GB2181554A - ECG machine - Google Patents

Abstract

A monitor comprises input means adapted for connection to a patient to receive electrical waveforms such as ECG or EEG, means (37) for storing the waveforms, and means 2 for displaying the waveforms, the display means being in the form of a liquid crystal display. The LCD display 2 may have a non-uniform pixel distribution, whereby a selected area eg a central strip of the display enhanced resolution. During recording events of interest may be provided with a marker for subsequent relocation, and the current screen on the LCD display can be arrested whilst recording continues. The heart rate of a patient based may be found by detecting QRS waves, inhibiting the QRS detection for a predetermined period following a QRS wave detection, and determining the average period between a predetermined number of the immediately previously detected QRS waves. <IMAGE>

Description

SPECIFICATION Monitors This invention relates to monitors for monitoring the electrical activity of certain parts of the human body, such as the heart and brain, and has for its objectto provide a more compact machine with improved operational facilities than hitherto. Accordingly, the invention has special reference to electrocardiograph (ECG) and electroencephalograph (EEG) monitors and will be discussed primarily in respect oftheformer although it will be appreciated that it has wider application.

According to a first aspect of the present invention there is provided a monitor comprising input means adapted for connection to a patientto receive, in use, electrical waveforms therefrom, means for recording the waveforms, and means for displaying the waveforms, the display means being in the form of a liquid crystal display.

The use of a liquid crystal displayforthe display means instead of a conventional cathode ray tube makes the machine much more compact. Preferably, the liquid crystal display has a non-uniform pixel distribution, whereby a selected area or areas of the display has or have an enhanced resolution compared with the remainder of the display. The selected area is preferably a band extending across the display and arranged to display the base line of the waveforms where medically significant phenomena occur.

According to a second aspect of the present invention there is provided a liquid crystal display having a non-uniform pixel distribution, the distribution being such that one or more areas of the display has or have an enhanced resolution compared with the remainder ofthe display.

In the context of ECG and EEG monitors, the display preferably has an enhanced resolution area in the form of a band extending across the display with a resolution of twice that of the remainder of the display.

According to a third aspect ofthe present invention there is provided a monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, means for displaying the waveforms, and means operable to arrest the display at any given instant whilst the recorder means continues to operate.

This aspect of the invention allows the machine operatorto dwell on a selected portion of the displayed waveform without interrupting the recording. If the selected portion or portions of the waveform is or are considered worthy of closer inspection, means may be provided for marking the recording and/or display, together with means for sensing a mark and arresting the display thereat when the recording is replayed. The MARK facility can be employed at anytime during operation ofthe machine and is not restricted to when the display has been arrested.

Thus according to a fourth aspect of the present invention there is provided a monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, means for displaying the waveforms, means operable to mark the recording and/or display of the waveforms, and means for sensing a mark and arresting the display thereatwhen the recording is replayed. The sensing means are operable to sense each successive mark and marks can also be made at the beginning and end of each recording.

According to a fifth aspect of the present invention there is provided a monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, means for displaying the waveforms, and first output means from which recorded waveforms are transmitted to a remote station.

The first output means is preferably optoisolated from a modem for connection to a telephone line.

In the case of an ECG monitor, the heart rate (in beats per minute) may be calculated and displayed on further display means which may also be used to display error and status information.

According to a sixth aspect of the present invention there is provided a monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, and means for deriving the heart rate of the patient, the means comprising detection means having a filter to which a received ECG waveform is applied, full wave rectifier means to which the output of the filter is applied, and threshold means to which the output of the full wave rectifier is applied, the threshold means providing an output signal representative of the occurrence of a QRS wave forming part of an ECG waveform when a predetermined threshold signal is exceeded by the output signal from the full wave rectifier, the calculation means further comprising means operable to inhibit the detection means for a predetermined period following a detected QRS wave, and means for determining the average period between a predetermined period following a detected QRS wave, and means for determining the average period between a predetermined number of immediately previously detected QRS waves to provide the patient's heart rate. By QRS wave is meantthe QRS complex which is a recognised phase of the electrocardiogram resulting from excitation ofthe ventricles.

Preferably, the filter is a bandpass digital filter which in one example may be a seven-bit, 2-pole, bandpass filterwith a centre frequency of 17Hz and a Q of 3.2. The duration of detection inhibition following a detected QRS wave is conveniently 200 ms. The threshold of the threshold means may be set by monitoring the period of detection inhibition to find a peak value, and setting the threshold at half the average of the peaks from a predetermined number of previous peaks. If a QRS wave is not detected for 2.55 s, then the threshold is halved but is not allowed to drop below a predetermined minimum value.

