WO2006057010A1

WO2006057010A1 - Method and device for measuring the respiratory rate of a mammal

Info

Publication number: WO2006057010A1
Application number: PCT/IS2005/000018
Authority: WO
Inventors: Bjorgvin Gudmundsson; Sveinbjorn Hoskuldsson; Birgir Thoroddsen; Kari Davidsson
Original assignee: Medcare Ehf
Priority date: 2004-09-02
Filing date: 2005-09-02
Publication date: 2006-06-01

Abstract

A device and a method for measuring the respiratory rate of a mammal, in particular a human, using Respiratory Inductive Plethysmograpb. (RIP) techniques. A wire loop is positioned around a body part of the mammal (the torso of a human) and forms a resonance circuit in combination with a capacitor. Changes in inductance of the loop relect changes in loop area and thereb changes in the circumference and volume of the body part . The changes in inductance are measured by means of measuring the resonance f requency . The invention uses the combination of an LF oscillator (1) with low power comumption and a HF oscillator (2) with high resolution . The HF oscillator is only turned on for a brief section of the sample period, thereby saving power while maintaining high resolution . A RIP device of the invention can be made waterproof by encapsulating belt and electronics using conventional moulding or over-moulding techniques.

Description

METHOD AND DEVICE FOR MEASURING THE RESPIRATORY RATE OF A MAMMAL

FIELD OF INVENTION

The present invention relates to a new method and device for measuring and processing plethysmography signals, in particular for measuring respiration movement and volume changes of a mammal, in particular a human, with Respiratory Inductive Plethysmography (RIP) technology.

TECHNICAL BACKGROUND AND PRIOR ART

Respiratory Inductive Plethysmography (RIP) technology is based on estimating the cross- sectional area of a wire loop by measuring its inductance. If the wire loop surrounds a body part of a patient, the measurement of the loop area is also the measurement of the cross- sectional area of the body part. Measuring the cross-sectional area of a person's thorax and abdomen gives a good indication of the volume changes of the body as the person inhales or exhales and thereby provides an indirect measurement of the person's respiratory effort and respiratory flow.

The inductance of a simple wire loop is uniformly proportional to the area of the loop.

Estimation of the loop area can therefore be performed by measuring the loop inductance. The conventional way to do so is to form a resonance circuit and measure a resonance frequency of the circuit. This is done by forming an LC-circuit, where L is the inductance of the loop and C is a fixed capacitor value. The resonance frequency can then by calculated as:

To measure the resonance frequency an oscillation circuit is typically used. Such circuitry uses feedback to maintain oscillation in the resonance circuit. The goal is to measure the frequency that appears in the resonance circuit, as it is an indirect measurement of the loop area.

The resonance frequency of a plethysmography wire loop varies as the person breaths since this causes the loop area to change. The electrical signal that appears is therefore a conventional FM signal. Since the changes in the loop area are relatively small, the frequency of the resonant circuit changes as little as 0.1 - 0.5 % as a person breaths under normal respiratory effort. In order to detect these changes the resolution of the frequency measurement must be as high as 1/100,000 to obtain a signal-to-noise ratio of 100:1. Furthermore, a good respiratory signal must have at least 20 samples per second, and it is therefore necessary to use a high stability oscillator operating at 2 MHz as reference. However, such a high frequency oscillator has relatively high power consumption. This is a disadvantage, in particular if it is desired to power the device using a battery rather than by using power supplied by a power grid.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a device and a method for measuring the respiratory rate of a mammal, in particular a human, using the RIP technology, wherein the power consumption is reduced as compared to prior art devices and methods, and wherein a high resolution is maintained.

