Method and apparatus for measuring fetal heart rate and an electroacoustic sensor for receiving fetal heart sounds

• US 6,245,025 B1
• Filed: 08/17/1998
• Issued: 06/12/2001
• Est. Priority Date: 08/10/1995
• Status: Expired due to Term

• Current Assignee: Remotaid Hungary KFT
• Original Assignee: Ákos Herman, Géza Balog, Imre Bartos, Kovács, Ferenc, Sándor Seres, Székely, István, Török, Miklós
• Inventors: Kovács, Ferenc, Balog, Géza, Székely, István, Bartos, Imre, Török, Miklós, Seres, Sándor, Herman, Ákos
• Primary Examiner(s): Hindenburg, Max
• Assistant Examiner(s): CARTER, RYAN CHRISTOPHER
• Application Number:
  US09/011,840
• Time in Patent Office: 1,030 Days
• Field of Search: 600/372, 600/382, 600/485, 600/500, 600/504, 600/508-509, 600/511, 600/528
• US Class Current: 600/500
• CPC Class Codes: A61B 5/02411 of foetuses
• CPC Class Codes: A61B 5/4362 Assessing foetal parameters
• CPC Class Codes: A61B 7/04 Electric stethoscopes microphones, acoustic transducers therefor H04R

Abstract

Method and apparatus for long-term, non-invasive measuring of fetal heart rate. The method utilizes the characteristic curves of first and second heart sound received by the preferred electroacoustical converter. The identification and distinction of first and second sounds, furthermore their time relation used for heartbeat identification highly increase the reliability of fetal heart rate determination. The distinction of first and second sound is based on the differences in frequency spectrums measured in a relatively short time window and by the estimation of the power peaks measured on two test frequencies chosen on the two ends of the frequency range of fetal heart sound. The method is implemented in an integrated apparatus to achieve low power consumption for battery-operated long-term measurements. Digital filtering and selective power estimation is applied for continuous computation of power time function on the two test frequencies. The test frequencies can be adaptively modified depending on individual fetal parameters to improve the distinction of the first and second sound. The apparatus is capable to registrate, store and transfer data of fetal heart rate and womb contractions to a personal computer.

Claims

1. A method for measurement of fetal heart rate on the basis of acoustic signals originating from heat activity of a fetus and detectable on a maternal abdomen, the method comprising the steps of:

measuring acoustic signals of fluctuating intensity originating from the heart activity of the fetus;

converting said acoustic signals into electric signals;

determining power levels (P_a, P_b) of said signals at a lower and at a higher test frequency (f_a,f_b), said lower and higher frequencies (f_a,f_b) being, respectively, in a lower and a higher frequency band of a frequency range of 20 to 80 Hz;

detecting and storing local peak values (P_a1, P_a2;

P_b1, P_b2) of said power levels (P_a, P_b) at different times (t_a1, t_a2;

t_b1, t_b2) during an inspection period (t_s) following the first one of said detected local peaks;

counting numbers (n_a, n_b) of said peaks of power levels during said inspection period t_s;

determining time differences between the times of detection of said peak values (P_a1, P_b1, P_a2, P_b2) within said inspection period (t_s);

identifying the detected peak as a said second sound, provided that said number (n_a) of local peaks of said power level (P_a) is one and said number (n_b) of local peaks of said power level (P_b) is one and said time difference (dt) between said peak values (P_a2, P_b2), is smaller than 15 ms;

identifying the detected peak as a said first sound, provided that said number (n_a) is one and for said time difference (dt) the criterion 15 ms<

dt<

40 ms is satisfied and for said power level (P_b) measured at said time (t_a1) corresponding of said peak values (P_a1) of said power level (P_a) the criterion (P_b(t_a1))/P_b1<

0.3 is satisfied;

identifying the detected peak as a said first sound provided that said number (n_a) is one and said number (n_b) is two and for said power level (P_b) measured at the time (t_a1) of said peak values (P_a1) of said power level (P_a) the criterion (P_b(t_a1))/P_b1<

0.3 is satisfied;

identifying the detected peak as a said first sound, provided that said number (n_a) is two and said number (n_b) is two;

classifying a pair of identified peaks as heartbeat, provided that the measured time difference (t_1,2) between the said first and second sound is in the range of 140 ms to 220 ms; and

calculating the fetal heart rate from the time difference between two consecutively identified heartbeats.

2. The method as claimed in claim 1 wherein said determination of power level (P_a) at said lower test frequency (f_a) includes averaging said power level (P_a) for a time window of 30 to 90 ms;

and

said determination of power level (P_b) at said higher test frequency (f_b) includes averaging said power level (P_b) for a time window of 20 to 60 ms.

