GB2256710A

GB2256710A - Heart monitoring arrangement

Info

Publication number: GB2256710A
Application number: GB9112488A
Authority: GB
Inventors: Gary Clifford Keall
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-06-11
Filing date: 1991-06-11
Publication date: 1992-12-16
Also published as: GB9112488D0

Abstract

An arrangement for monitoring the acoustic energy omitted by an artificial heart valve, for example, includes a transducer 1 for detecting the energy and converting it into electrical form. The signals are digitized at 3 and applied to two thresholding circuits 7 and 8. These circuits include adaptive threshold levels which permit useful data signals to be discriminated from noise for both low and high amplitude signals. Discrimination is such that the proportion of rejected signals is a substantially constant percentage of the amplitude distribution. The arrangement also includes means for displaying visually the acoustic signals emitted by the heart and enables a comparison to be made with data collected previously and stored at 24. The invention is also applicable to monitoring real heart valves. <IMAGE>

Description

Heart Monitoring Arrangement This invention relates to a heart monitoring arrangement and more particularly, but not exclusively to an arrangement which is suitable for detecting abnormalities in the operation of artificial heart valves.

A number of different types of artificial heart valves are available which emit characteristic acoustic signals during their operation the acoustic signature being governed by the type of valve used. A small amplitude signal is emitted during opening of a valve and a much larger amplitude signal is produced when the valve shuts.

The operation of prosthetic heart valves can be impaired by the presence of thrombosis which can ultimately lead to heart failure. However, the presence of a blood clot affects the high frequency acoustic signal emitted by the valve, being initially detectible by monitoring modification of the acoustic signal produced during opening of the valve.

The present invention arose from an attempt to provide an arrangement for monitoring heart function and more particularly to the functioning of prosthetic heart valves.

According to the invention, there is provided a heart monitoring arrangement comprising: a transducer for detecting acoustic energy emitted by a heart to be monitored and for converting the energy into electrical signals; thresholding means arranged to reject as noise signals having an amplitude less than a threshold level; means for changing the value of the threshold level in dependence on the amplitude distribution of the detected signals so as to maintain the proportion of the rejected signals at a substantially constant percentage of the amplitude distribution; and display means arranged to present a representation of detected data signals.

Although the invention arose from the consideration of monitoring artificial heart valves, it may also advantageously employed in monitoring natural hearts. By employing the invention, it is possible to discriminate data signals from background noise with sufficiently good performance that any abnormalities in the operation of a heart valve may be detected, even if these only appear, initially, as changes to the acoustic energy emitted during the small amplitude opening phase of the valve's operation.

Such information is of great value to a clinician examining a patient, for example, in controlling the dosage of any anti-clotting agents administered.

In a preferred arrangement, the threshold level is set so as to transmit as data signals only those having amplitudes falling within the top 20% of the amplitude distribution of detector signals. This ensures that all significant information is retained for further consideration.

It is further preferred that an arrangement in accordance with the invention includes a second thresholding means arranged to receive data signals from the first thresholding means and to apply a second threshold level to the received data signals such that only those signals exceeding the second threshold level are retained as data signals. By employing this feature, additional discrimination is available and ensures that only information bearing signals are retained for further analysis.

In one preferred embodiment of the invention, the second threshold level has a value which is selected from, two settings, to ensure that data is captured even when amplitude levels are low.

One way in which the invention may be performed is now described by way of example with reference to the accompanying drawing which is a schematic block diagram of heart monitoring equipment in accordance with the invention.

Heart monitoring equipment which is particularly suitable for detecting changes in the operation of artificial heart valves includes a piezoelectric transducer 1 which is configured so as to optimize the reception of acoustic energy emitted by a heart valve. The transducer 1 has a frequency response ranging from about 1kHz up to approximately 100 kHz and therefore detects acoustic signals emitted by the heart valve as it opens or closes.

During operation of the device, the transducer 1 is attached to the patient in the vicinity of the heart and the acoustic emissions detected and converted into electrical form. The output of the transducer 1 is applied to an amplifier and filter 2 which cuts out lower frequency signals having a frequency of less than 2 kHz. The output of the filter 2 is applied to an analogue to digital converter 3 having a sampling rate of 150 kHz. The digitized signals are then applied to a buffer 4 having two outputs 5 and 6 both of which carry the digitized signals.