According to a first aspect of the present invention the above and other objects are fulfilled by providing a circuit for measuring the respiratory rate of a mammal, the circuit comprising:

- a wire loop being adapted to be positioned around a body part of a mammal,

- a capacitor, said capacitor and said wire loop in combination forming a resonance circuit, and

- means for determining a resonance frequency of the resonance circuit, said determining means comprising: <

- a low frequency (LF) oscillator,

- a high frequency (HF) oscillator being adapted to be switched between an on- state and an off-state,

- means for determining a number of periods of the LF oscillator and a number of periods of the HF oscillator, where the periods of the LF oscillator, preceded and/or succeeded by the periods of the HF oscillator, correspond to the time duration of a number of periods of the resonance circuit, and

- means for determining the resonance frequency of the resonance circuit based on the determined numbers of periods of the LF oscillator and the HF oscillator. According to a second aspect of the present invention the above and other objects are fulfilled by providing a method for measuring the respiratory rate of a mammal, the method comprising the steps of:

- positioning a wire loop around a body part of a mammal, said wire forming a resonance circuit in combination with a capacitor, and

- determining a resonance frequency of the resonance circuit, said determination comprising the steps of:

- providing a low frequency (LF) oscillator,

- providing a high frequency (HF) oscillator being adapted to be switched between an on-state and an off-state,

- determining a number of periods of the LF oscillator and a number of periods of the HF oscillator, where the periods of the LF oscillator, preceded and/or succeeded by the periods of the HF oscillator, correspond to the time duration of a number of periods of the resonance circuit, and

- determining the resonance frequency of the resonance circuit based on the determined numbers of periods of the LF oscillator and the HF oscillator.

The mammal is preferably a human, but can alternatively be any other mammal of which it is desirable to determine the respiratory rate. The mammal may thus, e.g., be a pet, such as a dog, a cat, etc.

The body part is generally a body part which is related to a respiratory movement, typically the chest region or abdominal region of the torso. When the wire loop is positioned around such a body part, changes in the loop area will reflect changes in a cross-sectional area of the body part, and will therefore provide a measure for the respiratory rate of the mammal.

The LF oscillator is preferably a very stable and accurate oscillator having an oscillating frequency of e.g. approximately 20 samples per second. It may advantageously be a crystal based oscillator, e.g. a 32768 Hz crystal with an appropriate divider.

The HF oscillator preferably outputs a 1-5 MHz square wave. Preferably, the on-state is a state in which the HF oscillator is operating normally and the off-state is a state in which the power to the HF oscillator is completely switched off. Alternatively, the off-state may be a state in which the power to the HF oscillator is substantially reduced though not completely switched off, i.e. a sort of 'idle state'.

Preferably, the frequency of the LF oscillator is substantially lower than the frequency of the resonance circuit and the frequency of the HF oscillator is substantially higher than the frequency of the resonance circuit.

The present invention can be explained as follows. The key to measuring the RIP signal is to accurately measure the oscillation frequency of the wire loop. The key to doing that, using microprocessors, is to measure the period time of the signal, i.e. the time of one or more waves of the oscillating signal. The conventional way to do this is to monitor two frequencies; one of which is the frequency to be measured and the other is a known frequency, used as reference. To get high accuracy in the frequency measurement, the reference frequency must be high as the time resolution is directly proportional to the frequency. If the reference frequency is for example 5 MHz, one period of it is 1/5 NlHz = 200 ns, and this would be the time resolution of the period measured.

The problem with using such a high frequency is that the power consumption of the frequency generator is also directly proportional to the frequency. High reference frequency therefore means high power consumption.

The design according to the present invention solves this problem and makes it possible to measure the periods with high accuracy and at the same time low power consumption, therefore making the usage of battery driven RIP equipment much more feasible.

