Inductive plethysmographic transducers and electronic circuitry therefor

In an apparatus of the type comprising at least one inductance plethysmograph adapted for placement in encircling relation about a body portion of a subject, said inductive plethysmograph incorporating a conductor which comprises the inductance component of an LC oscillator circuit, the improvement comprising:

• said conductor having a plurality of turns whereby when said inductive plethysmograph is in encircling relation about said body portion said conductor encircles said body portion a plurality of times for improving the sensitivity of said apparatus,
• wherein said inductive plethysmograph comprises a stretchable substrate and wherein said conductor is arranged in a zigzag pattern on said stretchable substrate,
• wherein said inductive plethysmograph comprises two free ends joinable by mating connectors and further comprises lengths of conductor supported on said substrate with one end of each conductor length being joined to one connector and the other end being joined to the other connector, whereby when said connectors are joined said conductor having a plurality of turns is formed,
• wherein said conductor being electrically connected to said oscillator circuit through an impedance matching transformer with a secondary winding of the transformer being electrically connected to said conductor and a primary winding thereof being electrically connected to said oscillator circuit,
• wherein said conductor is electrically connected to said transformer via a connecting cable, said connecting cable being sufficiently long that said oscillator circuit and said transformer are disposed sufficiently remote from said plethysmograph to prevent said oscillator circuit and said transformer from contact with said plethysmograph or said subject when said subject is lying on said connecting cable, and
• wherein said transformer primary winding having an inductance greater than about ten times the reflected inductance of said conductor and said cable, whereby attenuation of changes int he inductance of said conductor and connecting cable is reduced.

In an apparatus of the type comprising at least one inductive plethysmograph for disposition about a subject and incorporating a conductor, said conductor comprising the inductance component of an LC oscillator circuit, said conductor being electrically connected to said oscillator circuit through an impedance matching transformer with a secondary winding of the transformer being connected to said conductor and a primary winding thereof being electrically connected to said oscillator circuit, the improvement comprising:

• said conductor being electrically connected to said transformer via a connecting cable comprising a coaxial cable comprising a center conductor and a screen, said connecting cable being sufficiently long that said oscillator circuit and said transformer are disposed sufficiently remote from said plethysmograph to prevent said oscillator circuit and said transformer from contact with said plethysmograph or said subject when said subject is lying on said connecting cable, wherein the ratio of a diameter of said screen to a diameter of said center conductor is minimized for reducing the inductance per unit length thereof, and said transformer primary winding having a first number of turns said secondary winding having a second number of turns, a ratio of the first number of turns to the second number of turns is selected so as to compensate for attenuation of changes in inductance of the conductor and the cable.