According to a seventh aspect of the present invention there is provided a method of calculating the heart rate of a patient comprising the steps of detecting QRS waves by passing an ECG waveform through a digital filter, then passing the filtered waveform to a full wave rectifier, applying the rectified waveform to threshold means, and producing an output from the threshold means when a predetermined threshold has been exceeded,the threshold output signal being representative of the occurrence of a QRS wave, inhibiting the QRS detection for a predetermined period following a QRS wave detection, and determining the average period between a predetermined number of the immediately previously detected QRS waves.

The various aspects of the present invention as set out above may be combined to provide a multi-functional machine. The monitor may include a printer adapted for connection to second output means, the latter being optoisolated from the printer. The printer may be a real time device.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an ECG monitor in accordance with the present invention, Figure2 is a block circuit diagram of the machine of Figure 1, Figure 3 is a diagrammatic representation of a component of Figure 1 ,and Figures4 and 5are respective enlarged fragmentary views of different portions of Figure 3.

Referring to Figure 1 ,the ECG monitor comprises a casing 1 in which is mounted a liquid crystal display (LCD) 2, and a 2 x 16 character alphanumeric liquid crystal display 3,togetherwith hardware and software components (not shown) which will be described hereinafter with reference to the block circuit diagram of Figure 2. A CONTRAST control 4 is provided forte LCD 2 in order to be able to compensate for changes in intensity ofthe displays due to ambient temperature variations. An ON/OFF switch 5 is also provided. A membrane key pad 6 is also provided on the casing 1, the key pad having fifteen keys 7 to 21.Extending from the casing are seven standard ECG cables or wires for connection to the patient via electrodes (notshown), the leads being marked conventionally as l, ll, IlI,aVR,aVF,aVLandVand providing input means forthe monitor.

The LCD 2 is shown diagrammatically in Figure 3 as having a non-uniform pixel distribution in that a central, lateral band 22 has a greater pixel density than the bands 23 on either side, thus increasing the resolution of the display in the band 22. In the preferred embodiment of the invention,the LCD 2 measures 160mm x 1 03mm and has an active display of 60mm in width and 53mm in length, the band 22 being 1 Omm in height.

As regards pixel distribution, Figure 4 indicates the size and spacing of the pixels 24 in the band 22, and Figure 4 indicates the size and spacing of the pixels 25 in the bands 23. The selected sizes of the pixels 24and 25 in the illustrated embodiment are such asto provide a resolution in the band 22 oftwicethat in the bands 23. More specificially, the display is made up of 64 (vertical) by 120 (horizontal) pixels with the band 22 having pixel density of 20 x 120 compared with that of 22 x 120 for each of the larger dimensioned bands 23.

Turning nowto Figure 2, the monitorcomprisesan analogue portion and a digital portion.Theanalogue portion comprises a defibrillation protection network 26 to which the ECG electrodes are connected so that this network receives the signalsfrom the ECG electrodes which are in contact with various parts ofthe patient's body. However, it should be noted that the defibrillation protection network may be incorporated within the cable for connection to the patient, ratherthan provided within the monitor. The outputs ofthe network 26 are connected to a switching matrix 27 which buffers the input signals, generates voltages forthe aVR, aVL and aVF leads, and switches a selected pair of signals into an instrumentation amplifier 28.The matrix 27 also provides an output of the common mode voltage on the patient, which voltage is amplified and inverted by a feedback amplifier 29 and fed back to the patient so as to reduce electrical inference.

The output of the instrumentation amplifier 28 is applied to a limiter 30 to counteractthe effect of pulses from a pacemaker, should the patient be fitted with such a device, and then filtered at 31 before being applied to a swtiched gain amplifier 32 which, under processor control (yetto be described) selects the required gain.

The output from the switched gain amplifier32 is applied to a track and hold amplifier33 converter (ADC) 34.

The track and hold amplifier33 is also under processor control. To ensure that the base line of the waveform is in the centre of the display 2 (and hence in the centre ofthe enhanced resolution band 21), the zero level ofthe ADC 34 is set up on a digital-to-analogue converter (DAC) 35. The value written into the DAC 35 is calculated for each gain setting when the monitor is first switched on.

