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A transparent celluloid ruler measuring 15 by 4 cm. has been devised, which enables one to determine auricular and ventricular rates and PR and QRS intervals directly from electrocardiographic tracings. The method generally employed atpresent involves the use of dividers, counting time intervals and subsequent mathematical calculations, for the determination of these values, especially rates. This comparatively cumbersome procedure, at its best, often leads to errors.
The principle on which this ruler was devised is quite simple. The motor speed of most electrocardiograph machines is so adjusted that 2.5 cm. of film is exposed per second. Hence, a record is produced in which the large interval, representing 0.20 second, covers a distance of 0.5 cm., while the small division, representing 0.04 second, extends over a distance of 0.1 cm. On this basis one can easily calculate the space covered by a single cardiac cycle at any heart rate. These values for a selected number of cardiac rates have been calculated and, as seen in the accompanying illustration, vertical lines have been constructed at the corresponding distances from an abscissa at the left end of the rate scale.
The rate scale is employed in the following manner: To determine the
Wirtschafter ZT. A RULER FOR DETERMINATION OF ELECTROCARDIOGRAPHIC VALUES. JAMA. 1936;106(17):1466–1467. doi:10.1001/jama.1936.92770170001011
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