Electrocardiography Brain Teaser

Intermittent Bachmann's Bundle (BB) Block; An Interatrial Conduction Defect (IACD) Associated with Both Sinus and Dome Dart Left Atrial (LA) P Wave; Sinoatrial Block (S-A Block)

The Journal of The Hoffman Heart September 2000
by James J. Purcell, MD

Special updates to this article:
"A Proposed Mechanism for the Formation of the “Dome-Dart” Left Atrial (LA) P Wave"  
The Rapunzel Solution: On the Formation of the Dome-Dart Left Atrial P-wave

Sinus P waves such as P1, P6, P7 and P19-P21 are 0.16 seconds in duration with an IACD manifesting two negative deflections in V1 separated by 0.08 seconds and a terminal negative deflection in Leads 2, 3, and aVF. The latter is due to retrograde activation of the LA (RALA)(1,2,3) and was something that could not have occurred unless there was either a total block(2) or a significantly greater degree of conduction delay in the path to the high left atrium(4), (BB), than there was in the path to the low LA, the part of the LA normally depolarized last(4). P11 and P22, which share the same contour in V1 and which occur at the expected time of the next sinus P wave are probably sinus P waves. These have almost the same contour in V1 for the first 0.08 seconds as sinus P waves that have RALA (because this represents mainly RA depolarization in P11 and P22). P11 and P22 are, however, shorter measuring 0.10 seconds because of the absence of RALA resulting from capture of the high LA via BB. Supporting this idea of normal conduction in BB in P22 is the increase in amplitude of the P wave in Leads 2 and 3 in P22 compared to P waves with RALA in these same leads in the preceding few beats and from the fact that P22 comes at the expected time of the next sinus node depolarization. Therfore, P22 is a sinus Pwave.  

Since the second atrium in the atrial conduction sequence starts to be depolarized at the interatrial septum the direction of conduction through this atrial chamber can therefore be used to determine on which side of the interatrial septum the second atrium lies. The small upright deflection seen in Lead I in P19-P21 coincident with the nadir of the RALA indicates right-to-left depolarization of the LA during RALA, confirming situs solitus.  

P4 and P12 have in addition to the dome dart configuration, which is considered definitive for an LA origin(6,7), an IACD with P4 being about 0.14 seconds. Confirmation that P2, P3, and P4 are LA P wave comes from the negative deflections in Lead 1 coincident with the peaks of the dart part of those P waves indicating a left-to-right depolarization of the RA, because in this case which is without a significant intra-atrial conduction delay(3) as can be seen from the normal duration of P22 but which is situs solitus left-to-right conduction through the entire second atrium in the atrial depolarization sequence identifies the latter chamber as the RA and the rhythm as left atrial. 

P9 occurs at the expected time of the next sinus P-wave, however, the P9-R9 interval is too short to conduct R9 as can be seen from the PR intervals of the immediately preceding sinus P-wave driven beats (with RALA). However, the P9-R9 interval is much the same as the dart-R intervals of LA P-waves 2,3,4 indicating that P9 is probably a RA fusion of a non conducted sinus P-wave and a LA P-wave. No junctional P-waves are seen in the EKG. 

The P13-P18 interval is four times the P13-P14 interval. LA P15 produces retrograde concealed conduction into the S-A junction causing first degree antegrade S-A conduction delay(9) of P16 which is perpetuated(9) in the SN17-P17 interval. Since the P11-P13 interval is almost exactly two times the P10-P11 interval retrograde sinus node (SN) depolarization following LA12 is unlikely while retrograde concealed conduction into the S-A junction as that following LA15 causing an antegrade exit block(13) of the next SN discharge is the most likely possibility including that of S-A interference dissociation.  

Click to view enlarged image of EKG


1. Scherf D, Cohen J. The atrioventricular node and selected cardiac arrhythmias. Grune and Stratton, New York, 1964:329.  

2. Bayes de Luna A, Cladellas M, Oter R, Torner P, et al. Interatrial conduction block and retrograde activation of the left atrium and paroxysmal supraventricular tachyarrhythmia. Eur Heart J 1988;9:1112-1118.  

3. Soejima K, Mitamura H, Miyazaki T, Akaishi M, et al. A case of widely split double P waves with marked intra-atrial conduction delay. J Cardiovasc Electrophysiol 1997;8:1296-1301.  

4. Willems JL, Robles de Medina EO, Bernard R, Coumel P, et al. Criteria for intraventricular conduction disturbances and pre-excitation. World Health Organizational/International Society and Federation for Cardiology Task Force Ad Hoc. J Am Coll Cardiol 1985;6:1261-75.  

5. Ogawa S, Dreifus LS, Osmick MJ. Longitudinal dissociation of Bachmann's bundle as a mechanism of paroxysmal supraventricular tachycardia. Am J Cardiol 1977;40:915-22.  

6. Harris BC, Shaver JA, Gray S 3d, Kroetz FW, Leonard JJ. Left atrial rhythm. Experimental production in man. Circulation 1968;37:1000-14.  

7. Beder SD, Gillette PC, Garson A Jr, McNamara DG. Clinical confirmation of ECG criteria for left atrial rhythm. Am Heart J 1982;103:848-52.  

8. Massumi RA, Sarin RK, Tawakkol AA, Rios JC, Jackson H. Time sequence of right and left atrial depolarization as a guide to the origin of the P waves. Am J Cardiol 1969;24:28-36.  

9. Purcell JJ. Junctional tachycardia. Circulation 1973;48:674-5  

10. Katz LN, Pick A. Clinical Electrocardiography. I The Arrhythmias. Lea and Febiger, Philadelphia 1956:532.  

11. Katz LN, Pick A. Clinical Electrocardiography. I The Arrhythmias. Lea and Febiger, Philadelphia 1956:245 (fig 124).  

12. Katz LN, Pick A. Clinical Electrocardiography. I The Arrhythmias. Lea and Febiger, Philadelphia 1956:318 (fig 171, Lead 1).  

13. Katz LN, Pick A. Clinical Electrocardiography. I The Arrhythmias. Lea and Febiger, Philadelphia 1956:662. 

Below is a link to a letter to the editor of the Journal Chest by Dr. Purcell which appeared in the September 2002 Issue:
Complete Interatrial Block Revisited