The signals on line 5 are applied to a thresholding circuit 7 in which signals having an amplitude value of less than a threshold level T1 are rejected as noise and the remainder transmitted as data signals. In this embodiment of the invention, the threshold level is set such that those signals having amplitudes falling in the lower 80% of the amplitude distribution are rejected as noise and only the upper 20% transmitted as carrying useful information.

The threshold level T1 is not constant but forms part of an adaptive thresholder so that as the overall amplitude of signals changes, the value Tl follows those changes so as to keep substantially constant the percentage of signals designated as containing useful data. As the closing signal from an artificial valve is appreciably greater in magnitude than the opening signal, by adapting the level T1 in this way useful information can be discriminated from the noise and background interference through all stages of the valve operation. The way in which the value of level T1 is changed is described in greater detail below.

The data signals from the thresholding circuit 7 are applied to a second thresholding circuit 8 where those signals having amplitudes less than a threshold value T2 are rejected as noise. The threshold level T2 is switched between two values, giving two degrees of further noise discrimination. These two values of T2 are both dependant on T1. In this particular embodiment of the invention, the threshold level T2 is set, at one value, to accept those signals lying within the upper 10% of the total noise distribution as information bearing signals and reject the remainder as noise and at its other value, to only accept those signals having amplitudes lying within the top 3% of the total noise distribution, as received at the buffer circuit 4.

Both values of the threshold level T2 change in accordance with changes of the total amplitude of received signals at the buffer 4, being adapted to follow the first threshold value T1. The particular one of the two T2 values which is applied depends on which part of the heart signature signal is being received.

The windowed output of the second thresholding circuit 8 is applied via a signal detector 9, which does not modify the data signal, to a first store 10, where it is held until further analysis is required and/or the output is to be displayed for visual interpretation. The digitized signals representative of the valve operation are stored in store 10 together with a value representing the time for which the signal is below the threshold before the next signal is stored. By not storing the signal below the threshold. the amount of memory required to store the useful signal is reduced to typically 100-200 Kbytes for a 10 second monitoring period so allowing several readings to be stored in computers of even modest memory capacity.

The duration of the useful signal is typically 2-5 ms per signal.

The signals rejected as containing only noise in the first thresholding circuit 7 are applied on its output line 12 to a noise processing circuit 13. This condenses the information contained in the noise signal which is then stored in a second store 14.

The signal detector 9 determines whether or not there is an output from the thresholding circuit 8 and indicates this status on its output line 11, this being used to determine which of the two values of T2 is applied at thresholding circuit 8.

The threshold levels T1 and T2 are set at circuits 15 and 16 respectively. The value of level T1 is initialised by an operator prior to using the equipment and is set at a level which, from previous operation of the equipment, is known to be within the range of amplitude values received.

The second threshold T2 level is dependent on level T1 and is a fixed multiple of level T1 for both the level T2 values, the multiples being different in each case. In this embodiment of the invention, the multiples by which the level T2 is set may be varied in accordance with the type of heart valve which is to be monitored.

The digitized signal on the output line 6 of buffer 4 is applied to a comparator 17. This compares the instantaneous signal amplitude with the threshold level T1 and gives an output which indicates whether T1 is larger or smaller than the signal on the comparator input. The output of the comparator 17 is applied to a processor 18 which calculates the change required in T1. If the signal on line 6 is larger than the threshold level T1, then T1 is increased, whereas if the comparison shows that the signal on line 6 is smaller than T1, the value of T1 is reduced.

The processor receives an input from a store 19 which holds an incremental value x which is variable, being dependent on T1, the value of T1 being applied on line 20 from the store 15. When the threshold level T1 is to be increased, it is increased by an amount of 4 multiplied by increment x. If the threshold level T1 is to be reduced, then it is reduced by the increment x. This adaptive arrangement ensures that values T1 and T2 follow the change in amplitude of the signals received by the transducer 1 so as to ensure that all useful information is retained.

