Demonstration of electrical and anatomic connections between Marshall bundles and left atrium in dogs: implications on the generation of P waves on surface electrocardiogram.
BACKGROUND
The muscle bundles within the ligament of Marshall (LOM) are electrically active. The importance of these muscle bundles (Marshall bundle [MB]) to atrial activation and the generation of the ECG P wave is unclear.


RESULTS
We used optical mapping techniques to study epicardial activation patterns in isolated perfused left atrium in four dogs. In another seven dogs, P waves were studied before and after in vivo radiofrequency (RF) ablation of the connection between coronary sinus (CS) and the LOM. Computerized mapping was performed before and after RF ablation. Optical mapping studies showed that CS pacing resulted in broad wavefronts propagating from the middle and distal LOM directly to the adjacent left atrium (LA). Serial sections showed direct connection between MB and LA near the orifice of the left superior pulmonary vein in two dogs. In vivo studies showed that MB potentials were recorded in three dogs. After ablation, the duration of P waves remained unchanged. In the other four dogs, MB potentials were not recorded. Computerized mapping showed that LA wavefronts propagated to the MB region via LA-MB connection and then excited the CS. After ablation, the activation of CS muscle sleeves is delayed, and P wave duration increased from 65.3 +/- 14.9 msec to 70.5 +/- 17.2 msec (P = 0.025).


CONCLUSIONS
In about half of the normal dogs, MB provides an electrical conduit between LA free wall and CS. Severing MB alters the atrial activation and lengthens the P wave. MB contributes to generation of the P wave on surface ECG.
Omichi
Chikaya
C
Division of Cardiology, Department of Medicine, Cedars-Sinai Medical Center, Los Angeles, California 90048-1865, USA.
Chou
Chung-Chuan
CC
Lee
Moon-Hyoung
MH
Chang
Che-Ming
CM
Lai
Angela C
AC
Hayashi
Hideki
H
Zhou
Shengmei
S
Miyauchi
Yasushi
Y
Okuyama
Yuji
Y
Hamabe
Akira
A
Hwang
Chun
C
Fishbein
Michael C
MC
Lin
Shien-Fong
SF
Karagueuzian
Hrayr S
HS
Chen
Peng-Sheng
PS
D000818:Animals; D002304:Cardiac Pacing, Artificial; D003326:Coronary Circulation / Q000502:physiology; D004285:Dogs; D004562:Electrocardiography; D004594:Electrophysiology; D006321:Heart / Q000033:anatomy & histology* / Q000502:physiology*; D006325:Heart Atria; D006801:Humans; D055095:Optics and Photonics; D014680:Veins