The idea is that there is no need to drive the high frequency reference all the time to get the high resolution. It is only required to know with high accuracy the start and/or stop time of the measured period. There is no need for the same resolution during the period time. This can be explained by imaging the measurement of the speed of a car driving a circle with a circumference of 100 km. To gain the best measurement, one needs to know the time when it crosses the start line with very high accuracy and also the time when it crosses again the line after finishingthe loop, for example using a fast clock with 1000 tics per second. Between those two events, it is however sufficient to use a slow clock e.g. with only 1 tick per second, as long as it is accurate. We would measure with the fast clock the time from when the car crosses the start line until the next tick of the slow clock occurs. Then the fast clock can be stopped until we see the car again to save power. When the car is in sight the fast clock is started again at the next tick of the slow clock and the fast clock is run until the car crosses the finishing line. The time resolution of this measurement is therefore the same as if the fast clock had been used all the time but the power consumption is considerably less. The trick is therefore to use a low-frequency reference in the middle of the period to keep track of time but start the high-frequency oscillator only at the beginning and at the end of the period to determine the timing of these events with high precision.

Thus, due to the fact that the HF oscillator is adapted to be switched between the on-state and the off-state, and due to the combined use of an LF oscillator with a low power consumption and a HF oscillator providing a high resolution measurement, it is possible to obtain a measurement with a relatively low power consumption while maintaining a relatively high resolution. As described above this is particularly advantageous when the device is powered by a battery, because it greatly reduces the rate at which it is necessary to change the battery, thereby avoiding the environmental and economical disadvantages involved in frequent battery changes.

The circuit may comprise means for calculating a loop area of the wire loop on the basis of the determined resonance frequency.

The circuit may further comprise means for switching the HF oscillator between the on-state and the off-state and means for controlling the switching means on the basis of a signal indicative of the beginning and/or ending of a period of the resonance circuit and/or on the basis of a signal indicative of the beginning and/or ending of a period of the LF oscillator.

In the present context the terms 'beginning of a period' and 'ending of a period' should be interpreted as a specific position in the period of the relevant signal where it has been decided that a period begins/ends. It could be any position in the period, such as a position of maximum or minimum signal (e.g. in case of a sinusoidal signal), or a position in which the signal value passes a certain level. In any case, once the position is defined, a period of a signal runs from this position until the next position in which the same conditions are fulfilled.

The beginning of a period may, thus, be defined as the position of a rising or falling edge in the corresponding signal or a position in which an output value of the corresponding signal exceeds or falls below a predefined value.

In one embodiment the control means may be adapted to control the switching means to switch the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the LF oscillator, and to control the switching means to switch the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the resonance circuit. In this embodiment the HF oscillator is turned on when a period of the LF oscillator begins and is switched off when the first succeeding period of the resonance circuit begins. When the next succeeding period of the LF oscillator begins the HF oscillator is once again switched on, etc. Thus, the HF oscillator will only be in the on-state during a fraction of a period of the resonance circuit in each sampling period. In case the period of the LF oscillator is substantially longer than the period of the resonance circuit the HF oscillator will be in the off-state most of the time, but will still ensure that the resolution of the measurement is relatively high.

Alternatively, the control means may be adapted to control the switching means to switch the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the resonance circuit, and to control the switching means to switch the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the LF circuit. This embodiment works in a very similar way as the embodiment described above, and the remarks set forth above are equally applicable here. The main difference is that in this case the HF oscillator is in the on-state during a fraction of the period of the LF oscillator, and it is therefore advantageous to use it when the period of the resonance circuit is substantially longer than the period of the LF oscillator.

The means for determining a number of periods of the LF oscillator and a number of periods of the HF oscillator may be adapted to perform the determination while the number of periods of the signal arising from the resonance circuit accumulates. This may, e.g., be obtained by means of at least one counter for counting said_, periods.

Similarly, the means for determining a resonance frequency of the resonance circuit may further comprise a counter for counting the number of periods of the resonance circuit.

The number of periods of the LF oscillator or the number of periods of the resonance circuit may be a predetermined number. Thus, it may be decided to count the number of periods of the resonance circuit during a predetermined number of periods of the LF oscillator or vice versa. A predetermined number may be 1, but it may alternatively be a larger number.

