The first derivative thoracic impedance cardiogram

When an alternating current of high frequency is applied to the thorax, the first derivative of the impedance to this current is affected by the cardiac cycle resulting in a characteristic wave form. Phonocardiograms, electrocardiograms, and first derivative thoracic impedance cardiograms were recorded simultaneously in 91 subjects. The first derivative thoracic impedance cardiograms were found to have sharply demarcated points which occur synchronously with the first heart sound, aortic second sound, pulmonic second sound, mitral opening snap, third heart sound, and fourth heart sound. The first derivative thoracic impedance cardiogram may thus be used not only as a reference tracing to help identify heart sounds on the phonocardiogram, but also for directly timing the intervals within the cardiac cycle.