[Apex cardiography calibrated by an infinite-time constant. Technic and methodology].

A new apexcardiographic transducer system is presented. The device features infinite time constant, absolute pressure calibration and measurement of thorax coupling pressure (PO). The sensing element is an estensimetric bridge, mounted on the internal surface of a rigid membrane, coupled to the chest by a 4 x 7 mm plastic cylinder. The system is linear within the range of 0 to 1350 mmHg; the frequency response of the transducer is linear between 0 and 3500 Hz. The electronic filtering circuits allow simultaneous output of apexcardiogram, first apexcardiographic derivative and three phonocardiographic bands. Measurements carried out on 6 volunteers using the new apparatus and a traditional transducer system demonstrate that control of the coupling pressure is necessary not only for absolute amplitude, but also for accurate amplitude ratio measurements because the transmission of pressure signal across the thorax changes with different coupling pressures.