The digital portion of the circuitry is based on a 65C02 microprocessor (CPU) 36with 10K (expandableto 16K) random access memory (RAM) and 8K read only memory (ROM), the memory being indicated at 37. As is conventional, each K of memory refers to 1024 bytes so that 10K = 10240 bytes. The two LCD's 2 and 3 are connected directly to a data bus 38. A parallel input/output and timer chip 39 is used to scan the key pad 6 and control the amplifiers 28,32 and 33 and switching matrix 27. A serial input/output device 41 provides an asynchronous communications interface adapter for printer and modem outputs which are fed via an optoisolator42. A power supply unit 43 provides a5 volt supply for the logic circuitry and a negative voltage rail forthe analogue components.

The alphanumeric LCD 3 is used to display the heart rate of the patient. The heart rate is calculated by analysing the ECG waveform to detect the QRS wave, which occurs once per beat. The displayed heart rate is updated every beat and is calculated from the average of the period between the immediately previous 3 beats. For example, if the period between four QRS waves are designated P1, P2 and P3, where P1, P2 and P3 are measured in seconds, then the heart rate is 180/(P1 + P2 + P3) beats per minute.

The QRS detection is performed by passing the received ECG waveform through a seven-bit, 2-pole, bandpass digital filter (not shown) with a centre frequency of 1 7 Hz and a Qof3.2. The resulting waveform is full-wave rectified and compared with a threshold. When the threshold is exceeded, a QRSwave is deemed to have occurred and further detection is inhibited for 200 ms. The threshold is set by monitoring the 200 ms following a ORS wave to find the peak value. The threshold is set to half the average of the peaks from the previous four QRS detections. If a QRS wave is not detected for 2.55 seconds then the threshold is halved. The threshold is never allowed to drop below a minimum value.

When a waveform is first displayed by the monitor the heart rate display 3 is inhibited to allowthe necessary history to build up.

The ECG monitorofthe illustrated embodiment is a stand alone machine and capturestheselected ECG waveform and displays it on the LCD 2, with the patient's heart rate being displayed on the LCD 3 as well as any error or status information. The waveform appearing on any of the seven leads i, ll, III, aVR, aVF, aVL and V can be selected at a sensitivity of 5, 10 or 20 mm/mV. The speed of recording is related to the paper speed in conventional recorders/printers and can be switched between 25 and 50 mm/s. A calibration signal (for example of 1 mv) can be injected into the front end of the instrumentation amplifier 28 to checkthe gain ofthe analogue section ofthe monitor.

The monitor has the capability of recording 90 seconds (expandable to 153 seconds) of ECG data and has the facility to record and review the data and to transmit itto a modem and/or printer. During the recording of a selected ECG waveform, events of interest can be marked to facilitate their reiocation subsequently, and the current screen on the LCD 2 can be arrested to examine the displayed portion of the waveform whilst recording continues. Should the available memory for the waveform be exceeded, then the last 90 seconds of data is kept and previous data lost.

The facility of being able to examine the current screen on the LCD 2 without interrupting recording is enhanced bythe non-uniform pixel distribution in the LCD 2 which provides a greater resolution at the base line of the waveform (i.e. in band 22)where medically significant phenomena occur. The sampling rate is 200 Hz and the signal is sampled every 5 ms but data is reduced using the Turning Point algorithm to obtain a storage display rate of 100 samples per second.

The general operation of the machine given above is augmented by the following description ofthe functions ofthe keys 7 to 21 of the key pad 6: 7 GAIN Selects the gain ofthe ECG amplifier 28 to 5,10Or20 mm/mV. Holding the button down for longerthan 1 second causes the indicated gain to increment. The selection is made when the key is released.

8 LEAD Selects one of the seven standard ECG leads orTwhich places a short circuit across the input of the ECG amplifier 28fortest purposes. Like the gain control, holding the key down increments the indicated lead and selection is made when the key is released.

9 9 OBSERVE Displays the selected waveform but does not record it.

10 RECORD Starts recording the current waveform into memory.

11 STOP Stops any functions and returns monitor to idle state.

12 PAUSE Halts the display but not the recording. Press PAUSE again to display currentwaveform.

13 SPEED Switches the speed from 25 to 50 mm/s or vice versa.

14 MARK Used to mark events of interest during recording to aid relocation using FIND key 15.

15 FIND During playback, will cause the display to centre on a MARK. Subsequent presses will advance the display to the next MARK. The start and end of the recording are permanently marked.

