A Proposed Mechanism for the Formation of the “Dome-Dart” Left Atrial (LA) P Wave in the Electrocardiogram on Page 8 of the Journal of the Hoffman Heart Institute of Connecticut Volume 6 #1, September 2000 (Reference 1)
A Thousand Points of Light
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"Electrocardiography Brain Teaser"
(the Journal of the Hoffman Heart Institute of Connecticut Volume 6 #1)
By James Purcell, MD
April 22, 2008
With a LA premature beat initiated subsequent to a sinus beat the order of depolarization of both the left and right atria (RA) are reversed, with areas in both atria last depolarized by the sinus node beat being the first depolarized by the LA premature beat. Therefore, since the LA cells have not achieved full repolarization, only a slow rising action potential will occur manifest on the electrocardiogram as a flat or low amplitude “dome” (Ref. 1 and 2) constituting the LA part of the LA P wave.
Depolarization which produces action potentials that are very small continue into the RA through a small area in the anteroinferior part of the interatrial septum (call this Point A). This produces a low or non-detectable amplitude response on the surface electrocardiogram such as was seen in a case of hyperkalemia (Ref. 3). This undetectable depolarization of the RA then proceeds in all directions in the RA and goes into areas where the relative refractoriness is decreasing because these are areas which were depolarized before Point A and which are now beginning recovery. The farther the depolarization proceeds from Point A the more that repolarization will have occurred. In this electrocardiogram it is very probable that the terminal part of the LA depolarization overlaps the initial part of the RA depolarization such as seen in Ref. 4, Fig. 2 where in a canine experiment the high RA depolarization precedes the depolarization of the LA appendage during a LA echo beat. Therefore, by the time full repolarization has occurred in the as yet undepolarized areas of the RA this undetectable depolarization in the RA will have reached parts of all areas of the RA making it possible for the remaining undepolarized areas of the RA to essentially depolarize simultaneously. This produces a summation of atrial action potentials which will now be at their maximum causing a peak in voltage in the electrocardiogram such as seen in Lead V1. This is called the RA “dart.” The relative refractory period following LA P2 may be shortened, but the same gradient of relative refractoriness which existed prior to LA P2 will exist prior to LA P3. Since the sequence of depolarization in LA P3 is the same as LA P2, a “dome-dart” P wave is again produced. This perpetuates and causes the “dome-dart” P wave in LA P4.
James J. Purcell, M.D.
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