Spread of activity through the atrioventricular node.

TRAXSMEMBRANE recording from single fibers of the atrioventricular (A-V) node of the rabbit heart has shown action potentials exhibiting rising phases with a much slower time course than those of plain heart muscle.' It has also been shown that the area in which such nodal potentials are recorded is not sharply delimited, regions being found at each side of the nodal area in which action potential shapes change gradually to those of the atrium or His bundle. An important fraction of the A-V delay was ascribed to a narrow region of the node where slowing of the action potential upstroke and of the propagation process are most conspicuous. DcterTiiinations of apparent propagation velocity in this area showed values as low as 0.02 M./sec., a figure in good agreement with that found subsequently by Scher and his colleagues, using a different technic. On the basis of these experimental facts, the conclusion has been reached that delay in the excitation of the His bundle is most probably explained by a low propagation velocity within the A-V node which has tentatively been attributed to decremental conduction' (see page 806). However, more precise data on the sequence of activation of the A-V node were needed before a further understanding of the mechanism of delay could be reached.