GB2077442A - Heart monitor - Google Patents

Abstract

A heart monitor has an output consisting of a audible tone which is frequency modulated according to the sensed heart action current. The amplitude may also be modulated to produce a quiet steady tone between heartbeats. The monitor may have a portable casing 1 on which chest electrodes 2 are mounted. <IMAGE>

Description

SPECIFICATION Heart monitor This invention relates to devices for monitoring the heart of a patient.

At present, physicians in general have to rely on stethoscopes. Conventional electronic heart monitors are too expensive for general use.

Consequently, in most cases, when a patient is dangerously ill the physician has no guidance, except what is available by use of a stethoscope and feeling the patient's pulse. Such indications are of only limited usefulness, because when a patient is seriously ill the pulse may be very weak and there may be no detectable pulse in the limbs.

According to the present invention, a heart monitor comprises electrodes for application to the body (in general the chest) of a patient, and circuitry for converting signals detected by the electrodes into an audio output indicative of the heart action current.

The electrical circuitry preferably comprises means for modulating the frequency of a tone, so that the audio output is a tone of which the frequency depends on the heart action currents.

Alternatively the output may be a pulse train of variable pulse frequency.

The audio output may be amplitude-modulated; however, the ear is more sensitive to small variations in frequency than to small variations in amplitude. Frequency modulation is therefore preferred.

The audio output may be modulated in both frequency and amplitude.

With such a monitor, the physician is freed from the need to feel the pulse or use a stethoscope constantly, and can attend to other functions of the patient while listening to the sound produced by the heart monitor. Conventional heart monitors provide a visual output on an oscilloscope screen, which requires constant observation whereas an audible monitor can be listened to while other things are being done, even in the presence of noise and conversation. Furthermore, audio output elements are small, cheap and light, whereas oscilloscopes are expensive and relatively bulky, so that the present monitor can be made smaller, and very much cheaper, than conventional electronic monitors, so much so that it can be regarded as an instrument for general use, to replace or supplement the stethoscope.

By the use of solid-state electronics and miniature loudspeakers or ear pieces, the present monitor is made easily portable, and therefore, easy to attach to the chest. It can be applied to and left in contact with the patient even when the patient is being transported.

In a preferred arrangement, the heart monitor itself has a casing designed to be applied to the chest of a patient, having one or more built-in electrodes for contact with the skin of the patient. It contains a primary or secondary battery, a voltage-controlled oscillator connected to the electrodes, and a miniature loudspeaker at the output of the oscillator.

Instead of or in addition to the loudspeaker, an earpiece connected by a flexible lead may be provided. Conventional low-pass or band-pass filtering circuitry or balancing circuitry is incorporated to eliminate interference due to mains hum. The entire instrument can be left resting on the patient's chest while the physician does other things.

The human ear is very sensitive to variations in frequency, and will therefore readily detect frequency variations in the monitor output due to quite small changes in the heart action signals detected by the monitor. With some training, users will be able to identify heart arrythmias, and in particular ventricular fibrillation. The accurate detection of fibrillation is essential to enable a decision to use a defibrillator to be made.

The invention will be further described with reference to the accompanying block diagram.

The schematically illustrated heart monitor has a portable casing 1 containing a primary or rechargeable battery and suitable power supply circuitry, which are not shown. The casing is designed so that it can be placed on the chest of a patient. Fixed on the casing are three sensing electrodes 2 placed to make contact with the chest of the patient. Alternatively, one or more electrodes may be separate from the casing and connected to the circuitry therein by suitable leads, or electrodes may be so mounted that they can be used separately, or used when attached to the casing.

The electrodes are connected, in a balanced configuration to eliminate noise, to a pre-amplifier 3.

The output of the pre-amlifier is connected by way of a band-pass filter 4 to signal-generating circuitry.

The filter is to eliminate noise, in particular mains hum, and may for example have a pass band of 0.5 to 20 Hz.

