James J. Purcell, M.D.
View a special update to this article: "A Proposed Mechanism for the Formation of the “Dome-Dart” Left Atrial (LA) 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 (The latter results from depressed conduction in BB(5).) resulting from capture of the high LA via BB. Consistent with this idea is the small upright deflection in Lead I which occurs 0.08 seconds later in P19-P21 relative to its RA predecessor than the small upright deflection in P22. The latter upright deflection is due to R-L depolarization of the lft atrium which can be seen from the fact that it occurs. 06 seconds after the beginning of hte P22 in Lead 2 and also from the fac t that there is an increase in amplitude of the Pwave in Leads 2 and 3 in P22 compared with the Pwave with RALA in these same leads because it indicates high to low left atrial depolarization as a result of conduction through BB.
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 a 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. LA P waves may be difficult to see in surface leads(8) and this may account for the lack of a discernable LA P wave in P3. LA9 is probably the same as LA3, but RA9 is probably a RA fusion beat.
The P13-P18 interval is four times the P13-P14 interval. The transseptal time from premature LA P15 allows the Sino Atiral Node (SAN) junction to be depolarized in its relative rather than its absolute refractory period causing retrograde concealed conduction into the sino atrial node junction. This in turn causes a first degree antigrade conduction delay(9) to p-16 which is perpetuated(9) in the lengthening of the interval to P-17. However, shortening of the refractory period due to quickening of depolarization(10,11,12) produces a shorter SA interval to P17 than the SN16-P16 interval and also a shorter transseptal time in P17 (i.e., an earlier RALA in P17 as measured from the beginning of P17 to the RALA) than with other P waves having RALA. Since the P11-P13 interval is almost exactly two times the P10-P11 interval retrograde sinus node depolarization following A12 is unlikely while retrograde concealed conduction into the SAN junction as that following LA15 causing an antegrade exit block(13) of the next SAN discharge is the most likely possibility including that of SAN junction interference.
References
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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.
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