The Inter-auricular Time Interval

In 1910, Wybauw (1) and Lewis (2) working independently, demonstrated that in the mammalian heart, the point of initial excitation corresponds to the location of the sino-auricular node. This finding has been confirmed by Eyster and Meek (3) and others. Although Erlanger (4) and his co-workers have placed a different interpretation on the records of the above named investigators, the view is generally held that the normal seat of impulse formation, is the sino-auricular node and more particularly the head of this node. Granting that this view is essentially correct, the excitatory process is then propagated from this area by muscle tissue to all parts of the heart. If the rate of propagation through auricular muscle is the same throughout, it may be expected that parts at a sufficiently greater distance from the sinoauricular node than nearby parts, would show an appreciable time difference in the onset of their contraction. Thus the left auricle, though in direct and common muscular connection with the right auricle, may be expected to contract later than the right. It is generally taught, however, that the contractions of the two auricles are synchronous. Yet Chauveau (5), in 1894, stated that “in the horse, the right auricle begins its systole an appreciable time before that of the left auricle.” Later Fredericq (6) took up the question and demonstrated the same fact in the dog’s heart. Fredericq’s pupils-Schmidt-Nielsen (7)) Stassen @)-confirmed his findings. The correctness of this statement has, however, been disputed by Hirschfelder and Eyster (9) who attribute Fredericq’s results to the use of excised hearts; they believe that the heart in situ maintained under normal conditions, does not show any time interval in its auricular contractions and that such time