The signal-generating circuitry consists primarily of a voltage-two-frequency converter 5, the output frequency of which depends on the heart action currents sensed by the electrodes. The output signal from the converter is therefore a tone of varying frequency corresponding to the instantaneous heart action current, and is an audible analogue of the trace conventionally displayed on A ECG screen.

This signal is applied, by way of a suitable power stage 6, to a small loudspeaker 7 mounted in the casing, and/or to a separate ear piece. The casing can be made quite small, e.g. a few mm. thick and a few cm. wide (to give the electrodes 2 adequate spacing).

In practice, if the output is simply a cone with a frequency modulated according to the sensed input signals, as described above, the steady cone present between heartbeats may have to be irritatingly loud, in order to ensure a clearly audible signal indicating a heartbeat, and the output cone representing the R wave may not be as distinct as a user would like. In order to produce an output sound in which the modulated cone representing the heartbeat is clearly distinct and emphasised relative to the steady tone between heartbeats, the illustrated signal generating circuitry also includes means for modulating the amplitude of the audio signal. The frequencymodulated signal from the converter circuit 5 passes through a voltage-controlled amplifier 8 forming an automatic gain control circuit.The gain is controlled by the output of a full-wave rectifier 9 the input to which is the pre-amplified and filtered signal from the electrodes.

As a result, as the input at the electrodes deviates from its base level, the sound produced by the monitor is a tone which changes in pitch and increases in volume, producing a very distinct and well-defined output signal. By way of example, the tone may increase in pitch for a positive deviation of the input signal, and decrease in pitch for a negative deviation; the tone becomes louder in proportion to the amount of the positive or negative deviation of the input signal from the base level.

What the user hears is a quiet steady tone between heartbeats, which indicates that the instrument is operating, and repetitive cone variations representing the heart action, of increased amplitude relative to the normal steady tone and of frequency (above or below the steady tone) dependent on the heart action current, that is to say, on the voltage picked up by the sensing electrodes. The heartbeat signal sounds like a brief warbling. Because it directly follows the trend of the heart action currents, the signal heard by the user gives an indication of abnormal heart action. In the case of asystole, only the steady tone is heard. Fibrillation prodUces a very distinctive pattern of relatively rapid tone frequency variation. These abnormalities are immediately obvious even to relatively untrained users. With training, more subtle variations in heart action can be detected in the frequency modulation of the sound heard.

Claims (11)

1. A heart monitor comprising sensing electrodes for application to a patient's body for sensing heart action currents, electrical circuitry connected to the sensing electrodes and adapted to generate a signal having a parameter dependent on the sensed heart action current, and an audio transducer connected to an output of the said circuitry for audibly reproducing the said signal.

2. A heart monitor as claimed in Claim 1 in which the said signal has an amplitude dependent on the sensed heart action current.

3. A heart monitor as claimed in Claim 1 or 2 in which the said signal has a frequency dependent on the sensed heart action current.

4. A heart monitor as claimed in Claim 3 in which the signal is a tone of variable frequency.

5. A heart monitor as claimed in Claim 3 in which the signal is a pulse train of variable pulse frequency.

6. A heart monitor as claimed in Claim 3, 4 or 5, in which the said circuitry includes a voltage-tofrequency convertor coupled to the electrodes.

7. A heart monitor as claimed in Claim 6 in which the said circuitry includes a full-wave rectifier with its input connected to the electrodes, and means re sponsive to the rectifier output for controlling the signal amplitude according to the rectifier output.

8. A heart monitor as claimed in any of the preceding Claims having three electrodes connected in a balanced configuration.

9. A heart monitor as claimed in any of the preceding Claims including a band-pass or low-pass filtering means connected between the electrodes and the signal-generating circuitry.

10. A heart monitor as claimed in any of the preceding Claims comprising a casing containing the monitor circuitry, at least one of the electrodes being mounted on the exterior of the casing for making contact with the skin bf the body.

11. A heart monitor substantially as herein described with reference to the accompanying drawing.