AN INK-WRITING CARDIOCHRONOGRAPH FOR THE STUDY OF THE ACTIVITY OF THE HUMAN AUTONOMIC NERVOUS SYSTEM

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Abstract
A study of the fluctuations of cardiac period from beat to beat gives information as to the activity of the vagus and sympathetic nerves which control heart rate. The instrument uses the electrocardiac voltages (Q.R.S. complex) between right arm and left leg, with the left arm grounded to the instrument. The out-of-phase voltages are amplified first by a differential amplifier, of high discrimination, then by a two stage pulse amplifier. The output is further amplified and distorted by a 'pulse-shaping' circuit to give a positive-going triggering impulse. This impulse, by 'striking' the thyratron of a 'sweep generator', causes the discharge of a condenser which has been slowly charging at a nearly constant rate. A recording milliammeter, of the ink-writing type, is connected to the condenser through a 'sweep amplifier' so that it draws a series of transverse lines on the record, the length of each being determined by the duration of the cardiac period before the next triggering impulse arrives. Over a long period of time, every successive cardiac period is recorded in a comparatively short record. Results have already shown the astonishing variability of the normal cardiac rhythm.

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