It is nearly half a century since Einthoven first employed the letters P, Q, R, S and T to designate the component deflections of the curves which he obtained by computing and eliminating the distortion present in records of the normal heart beat taken with the capillary electrometer. After he had invented the string galvanometer and was able to record the human electrocardiogram in undistorted form he continued to use these symbols and eventually added to their number by assigning the letter U to the low voltage deflection often present in early diastole and by accepting the designation Ta, previously employed by Hering, for the inconspicuous final component of the auricular complex.
This system of nomenclature has been in practically universal use since the very beginning, and it is permanently embedded in a vast and important literature, which all serious students of electrocardiography must frequently consult. In spite of
Barnes AR, Katz LN, Levine SA, Pardee HEB, White PD, Wilson FN. THE STANDARDIZATION OF ELECTROCARDIOGRAPHIC NOMENCLATURE: REPORT OF COMMITTEE OF THE AMERICAN HEART ASSOCIATION. JAMA. 1943;121(17):1347–1349. doi:10.1001/jama.1943.62840170006009
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