The circuit of the invention may advantageously be incorporated in a Respiratory Inductive Plethysmograph (RIP). In this case the wire loop of the circuit is embedded in a belt being adapted to be positioned around a body part of a mammal.

The circuit may be connected to a microprocessor being adapted to process signals obtained by the circuit, and it may comprise a battery for powering the RIP. The RIP may be adapted to deliver an output for an external device for further processing of the output. The external device may, e.g., be a computer device or a monitor for displaying output, e.g. in a graphical form.

The belt may be made waterproof in order to allow for washing of the belt. This can be obtained by standard moulding or over-moulding techniques, where the battery and any electronics, such as a microprocessor, are positioned on the belt which is subsequently encapsulated using a standard technique, such as injection moulding or over-moulding. Furthermore any cables may be removable with a plug, thereby improving the waterproof properties of the belt.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a functional diagram of a Direct Digitized Respiratory Inductive Plethysmograph (DDRIP), and

Fig. 2 shows a circuit diagram of a low power RIP oscillator.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a functional diagram of a Direct Digitized Respiratory Inductive Plethysmograph (DDRIP). The key elements of the circuit are an ultra-stable sample generator 1, a gated high-frequency (HF) generator 2, and a microcontroller 3.

The sample generator 1 outputs reference pulses at a desired sampling rate, which is typically approximately 20 samples per second in order to obtain a good quality RIP. For low power consumption a 32768 Hz crystal can advantageously 'be used with an appropriate divider to generate a low frequency (LF) sampling signal.

The signal lines at the bottom of the figure illustrate various signals used for determining the frequency of the resonance circuit. The LF sampling signal is illustrated in the upper signal line designated "Sample'. The next signal line, designated 'Riplnput' shows the signal coming from the belt. This is the 'unknown' signal, i.e. the signal for which we want to find the frequency. The 'Riplnput' signal may e.g. be supplied by a low power RIP oscillator as the one illustrated in Fig. 2, but it may alternatively be supplied by a standard LC oscillator using linear amplifiers and sinusoidal excitation. The 'Riplnput' signal is fed to Counter2 of the microcontroller 3. The third signal line, designated 'HFoscON', shows when the HF oscillator 2 is in the on state and when it is in the off state. In the present example the HF oscillator 2 is in the on state when the signal is 'high' and in the off state otherwise. The last signal line, designated 'HFOUT, shows the output of the HF oscillator 2. This signal is fed to Counted of the microcontroller 3.

The HF oscillator 2 should typically output a 1-5 MHz square wave and can be as simple as one 74HC132 gate with an RC network as feedback.

As illustrated by the signal lines at the bottom of the figure, each sample starts when a rising edge of the 'Sample' signal sets a flip-flop 4 and turns the HF oscillator 2 on. At the next rising edge of the 'Riplnput' signal the flip-flop 4 is reset and the HF oscillator 2 turned off. Then the microcontroller 3 reads the values of Counterl and Counter2, and knowing the sampling rate and frequency of the HF oscillator 2, the unknown frequency of the 'Riplnput' signal can now be calculated.

If the HF oscillator 2 is an RC based oscillator or another 'unstable' kind of oscillator, then a calibration measurement must be performed periodically. A 'Calibration' output of the microcontroller 3 is then raised for a fixed period, thereby turning the HF oscillator 2 on for this period, in order to find the actual operating frequency of the HF oscillator 2.

Thus, an LF reference is used in the middle of the measuring period to keep track of time, thereby saving power, and the HF oscillator 2 is only turned on at the beginning and/or end of the measuring period in order to obtain a high resolution. Thereby a low power consumption combined with a high resolution is obtained.

Fig. 2 shows a circuit diagram of a Low Power RIP oscillator suitable for supplying the 'Riplnput' signal of Fig. 2. Using this Low Power RIP oscillator instead of a conventional LC oscillator further decreases the power consumption of the RIP. The signal from the belt is applied at the 'Riplnput', and the signal produced at 'RipOutput' is fed to the circuit of Fig. 1 as 'Riplnput', such as via an appropriate divider for reducing the supplied frequency in a controlled manner.

