Determining respiratory rate via impedance pneumography
A method and system for determining a respiratory rate of a user are disclosed. The method comprises measuring a differential voltage across first and second electrodes of a sensor device coupled to the user. The method includes sampling the differential voltage using an analog-to-digital converter to produce an output signal. The method includes processing the output signal to detect a breath of the user based on a positive voltage transition through a midpoint, wherein the breath of the user is utilized to determine the respiratory rate of the user.
1. A method for continuous monitoring of a respiratory rate of a user using a wearable sensor device, wherein the wearable sensor device comprises a memory, a processing unit coupled to the memory and executing instructions stored therein, an analog-to-digital converter coupled to the memory, a transmitter coupled to the memory and first and second electrodes coupled to the analog-to-digital converter and the method comprising:
measuring a differential voltage across the first and second electrodes of the wearable sensor device coupled to the user, wherein a voltage signal of known impedance is used as an input signal;
sampling the measured differential voltage using the analog-to-digital converter to produce an output signal; and
detecting a breath of the user, wherein detecting the breath comprises:
determining a local maximum using the output signal,
determining transitioning of the samples from below a midpoint value to above the midpoint value using the output signal, when the local maximum of the output signal is above a midpoint between maximum value of previous breath and minimum value of previous breath, wherein a positive voltage transition through the midpoint indicates the breath of the user, and
comparing the local maximum with a predetermined threshold when the local maximum of the output signal is below the midpoint between the previous breath's maximum and minimum value, wherein the local maximum above the predetermined threshold indicates the breath of the user;
wherein the breath of the user is utilized to determine the respiratory rate of the user, which is transmitted to another user or device by the transmitter.
2. The method of claim 1 , further comprising:
applying a voltage signal to the first and the second electrodes, wherein the voltage signal is a high frequency square wave or a pulse signal with rise and fall times such that an impedance of the first and second electrodes is lower than a body impedance.
3. The method of claim 1 , wherein the sampling further comprises:
sampling peaks of the differential voltage; and
downsampling the sampled peaks to provide a voltage sample (a n ) as the output signal.
4. The method of claim 3 , wherein the peaks of the differential voltage are sampled at 1 mega sample per second (MS/s) and the sampled peaks are downsampled to 12.5 samples per second.
5. The method of claim 1 , wherein the processing further comprises:
applying a first filter to the output signal to remove any of noise and artifacts thereby producing a second output signal (a p,n ); and
applying a second filter to the second output signal (a p,n ) to compute an L-tap moving average of the second output signal thereby producing a third output signal (b n ).
6. The method of claim 5 , wherein the first filter is a second order Butterworth filter with a passband from 0.06 Hertz (Hz) to 1.1 Hz and the second filter is a finite impulse response (FIR) filter where L=6.
7. The method of claim 5 , wherein the midpoint is a midpoint between maximum (b n,max ) and minimum (b n,min ) values of the third output signal (b n ).
8. The method of claim 7 , wherein the minimum value (b n,min ) is set to b n-1-N when the following conditions are satisfied:
( b n-N )−( b n-1-N) >hysteresis threshold (tau);
( b n-2-N )−( b n-1-N )>hysteresis threshold (tau); and
b n-1-N <b n,min .
9. The method of claim 7 , wherein the maximum value (b n,max ) is set to b n-1-N when the following conditions are satisfied:
( b n-1-N )−( b n-N )>hysteresis threshold (tau);
( b n-1-N )−( b n-2-N )>hysteresis threshold (tau); and
b n-1-N >b n,max .
10. The method of claim 9 , further comprising:
waiting a predetermined time period (N) before calculating the positive voltage transition through the midpoint, wherein the breath of the user is detected when the following conditions are satisfied:
( b n-1-N )−( b n-2-N )>hysteresis threshold (tau);
b n,max >b n,min ;
b n-N >( b n,max +b n,min )/2; and
b n-1-N >( b n,max +b n,min )/2.
11. The method of claim 5 , further comprising:
calculating breath depth by using minimum (a p,n,min ) and maximum (a a,p,max ) values of the second output signal (a p,n ); and
calculating breath duration by comparing time values of the second output signal (a p,n for breath m 2 ) and a preceding second output signal (a p,n for breath m 1 ).
12. A system for continuous monitoring a respiratory rate of a user using a wearable sensor device, the system comprising:
the wearable sensor device comprising a memory, a processing unit coupled to the memory and executing instructions stored therein, a transmitter coupled to the memory and first and second electrodes coupled to the user, wherein a voltage signal of known impedance is used as an input signal, and wherein the wearable sensor device measures a differential voltage across the first and the second electrodes; and
an analog-to-digital converter coupled to the first and second electrodes of the wearable sensor device, wherein the analog-to-digital converter samples the differential voltage measured by the first and second electrodes to produce an output signal;
wherein the processing unit coupled to the wearable sensor device processes the output signal to detect a breath of the user by executing instructions comprising:
determine a local maximum using the output signal,
determine transitioning of the samples from below a midpoint value to above the midpoint value using the output signal, when the local maximum of the output signal is above a midpoint between maximum value of previous breath and minimum value of previous breath, wherein a positive voltage transition through the midpoint indicates the breath of the user; and
compare the local maximum with a predetermined threshold when the local maximum of the output signal is below the midpoint between the previous breath's maximum and minimum value, wherein the local maximum above the predetermined threshold indicates the breath of the user;
wherein the breath of the user is utilized to determine the respiratory rate of the user, which is transmitted to another user or device by the transmitter.
13. The system of claim 12 , further comprising:
a voltage generator coupled to the wearable sensor device, wherein the voltage generator applies a voltage signal to the first and the second electrodes, wherein the voltage signal is a high frequency square wave or a pulse signal with rise and fall times such that an impedance of the first and second electrodes is lower than a body impedance.
14. The system of claim 12 , wherein the analog-to-digital converter further:
samples peaks of the differential voltage; and
downsamples the sampled peaks to provide a voltage sample (an) as the output signal.
15. The system of claim 14 , wherein the peaks of the differential voltage are sampled at 1 mega sample per second (MS/s) and the sampled peaks are downsampled to 12.5 samples per second.
16. The system of claim 12 , wherein the wearable sensor device further:
applies a first filter to the output signal to remove any of noise and artifacts which produces a second output signal (a p,n ); and
applies a second filter to the second output signal (a p,n ) to compute an L-tap moving average of the second output signal which produces a third output signal (b n ).
17. The system of claim 16 , wherein the midpoint is a midpoint between maximum (b n,max ) and minimum (b n,min ) values of the third output signal (b n ).
18. The system of claim 17 , wherein the wearable sensor device further:
waits a predetermined time period (N) before calculating the positive voltage transition through the midpoint, wherein the breath of the user is detected when the following conditions are satisfied:
( b n-1-N )−( b n-2-N )>hysteresis threshold (tau);
b n,max >b n,min ;
b n-N >( b n,max +b n,min )/2; and
b n-1-N >( b n,max +b n,min )/2.