Distortion of non-invasive cardiac pulse curves. A capillary-damped pick-up and a calibration unit forapex cardiograms and other pulse curves.

A calibration unit and a capillary-damped pick-up, of funnel type, intended for apex cardiograms and other non-invasive cardiac pulse curves are presented. The physical properties of a recording system, suitable for non-invasive cardiac diagnostic techniques, are reviewed. In 4 identical recording units consisting of a funnel with and without capillary damping, connected via air transmission to a transducer, the overamplification in the underdamped systems varied between 2-4 and 4-3, and in the capillary-damped systems between 1-1 and 1-2. The capillary damping gave an approximately linear relation between phase shift and frequency in the high frequency range with a calculated delay of about 4ms. The low frequency time constant in the systems varied between 1-9 and 4-6 s, depending on the amplifier setting used, which for sine curves of frequency 0-5 and 2 Hz means an apparent prematurity for their maxima and minima varying between 53 and 1 ms, this being larger with lower frequency time constants and lower frequency curves.