The Low Power RIP oscillator of Fig. 2 is based on two cascaded inverters or inverting gates U2, U3 with positive feedback. The inverters U2, U3 are preferably of the type 74HC04. The key to the low power consumption of 150-250 μA (micro amperes) is the unique way of draining the power from a stable low voltage reference Vosc through a resistor Rl having a resistance of 1-5 kΩ. This limits the power used by regulating the supply voltage of the inverters U2, U3 to the lowest operating point. Thereby a low power operation can be combined with the ability to operate with belts of various sizes. When the oscillation starts at full voltage, Vosc = 1.8 V, the power consumption is at maximum and drains the decoupling capacitor C3 quickly. As the operating voltage drops, the current drops as well, and at some point a balance is reached. The position of the balance point depends on many factors, such as the oscillation frequency, the Q-factor of the resonance circuit, the surrounding temperature, the type of inverter 112, U3, etc. The mechanism is self regulating in the sense that if some external factor tends to slow down the oscillation, the voltage will rise until a new balance point is reached.

Another great benefit of this method is that when the supply voltage goes down, the inverters U2, U3 get Mess digital' and 'more linear'. This is an advantage when making an oscillator.

An important aspect of the present invention is how the logical gates are being used to drive the LC resonance circuitry with high stability. This can be explained as follows.

When designing an oscillator circuit, the requirements of the Barkhausen criteria must be met. This means that the so-called loop gain must be equal to 1 and the loop phase margin must be 360°.

The normal approach when designing an LC oscillator is to use linear amplifiers and sinusoidal excitation to get the preferred stability. This requires that that the loop gain must be regulated to be equal to 1 to meet the Barkhausen criteria. The gain controllers used to obtain this normally contain both a large number of components and have high power consumption.

The benefits of using logic gates instead of linear circuitry are a much smaller number of components, much lower power consumption, and much lower 'Cost of Goods Sold' (COGS). Therefore, this is the normal approach in crystal based oscillators like the ones used in watches and computers, and it works fine in those cases since the crystal resonance frequency is very precisely determined and provides the stability required.

However, driving an LC oscillator using logic gates is difficult because the harmonics created by the square-waved output of the logic gates disturb the oscillation.

The design according to the present invention combines the benefits of the logic gate oscillators and the linear oscillators by lowering the operation voltage of the logic gates below their specified lower limit. In the example illustrated in Fig. 2 this is done by connecting resistor Rl in series with the supply pins, thereby transforming the function of the logic gates to be more linear and self regulating in the LC oscillator. As mentioned above, the benefits are linear signal quality, but logic gate power consumption and component costs are increased.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

1. A circuit for measuring the respiratory rate of a mammal, the circuit comprising:

- a wire loop being adapted to be positioned around a body part of a mammal,

- means for determining a resonance frequency of the resonance circuit, said determining means comprising:

- a low frequency (LF) oscillator,

- means for determining the resonance frequency of the resonance circuit based on the determined numbers of periods of the LF oscillator and the HF oscillator.

2. The circuit according to claim 1, further comprising means for calculating a loop area of the wire loop on the basis of the determined resonance frequency.

3. The circuit according to claim 1 or 2, further comprising means for switching the HF oscillator between the on state and the off state and means for controlling the switching means on the basis of a signal indicative of the beginning and/or ending of a period of the resonance circuit.

4. The circuit according to claim 3, wherein the control means is further adapted to control the switching means on the basis of a signal indicative of the beginning and/or ending of a period of the LF oscillator.

5. The circuit according to claim 4, wherein the control means is adapted to control the switching means to switch the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the LF oscillator, and to control the switching means to switch the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the resonance circuit.

6. The circuit according to claim 4, wherein the control means is adapted to control the switching means to switch the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the resonance circuit, and to control the switching means to switch the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the LF circuit.

