Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1998-08-17
2001-06-12
Hindenburg, Max (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S511000
Reexamination Certificate
active
06245025
ABSTRACT:
TECHNICAL FIELD
This invention relates to particularities of monitoring the well-being of a fetus on the basis of fetal heart activity.
BACKGROUND OF THE INVENTION
The widely used CTG equipment for monitoring of fetal heart activity apply ultrasound Doppler technique, where an ultrasonic beam is directed to the fetus, the reflection of which determines the heart movement and hereby fetal heart rate (FHR). However, these equipment are unsuitable for long-time monitoring without inspection, since uncontrolled high-dose exposure may have some harmful effects.
A further technique for this purpose is the phonocardiography (PCG) where acoustic waves excited by heart movement are detected. In the case of fetal heart sounds, however, difficulties arise in the detection and identification of the signals. Acoustic waves of fetal origin reach the sensor placed on the maternal abdomen through a complex transmission path, where significant spectrum variation occurs. Furthermore, disturbances of maternal digestive organs may hinder signal detection. Finally, fetus movements result in the displacement of the optimal sensing point and lead to the decrease of signal level.
An essential advantage of the acoustic method is that the passive manner of the sensing is harmless to the fetus even at very long monitoring time, which makes possible home care. The detection and processing of acoustic signals of fetal heart are dealt with by many researchers.
A fetal monitor is announced in U.S. Pat. No. 2,536,527 to Appel. The invention serves for monitoring fetal condition during delivery. A microphone applied to the stethoscope produces a signal which is amplified, filtered, rectified and used to indicate abnormally high or low FHRs.
U.S. Pat. No. 3,187,098 to Farrar describes a fetal heartbeat detector, which uses a cantilevered piezoelectric crystal mounted within a contacting slab. A fetal monitor is given in U.S. Pat. No. 3,409,737 to Settler et al. This monitor is used with a belt having three microphones. An amplifier is used to selectively amplify the fetal heartbeat and remove the maternal heartbeat.
U.S. Pat. No. 4,781,200 to Baker uses a sensor belt wearing twelve sensors, the detected signal of which are compared to cancel disturbances and trace fetal position. The signal processing is carried out by the conventional FFT method for 128 points applied in every ⅛ seconds, delivering the frequency spectra for selection of coincided components of the sensors. However, because of the relatively long time period of analysis the fast variations in spectral power density are averaged. Thus short time characteristics of frequency components cannot be correctly detected using this method. U.S. Pat. No. 5,140,992 to Zuckerwar et al. uses a belt wearing more piezoelectric polymer film sensors for fetal heartbeat indication.
All of this invention have the common insufficiency that they do not distinguish perfectly the first and second sound and thus they cannot apply this additional information to the identification of fetal heartbeat sound. In addition, a lot of computations are carried out in signal processing, which can be saved without the deterioration of the reliable sound identification. As a consequence, these instruments require high supply current and thus they are for battery operation unsuitable.
Reliable identification of fetal heartbeat is of vital importance at FHR measurement. Consequently, there is need for a solution that enables reliable identification of fetal heartbeats, does not require bulky hardware and allows battery operation even without medical supervision.
GENERAL DISCLOSURE OF THE INVENTION
In a first aspect the aim of the invention is to provide a method for identifying fetal heart sounds with improved reliability. According to a second aspect, the method should be implemented on a low-power electronic circuit to allow designing a portable apparatus suitable for long-term home monitoring. According to a further aspect, the apparatus should enable easy finding of the correct position of the sensor on the maternal abdomen. Advantageously, simultaneous measurement of maternal womb contractions should be possible by using a conventional tocodynameter. According to a further aim of the invention a sensor should be applied that enables improved acoustical detection of fetal heart activity.
The above and other aims can be achieved by the invented method for measuring fetal heart rate on the basis of acoustic signals detected at the maternal abdomen. The invented method comprises the following steps:
converting said acoustic signals into electric signals;
determining power levels P
a
(t) and P
b
(t) of said signals of fluctuating intensity at test frequencies f
a
and f
b
being in a lower and higher frequency band of the range of 20 to 80 Hz;
detecting and storing local peak values P
a1
and P
a2
; as well as P
b1
and P
b2
of said power level and relevant times t
a1
and t
a2
; as well as t
b1
and t
b2
for an inspection period t
s
following the first one of said detected local peaks;
counting numbers n
a
and n
b
of said peaks of power levels during said inspection period t
s
;
storing the value of said power level P
b
(t
a1
) when a peak value P
a1
at said lower test frequency f
1
is detected;
determining time differences dt=t
a1
−t
b1
and dt=t
a2
−t
b2
between detection of first peak values P
a1
and P
b1
, or P
a2
and P
b2
within said inspection period;
classifying detected peaks on the basis of numbers n
a
, n
b
, values and timing of said peaks of power levels, and identifying peaks meeting preset criteria as first and second sounds, respectively;
measuring the time difference between an identified first sound and a subsequent second sound representing the closing time;
identifying said first and second peak as a matching pair of sounds of a heartbeat if said closing time is in the range of 140 to 220 ms; and
calculating fetal heart rate from a time difference between two consecutively identified heartbeats.
The detected peaks are classified on the basis of the following criteria:
if one local peak is detected on power level P
a
(t) as well on power level P
b
(t), that means n
a
=1 and n
b
=1, and for the time difference dt=t
a2
−t
b2
between the two peaks the criterion −15 ms<(t
a2
−t
b2
)<15 ms is satisfied, then the detected peak is a second sound (see
FIG. 3
a,
right side),
if one local peak is detected on power level P
a
(t) as well on power level P
b
(t), that means n
a
=1 and n
b
=1, and for the time difference dt=t
a1
−t
b1
between the two peaks 15 ms<dt<40 ms, furthermore, for the power level P
b
(t) the P
b
(t
a1
)/P
b
(t
b1
)<0.3 criterion is satisfied, then the detected peak is a first sound (see
FIG. 3
a, left side),
if one local peak is detected on power level P
a
(t) and two peaks on power level P
b
(t), that means n
a
=1 and n
b
=2, and for the peak values on the power level P
b
(t) the P
b
(t
a1
)/P
b
(t
b1
)<0.3 criterion is satisfied, then the detected peak is a first sound (see
FIG. 3
b
),
if two local peaks is detected on power level P
a
(t) and one peak on power level P
b
(t), that means n
a
=2 and n
b
=1, and for time difference dt=t
a1
−t
b1
between the two peaks the criterion −15 ms<(t
a2
−t
b2
)<15 ms is satisfied, then the detected peak is a first sound (see
FIG. 3
c
),
if two local peaks is detected on power level P
a
(t) as well on power level P
b
(t), that means n
a
=2 and n
b
=2, then the detected peak is a first sound (see
FIG. 3
d
),
Peaks not meeting criteria are classified as undefined sounds. This kind of classification enables identification of heart sounds with an improved reliability. Further criteria, especially for non-typical cases can also be of use. Preferably, said estimation of power P
a
(t) at said lower test frequency fa comprises averaging for a time window of 30 to 90 ms; said estimation of power level P
b
(t) at said
Balog Géza
Bartos Imre
Herman Ákos
Kovacs Ferenc
Seres Sándor
Carter Ryan
Hindenburg Max
Kolisch Hartwell Dickinson & McCormack & Heuser
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