ECG and polygraph

“The polygraph and electrocardiograph record fundamentally different signals; this difference may have accounted for much of the difficulty in moving perceptually from one to the other. The polygraph records the mechanical events produced by the beating heart as reflected in the pulse waves of the veins and arteries. It is an amplifier of the mechanical waves that can be seen in the veins of the neck or felt in the arteries of the neck or wrist. The polygraph does not change the type of signal which is being recorded; the mechanical motion of the blood vessel (artery or vein) is reflected – via a series of cups, tubes and levers – in the mechanical motion of the pen on the paper. The electrocardiograph, however, detects electrical signals produced by the heart. Unlike the polygraph, it does change the type of signal used. The electrocardiograph detects an electrical signal far too small to be appreciated by human senses. It changes this electrical signal generated by the heart into a mechanical signal which can be recorded, thus it acts as a transducer. In Einthoven’s experiments, first the capillary electrometer and later the string galvanometer (the key element of the electrocardiograph) acted as transducers, changing electrical signals into mechanical ones. [Thomas] Lewis, however, used the electrocardiograph methodologically as though it were an amplifier of palpable or visible pulse waves rather than a transducer of electrical signals” (Howell V, 94-95). . . . [Lewis and Mackenzie used the electrocardiograph as though it were a superior polygraph and thereby evinced] “an important misconception of the ECG. For, while one could feel the arterial pulses and see the venous pulses recorded by the polygraph, one could neither feel nor see the electrical currents recorded by the electrocardiography. The a, c, and v waves were within reach of the unaided senses and could be analyzed by the skilled observer without instrumental aids; the P, QRS and T waves were not. Without the electrocardiograph, no amount of training enabled the practitioner to determine the form of the T wave” (96).