The cardiac arrhythmias of childhood are largely confined to those alterations in rhythm which are governed by the rate of stimulus production in the sino-atrial node. This function is stimulated or depressed by the influence of extracardiac nerves, the vagus and the sympathetic, which in the young are especially active. Of the arrhythmias arising in the sino-atrial node, the simple respiratory arrhythmia which characterizes Mackenzie's "youthful heart" is the most common. There is another type of sinus arrhythmia which is independent of the respiration, but which still has a peculiar phasic character. The cardiac cycles increase progressively in length until the rate is notably retarded, then become shorter until the more rapid rhythm is again established. The rhythm of the sinus node may be interrupted by excitation waves arising at some lower point in the primitive cardiac tissue and retrograde stimulation disturb the fundamental rhythm, or the focus of stimulus
BROWN NW. SINO-ATRIAL HEART BLOCK IN A CHILD: WITH OBSERVATIONS ON THE EFFECTS OF ATROPIN AND VAGUS STIMULATION. Arch Intern Med (Chic). 1919;24(4):458–470. doi:10.1001/archinte.1919.00090270104010
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