Conal contribution to the electrocardiogram of chick embryo hearts

Contraction of the conus arterisus (conus) is visibly delayed in the tubular heart of the chick embryo, and since the electromechanical coupling interval of isolated conal segment is similar to other areas of the embryonic heart, the delayed contraction of the conus indicated an impedance in the spread of the depolarization wave across the ventriculo-conal (VC) junction. A surface electrode placed on the conus of hearts obtained from the 72-hour chick embryo records a deflection, termed the C wave, appearing between the QRS complex and the T wave. To establish that the C wave arose from below the atrioventricular sulcus, atria were removed from a series of hearts from 96-hour embryos and the electrogram revealed a C wave without a P wave. The VC junction responds to digitoxin just as the atrioventricular tissue. Digitoxin prolongs the interval between the QRS and the C wave prior to a complete block in a manner analogous to the PR interval increase prior to the development of the Wenckeback phenomenon or complete atrioventricular block. As the embryonic heart developed after 96 hours, the C wave was no longer identifiable in the ECG.