16 RESET Causes the trace to centre in the middle of the display.

17 1 mV Injects a 1 mV calibration signal into the ECG amplifier 28to check gain thereof.

18 & 19 < ii > These two keys are used to scroll backwards and forwards through recorded data.

20 PRINT If operated while observing or recording, the current waveform is sent to a real time printer.

If operated from the idle state, then recorded data is sent to either a real time printer ora standard printer.

21 TRANSMIT Sends the data currently in memory to a modem fortransmission to a remote location.

It will be seen that the present invention provides an ECG monitor which has a number offacilitieswhich make the monitor very versatile and yet still be of an overall size which makes it portable. Thus the monitor can be carried in ambulances, by general practitioners, etc. and the ability to transmit the recorded waveforms to a remote station, such as hospital to which a patient might betaken, is particularly useful. Also the enhanced resolution over a portion ofthe waveform display is a significant advance in the art as well as the facility of being able to arrest the display and mark it (and the recording) for future reference.

Claims (19)

1. A monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, and means for displaying the waveforms, the displaying being in the form of a liquid crystal display.

2. A monitor according to claim 1, wherein the liquid crystal display has a non-uniform pixel distribution, whereby a selective area or areas of the display has or have an enhanced resolution compared with the remainder of the display.

3. A monitor according to claim 2, wherein the selected area is a band extending across the display and arranged to display the base line of the waveforms where medically significant phenomenon occur.

4. A liquid crystal display have a non-uniform pixel distribution, the distribution being such that one or more areas ofthe display has or have an enhanced resolution compared with the remainder of the display.

5. A liquid crystal display according to claim 4, wherein said area is in the form of a band extending across the display and has a resolution twice that ofthe remainder of the display.

6. A monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, meansfordisplaying the waveforms, and means operable to arrestthe display at any given instant whilst the recorder means continues to operate.

7. A monitor according to claim 6, wherein means are provided for marking the recording and/or display, togetherwith means for sensing a mark and arresting the displaythereatwhen the recording is replayed.

8. A monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, means for displaying the waveforms, means operable to mark the recording and/or display of the waveforms, and meansforsensing a mark and arresting the display thereat when the recording is being played.

9. A monitor according to claim 8, wherein the sensing means are operable to sense each successive mark.

10. A monitor according to claim 8 or 9, wherein marks are made at the beginning and end of each recording.

11. A monitor comprising input means adapted for connection to the patient to receive, in use, electrical waveforms therefrom, means for recording the waveforms, means for displaying the waveforms, and first output means from which recorded waveforms are transmitted to a remote station.

12. A monitor according to claim 11,wherein the first outut means is optoisolated from a modem for connection to a telephone line.

13. A monitor comprising input means adapted for connection to a patient to receive, in use, electrical waveforms therefrom, and means for deriving the heart rate of the patient, the means comprising detection means having a filter to which a received ECG waveform is applied, full wave rectifying means to which the output ofthe filter is applied, and threshold means to which the output ofthefull wave rectifier is applied,the threshold means providing an output signal representative of the occurance of a QRS wave forming part of an ECG waveform when a predetermined threshold signal is exceeded by the output signal from thefull wave rectifier, the calculation means further comprising means operable to inhibit the detection means for a predetermined period following a detected QRS wave, and means for determining the average period between a predetermined number of immediately previously detected ORS waves to provide the patient's heart rate.

14. A monitor according to claim 13, wherein the filter is a bandpass digital filter.

15. A monitor according to claim 14, wherein the filter is a seven-bit, 2-pole, bandpass filterwith a centre frequency of 1 7Hz and a Q of 3.2.

16. A monitor according to claim 14 or 15, wherein the QRS wave is 200ms.

17. A method of calculating the heart rate of a patient comprising steps of detecting QRS waves by passing an ECG waveform through a digital filter, passing the filtered waveform to a full wave rectifier, applying the rectified waveform to threshold means, and producing an output from the threshold means when a predetermined threshold has been exceeded, the threshold output signal being representative of the occurrance of a QRS wave, inhibiting the QRS detection for a predetermined period following a QRSwave detection, and determining the average period between a predetermined number of the immediately previously detected QRS waves.

18. A method substantially as herein particularly described with reference to the accompanying drawings.

19. Apparatus substantially as herein particularly described with reference to the accompanying drawings.