3. The method as claimed in claim 1 or 2, wherein local peak values (P_a1, P_a2;

P_b1, P_b2) of power levels (P_a, P_b) are detected if their values exceed the {fraction (1/10)} part of the average of previously detected peak values.

4. The method as claimed in claim 1, wherein said lower testing frequency band ranges from 25 to 35 Hz and said upper testing frequency band ranges from 55 to 65 Hz.

5. The method as claimed in claim 1 wherein said testing frequencies (f_a,f_b) are predetermined according to the time of gestation and estimated weight of fetus, and said predetermined test frequencies are applied as starting values at the beginning of the measurement.

6. The method as claimed in claim 1 wherein the values of said testing frequencies (f_a,f_b) are continuously adjusted to obtain maximum values for said local peaks of said signal power.

7. The method as claimed in claim 1 wherein said inspection time (t_s) is longer than 80 ms and shorter than 120 ms.

8. The method as claimed in claim 1 wherein said inspection time (t_s) is shortened if identification of said first sound or said second sound is completed.

9. The method as claimed claim 1 wherein the time difference (t_1,2) between the first and second sound is longer than 140 ms and shorter than 220 ms. and said time difference (t_1,2) is continuously adjusted on a previously calculated average of the time difference (t_1,2).

10. An apparatus for measuring fetal heart rate, the apparatus comprising:

an acoustic sensor (1) for sensing acoustic vibrations derived from a fetus, said sensor adapted to be placed on a maternal abdomen;

an analog unit (2) for amplifying and enhancing frequencies characteristic to fetal heart activity, the analog unit (2) including an amplifier (4) having adjustable gain for adjusting a signal level to be in a conversion range of an analog to digital converter (61) connected to the amplifier (4) which converts analog signals to digital sample values;

a first frequency selective power estimation unit (15, 16) for determining signal level at a lower test frequency (f_a) within a frequency range from 25 to 35 Hz;

a second frequency selective power estimation unit (17) for determining the signal level at a higher test frequency (f_b) with a frequency range from 55 to 65 Hz;

a first and second peak detector (42, 56) for monitoring said signal levels fluctuating in time and estimated by said first and second power estimation units (15, 16, 17) and establishing occurrence of local peaks of said signal levels;

an identification unit for identifying first and second heart sounds based upon the time and the sequence of said local peaks and for computing fetal heart rate according to the method of claim 1;

a storage means (8) for storing measured fetal heart rate values; and

a communication port (10) for making said stored fetal heart rate values accessible for further evaluation.

11. The apparatus as claimed in claim 10, wherein said peak detectors (42, 56) include comparators (43, 44, 45, 46) connected to parallel outputs of said fourth and sixth register files (41, 55);

and

an AND gate (47) connected to outputs of said comparators (43, 44, 45 and 46) indicating the local peak of said power level (P_a, P_b).

12. The apparatus as claimed in claim 10, wherein said sound unit (5) includes a switch (62) adapted to turn on and off transponded and audible heart sounds.

13. The apparatus as claimed in claim 10, wherein said serial communication port (10) is adapted to be connected to a line printer.

14. The apparatus as claimed in claim 10, wherein the acoustic sensor (1) includes a hollow body (27) and an electroacoustic converter (24), located inside said hollow body (27) and having a sensitive membrane;

wherein said hollow body (27) is partitioned into a first, a second, and a third chamber (19, 20, 25);

said first chamber (19) being open at one side, and coupled to said second chamber (20) via an opening (21) formed in the partition wall between said first and second chambers (19 and 20), and having a first boring (23) on its sidewall communicating with the atmosphere;

said second chamber (20) having an upper partition wall formed by a sensitive membrane of an electroacoustic converter (24); and

said third chamber (19) communicating with the atmosphere through a second boring (26) formed in the sidewall of the hollow body (27).

15. The apparatus as claimed in claim 14, wherein said electroacoustic converter (24) is an electrodynamic microphone having an elastic membrane.

16. The apparatus as claimed in claim 15, wherein an elastic ring (22) is provided at the periphery of said open side of said first chamber (19).

17. The apparatus as claimed in claim 10, further comprising a tocodynameter (9) for continuous measurement of maternal womb contractions.

18. The apparatus as claimed in claim 10, wherein said analog unit (2) includes an active filter (3) connected to said sensor (1), said amplifier (4) being connected to said active filter (3) and a sound unit (5) which is connected to said amplifier (4), the apparatus thereby amplifying, filtering and frequency transponding the input signal.

19. The apparatus as claimed in claim 10, wherein said analog to digital converter (61) is included in a microcontroller, said microcontroller (6) being adapted to evaluate data and control peripheral units.

20. The apparatus as claimed in claim 19, further comparing a standardized modem adapted to effect data transfer for telemetric measurements and for communicating with the microcontroller (6).