7. The circuit according to claim 5 or 6, wherein at least one of the signals indicative of the beginning of a period is generated when it is determined that a rising or falling edge is occurring in the corresponding signal.

8. The circuit according to any of claims 5-7, wherein at least one of the signals indicative of the beginning of a period is generated when it is determined that an output value of the corresponding signal exceeds or falls below a predefined value.

9. The circuit according to any of the preceding claims, wherein the means for determining a number of periods of the LF oscillator and a number of periods of the HF oscillator is adapted to perform the determination while the number of periods of the signal arising from the resonance circuit accumulates.

10. The circuit according to claim 9, wherein the means for determining a number of periods of the LF oscillator and a number of periods of the HF oscillator comprises at least one counter for counting said periods.

11. The circuit according to any of the preceding claims, wherein the means for determining a resonance frequency of the resonance circuit further comprises a counter for counting the number of periods of the resonance circuit.

12. The circuit according to any of the preceding claims, wherein the number of periods of the LF oscillator is a predetermined number.

13. The circuit according to any of claims 1-11, wherein the number of periods of the resonance circuit is a predetermined number.

14. A Respiratory Inductive Plethysmograph (RIP) comprising a circuit according to any of the preceding claims and wherein the wire loop of the circuit is embedded in a belt being adapted to be positioned around a body part of a mammal.

15. The RIP according to claim 14, wherein the circuit is connected to a microprocessor being adapted to process signals obtained by the circuit.

16. The RIP according to claim 14 or 15, further comprising a battery for powering the RIP.

17. The RIP according to any of claims 14-16, the RIP being adapted to deliver an output for an external device for further processing of the output.

18. A method for measuring the respiratory rate of a mammal, the method comprising the steps of:

- providing a low frequency (LF) oscillator,

19. The method according to claim 18, further comprising the step of calculating a loop area of the wire loop on the basis of the determined resonance frequency.

20. The method according to claim 18 or 19, further comprising the step of switching the HF oscillator between the on-state and the off-state on the basis of a signal indicative of the beginning and/or ending of a period of the resonance circuit.

21. The method according to claim 20, wherein the switching step is further performed on the basis of a signal indicative of the beginning and/or ending of a period of the LF oscillator.

22. The method according to claim 21, wherein the switching step comprises switching the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the LF oscillator, and switching the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the resonance circuit.

23. The method according to claim 21, wherein the switching step comprises switching the HF oscillator to the on-state on the basis of a signal indicative of the beginning of a period of the resonance circuit, and switching the HF oscillator to the off-state on the basis of a signal indicative of the beginning of a period of the LF circuit.

24. The method according to claim 22 or 23, further comprising the step of generating at least one of the signals indicative of the beginning of a period when it is determined that a rising or falling edge is occurring in the corresponding signal.

25. The method according to any of claims 22-24, further comprising the step of generating at least one of the signals indicative of the beginning of a period when it is determined that an output value of the corresponding signal exceeds or falls below a predefined value.

26. The method according to any of claims 18-25, wherein the step of determining a number of periods of the LF oscillator and a number of periods of the HF oscillator is performed while the number of periods of the signal arising from the resonance circuit accumulates.

27. The method according to claim 26, wherein the step of determining a number of periods of the LF oscillator and a number of periods of the HF oscillator comprises counting said periods.

28. The method according to any claims 18-27, wherein the step of determining a resonance frequency of the resonance circuit further comprises counting the number of periods of the resonance circuit.

29. The method according to any of claims 18-28, wherein the number of periods of the LF oscillator is a predetermined number.

30. The method according to any of claims 18-28, wherein the number of periods of the resonance circuit is a predetermined number.

31. The method according to any of claims 18-30, further comprising the step of processing the signals obtained by means of the method in a microprocessor.

32. The method according to any of claims 18-31, further comprising the step of delivering an output to an external device for further processing of the output.