Apparatus and method for adapting a piezoelectric respiratory sensing belt to a respiratory inductance plethysmography polysomnograph
Circuits for rendering piezo-based respiratory belts compatible with polysomnograph (PSG) machines designed for use with respiratory induction belts (RIPs) comprise an instrumentation amplifier adapted to be connected to a piezoelectric transducer and providing an AC output signal to a low-pass filter. In a first embodiment, the low-pass filter output is applied to an input of a microcontroller's A to D converter and the resulting digitized samples are used to vary the resistance of a digital potentiometer whose wiper terminal is coupled in series with an inductor so as to emulate the presence of a RIP belt to the PSG machine. In a second embodiment, the low-pass filter output is used to drive the primary of a transformer so as to vary the permeability of the transformer's ferrite core in a way that emulates the performance of a RIP belt to the PSG.
1. A circuit interfacing a polyvinylidene fluoride (PVDF) piezoelectric transducer to a respiratory inductance (RIP) input to a polysomnograph machine comprising:
a) a PVDF transducer adapted to be applied to a patient for sensing respiratory activity and producing a voltage output signal proportional to the respiratory activity;
b) an instrumentation amplifier having an input coupled to receive said voltage output signal from the PVDF transducer and operative to reduce common mode noise in said voltage output signal, said instrumentation amplifier having an output feeding a low-pass filter stage; and
c) means coupled to an output from said low-pass filter stage for modulating an inductance element to produce an input signal compatible with a RIP machine.
2. The circuit of claim 1 wherein the PVDF transducer is affixed to an elastic belt adapted to surround the torso of the patient proximate at least one of his or her chest and abdomen.
3. The circuit of claim 1 wherein the instrumentation amplifier produces a gain in a range of from 2 to 10.
4. The circuit of claim 1 wherein the instrumentation amplifier produces a gain of about 6.4.
5. The circuit of claim 1 wherein the low-pass filter stage comprises a 3rd order Butterworth filter having a cut-off frequency in the range of from 0.5 Hz to 5 Hz.
6. The circuit of claim 1 wherein the means coupled to an output from the low-pass filter stage for modulating an inductance element comprises a programmable microcontroller having an analog-to-digital converter (ADC) that is coupled to receive the output from the low-pass filter stage and produce digital patterns representative of the amplitude of the output from the low-pass filter at discrete time intervals;
a digital potentiometer coupled to receive said digital patterns and producing a resistance variation in relation to the digital patterns being received; and
an inductor coupled to a wiper terminal of the digital potentiometer, said inductor having a nominal value corresponding to an inductance value presented to a RIP machine and changes from said nominal value being due to said resistance variations.
7. The circuit of claim 6 wherein the programmable microcontroller further comprises a look-up table for mapping ADC codes to said digital patterns.
8. The circuit of claim 6 and further including a fixed resistor serially connected between the wiper terminal and the inductor.
9. The circuit of claim 1 wherein the means coupled to an output from said low-pass filter stage for modulating an inductance element comprises: a first operational amplifier with its inverting input AC coupled to the low-pass filter output and an optional voltage divider coupled to the non-inverting input thereof and a second operational amplifier current driver connected to an output of the first operational amplifier and having an output connected to drive a primary winding of a ferrite core transformer and with a secondary winding of the transformer adapted for connection to RIP inputs of a PSG machine.
10. The circuit of claim 9 with said optional voltage divider removed.