21. The apparatus as claimed in claim 19, wherein at least one of said first frequency selective power estimation unit (15, 16), said second frequency selective power estimation unit 17, said first peak detector (42) and said second peak detector (56) is included in the microcontroller (6).

22. The apparatus as claimed in claim 19, wherein at least one of said first frequency selective power estimation unit (15, 16), said second frequency selective power estimation unit (17), said first peak detector (42), said second peak detector (56 ) is included in an application-specific processor (7) connected to the microcontroller (6), said application specific processor (7) further comprising a gain control circuit (18) connected to the gain control input of said amplifier (4).

23. The apparatus as claimed in claim 17, wherein said storage means (8) includes a non-volatile memory (8) connected to said microcontroller (6) for storing measured fetal heart rate data and addition data provided by said tocodynameter (9).

24. The apparatus as claimed claim 19, wherein said communication port (10) is a serial communication port connected to the microcontroller (6) for connecting said apparatus to an additional computer via a standard serial communication line.

25. The apparatus as claimed in claim 19, further comprising an input device (12) connected to the microcontroller (6) for indicating subjective feeling of fetal movement.

26. The apparatus as claimed in claim 19, further comprising a light emitting diode (13, 14) connected to the microcontroller (6) for indicating amplifier overdrive and extremely low input signal level.

27. The apparatus as claimed in claim 26, wherein at least one of said frequency selective power estimation units (15, 16;

17) includes a digital filter (15) adapted to select frequencies within said frequency range and said power estimation unit is connected to the output of said digital filter (15).

28. The apparatus as claimed in claim 27, wherein said digital filter (15) includes:

a register file (28) having serial-in, parallel-out structure and containing actual input data and preceding data to be considered on the course of digital filtering;

a segmented decoder (29) consisting of decoder segments for separately decoding parallel outputs of said register file (28);

a segmented look-up table (30) storing words to be designated separately by said segmented decoder (29) consisting of products (a_kd<sub>n−

k</sub>) where n is the number of sampling, d<sub>n−

k</sub>, is the (n−

k)th previously sampled input data, a_k are filter coefficients with k being in the range of k=0 to M, where M represents the order of the filter;

a second register file (33) having serial-in, parallel-out structure and containing preceding filtered data used for actual filtering;

a second segmented decoder (32) consisting of decoder segments for separately decoding parallel outputs of said second register file (33);

a second segmented look-up table (31) storing words to be designated separately by said segmented decoder (32) consisting of products (b_ky<sub>n−

k</sub>), where y<sub>n−

k</sub> are (n−

k)th previous filtered data, b_k are filter coefficients with k being in the range of k=1 to N, where N corresponds to the order of the filter;

a summator (24) for consecutively reading out and summing each designed word of said table segments according to the following formula;

$y_n=\underset{k=0}{\overset{M}{\sum <br><br>}}<br><br>a_k<br><br>d_{n-k}+\underset{k=1}{\overset{N}{\sum <br><br>}}<br><br>b_k<br><br>y_{n-k},$

wherein y_n is the actual filtered data, and the meaning of other designations are the same as above.

29. The apparatus as claimed in claim 21, wherein said power estimator (16) comprises:

a decoder (35) for decoding filtered data as an address;

a look-up table (36) addressed by said decoder (35) for containing squared values of said addresses;

a summator (37) connected to said look-up table (36) for summing values consecutively rad out of said table (36);

a scanner (38) controlling repetitive and consecutive addressing and summing said squared values by said summator (37);

a third register file (39) connected to said summator (37) and having serial-in, parallel-out structure and containing new summed data in its upper register and the oldest data in its lowest register;

a replacer (40) having a first input connected to the upper register of said third register file (39), a second input connected to the lower register of the register file, and a third input connected to a fourth register file (41), said replacer (40) being adapted for adding actual values at its first input to and subtracting actual values at its second input from the value received from said fourth register file (41);

the fourth register file (41) being connected to an output of said replacer (40) and having a serial-in, parallel-out structure storing calculated power values and shifting its contents down step by step and having parallel outputs, the first of them connected to said third input of said replacer (40) and all inputs being connected to a peak detector (42) adapted to transfer contents of said microcontroller (6).

30. The apparatus as claimed in claim 29, wherein said second frequency selective power estimator unit (17) includes:

a fifth register file (48) having a serial-in, parallel-out structure and storing input data (d_i) measured in a time window;

a decoder (49) for decoding data (d_i) stored in the fifth register file (48) and designating corresponding words of a second look-up table (50) containing products according to the following formulae;

31. The apparatus as claimed in claim 29, wherein said decoders (29, 32) and summator (34) are adapted to be turned in sleep mode to reduce power consumption when no heartbeat is expected.