The acoustic emissions from a heart depend on the type of artificial valve which is fitted. However, in each case, and in the case of natural hearts also, the initial signals received when a valve opens are smaller than the signals emitted during closing of the valve, being typically 30 times less in amplitude. When no signal is detected at 9, then the value T2 is set so as to accept the upper 10% of the initial signal distribution. Once the signal detector 9 monitors the presence of useful information at the output of thresholding circuit 8, then the value T2 is reset and is increased so that only the largest signal values are passed, falling within the upper 3% of the total signal distribution. This ensures that useful information for contained in signals of low amplitude level, as with the opening of the valve, can be detected.

The equipment in accordance with the invention includes a frequency analyser 21 which receives the thresholded data signals from the first store 10 and determines the frequency distribution thereof. The proportion of the signals falling within the different frequency bands can give useful information regarding the functioning of the heart valve.

The equipment includes a display 22 to which information regarding the output detected by transducer 21 can be transmitted for visual inspection. The display can be controlled so as to show only the information contained within the first store 10 but can also be adjusted so as to insert the noise information held in the second store 14 to give a clinician operating the device an understanding of the signal to noise level encountered. The equipment is also capable of receiving an input from an electro-cardiograph (ECG) machine and for displaying it adjacent to the representation of acoustic signals so that a clinician may compare the two. Furthermore, a store 24 is included which enables the clinician to recall previously stored acoustic data, perhaps recorded at a visit some months previously, and compare any changes which have occurred in the intervening period or to compare the output from a valve with a "standard" output likely to be detected from the type of artificial valve installed. There is also a provision for an unfiltered transducer output to be displayed with the processed data. The type of display

Claims

CLAIMS 1. A heart monitoring arrangement comprising: a transducer for detecting acoustic energy emitted by a heart to be monitored and for converting the energy into electrical signals; thresholding means arranged to reject as noise signals having an amplitude less than a threshold level; means for changing the value of the threshold level in dependence on the amplitude distribution of the detected signals so as to maintain the proportion of of the rejected signals at a substantially constant percentage of the amplitude distribution; and display means arranged to present a representation of detected data signals.
2. An arrangement as claimed in claim 1 and including storage means for storing data signals prior to them being applied to the display means.
3. An arrangement as claimed in claim 1 or 2 wherein the representation presented by the display means is a visual representation.
4. An arrangement as claimed in claim 1, 2 or 3 and including storage means for storing noise signals after their rejection by the thresholding means.
5. An arrangement as claimed in any preceding claim and comprising means for recombining the noise signals with data signals for display by said display means.
6. An arrangement as claimed in any preceding claim wherein the threshold level is set so as to transmit as data signals only those having amplitudes falling within the top 20% of the amplitude distribution of detected signals.
7. An arrangement as claimed in claim 6 and wherein the means for changing the value of the threshold level comprises a comparator for comparing the amplitude of the received signal with the threshold level and, if the comparison shows that the signal is larger than the threshold level, increasing the value of the threshold level by an amount of approximately four multiplied by an incremental value and, if less, decreases the threshold value by the incremental value.
8. An arrangement as claimed in claim 7 wherein the incremental value is dependent on the value of the threshold level.
9. An arrangement as claimed in any preceding claim and including a second thresholding means arranged to receive data signals from the first thresholding means and to apply a second threshold level to the received data signals such that only those signals exceeding the second threshold level are retained as data signals.
10. An arrangement as claim in claim 9 wherein the second threshold level has a value such that only those data signals having an amplitude falling within the top 10% of the amplitude distribution of the detected signals are transmitted as data signals.
11. An arrangement as claimed in claim 9 or 10 wherein the second threshold level has a value which is selected from two settings.
12. An arrangement as claimed in claim 9, 10 or 11 wherein the second threshold level has a value which is dependent on the value of the first threshold level.
13. An arrangement as claimed in any preceding claim and including means for accepting a signal from electro-cardiograph equipment and for displaying it with the said representation of detected data signals.
14. An arrangement as claimed in any preceding claim and including means for analysing the frequency spectrum of the received signals.
15. An arrangement as claimed in any preceding claim and including storage means for storing data relating to acoustic energy detected at a previous time.
16. An arrangement as claimed in any preceding claim and including means for setting the threshold level or levels in accordance with a type of heart or heart valve to be monitored.
17. Heart monitoring equipment substantially as illustrated in and described with reference to the accompanying Figure.