram conduction time by 21ms, observed at a diastolic interval of 137ms. However, the QRS duration and stimulus to left ventricular electrogram conduction time were increased by 29 and 24ms, respectively, at a diastolic interval of 0ms (premature stimulation occurred before 90% repolarization of the preceding MAP; Fig. 2). Shortening of the QRS duration was observed at a mean diastolic interval of 110±9ms, which corresponded to the mean MAP duration [(S1S1 interval − diastolic interval)/S1S1 interval] of 68.5±2.8%. QRS duration did not increase in patients 1, 6, 7, 8, and 9, and increased by 6ms in patients 2 and 10, but none of the 7 patients showed an increase≥10ms at shorter pacing cycle lengths.
Discussion We showed that shortening of the QRS duration occurred at a pacing cycle length ≤400ms in 29% of the patients and that shortening of the QRS duration and stimulus to left ventricular conduction time occurred in one patient in whom programmed ventricular stimulation was performed. Supernormal conduction is defined as conduction that is more rapid than expected or as the presence of conduction when block is anticipated. True supernormal conduction may be defined as a shortening of the conduction time in premature beats as compared with the regular conduction of later stimuli in the normal heart. This decrease in conduction time might be caused by the period of supernormal excitability. A previous study of the intact canine heart by Puech et al.  showed that supernormal conduction in the RV, measured from pacing spike to the onset of the MAP, occurred at an S1S2/S1S1 ratio between 66.1±0.9% and 77.9±8.2%. Our finding that MAP duration/S1S1 interval was 68.5±2.8% is consistent with that of their study. In contrast, another study reported that intraventricular conduction time and the major axis dimension of the left ventricle were shortened to a similar magnitude and at a similar time in the cardiac cycle. Therefore, shortening of intraventricular conduction time might result, at least in part, from a shortened dimensional pathway length from the site of the stimulating impulse propagation to the recording electrodes . However, Puech et al . showed “supernormal conduction” at the RV epicardium, but we found that only 29% of the patients exhibited shortening of the paced QRS duration during rapid ventricular pacing. Therefore, further studies are needed to clarify the association between true “supernormal conduction” and shortening of the conduction distance in the shortening of the QRS duration in the intact heart.
Conflict of interest
Introduction Long RP tachycardia is an uncommon form of supraventricular tachycardia (SVT) that exhibits a longer RP interval than PR interval on 12-lead electrocardiograms (ECG). The differential diagnosis of a long RP tachycardia includes the uncommon form of AV nodal reentrant tachycardia (AVNRT), the permanent form of reciprocating tachycardia (PJRT), and atrial tachycardia (AT). AT accounts for majority of long RP tachycardia cases [1,2]. In addition, although there are some P-wave features associated with differential diagnosis, we are unaware of studies giving clear numeric guidelines that can differentiate AT from other tachycardias. Therefore, the aim of the present study was to investigate the incidence and electrocardiographic features of long RP tachycardias.