Physical Principles of the Displacement Cardiograph Including a New Device Sensitive to Variations in Torso Resistivity

The displacement cardiograph (DCG) is a noncontacting device which senses displacements associated with the cardiac cycle through a high-frequency electromagnetic field interaction between a sensing coil and the thorax. The coil is located in the tuning circuit of an oscillator, and perturbations in the oscillation frequency, i.e., frequency quency modulation (FM), resulting from tissue displacements are monitored. The device, originally described by Vas, is similar to earlier devices reported by others. Separation of capacitive and inductive effects with a capacitive shield indicates that the principal interaction is through capacitance, rather than inductance. Experimental and theoretical results suggest that the DCG is relatively insensitive to internal movements as compared to displacements of the precordium. A new technique for monitoring cardiac activity is reported which utilizes the amplitude modulation (AM) of the oscillator. With capacitive shielding, we have successfully recorded waveforms which result from the inductively coupled, reflected resistance in the coil. This AM device does respond significantly to internal movements.