GB1568557A - Cardia frequency meter - Google Patents

Description

(54) CARDIAC FREQUENCY METER (71) We, ASSISTANCE TECH NIQUE MEDICALE SERDAL S.A., a Society' Anonyme organised under the laws of France of Zone Industrielle de Coignieres-Maurepas, B6ite Postale No.

45-78310-Maurepas, France, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a cardiac frequency meter especially applicable for the surveillance of subjects who have implanted cardiac stimulators.

Known cardiac frequency meters determine the heartbeat of patients from the R wave of the electrocardiogram. This does not give compete satisfaction when it is necessary to survey a patient with an implanted cardiac stimulator. Indeed, the R wave of the electrocardiogram and the terminal phase of the stimulating pulse are very similar and it is difficult, indeed impossible, to distinguish them by an electronic filter. Thus in certain cases, the cardiac frequency meter indicates the fixed frequency of the cardiac stimulator and not the true frequency of the patient's heart.

Certain extreme cases have even been encountered where the cardiac frequency meter indicated a normal cardiac frequency when the heart of the patient had stopped.

The object of the present invention is to avoid this drawback by producing a cardiac frequency meter of improved dependability due to the fact that it counts the cardiac rhythm against a phenomenon of mechanical origin and not electrical, this is to say the contraction of the myocardium shown by the variation of the associated impedance.

According to the present invention there is provided a cardiac frequency meter comprising four electrodes for placement on the chest of the subject to be surveyed, namely two electrodes connected to a high frequency generator for feeding a high frequency current, and, for placement between these feed electrodes two exploratory electrodes connected to a channel for detecting, filtering and forming the rheocardiographic signal, this channel being connected to a pulse counter, there being provided a channel for determining variations of pulmonary impedance connected to the two feed electrodes and delivering a pulmonary rheographic signal and, in the detecting channel, a combining stage in which the pulmonary rheographic signal is subtracted from the rheographic signal.

The cardiac frequency meter according to the invention provides a dependability of counting which is substantially superior to that of known cardiac frequency meters triggered by the wave R of the electrocardiogram.

With non-stimulated subjects, the cardiac frequency meters according to the invention assure a precision of counting wholly comparable with that given by known cardiac frequency meters.

An example of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a block diagram of a cardiac frequency meter according to the invention Fig. 2 is a diagram showing the positions of various electrodes on the body of a subject; and Fig. 3 is an electrical circuit diagram showing the equivalent thoracic and cardiac impedances.

The cardiac frequency meter according to the invention works on the principle that the cardiac rheograph consists of the registration of the variations in impedance of the heart during its contractions. The cardiac rheograph consists of the feeding of a current of constant predetermined frequency, by means of two electrodes I" 12, adhered to the chest of a patient under surveillance. These electrodes I" 12, are connected respectively, by the intermediary of limited resistances R, and R2 (having, for example a value of 2.7k), to two output terminals of a generator 1 supplying an alternating voltage of 25 volts between these terminals. An alternating voltage modified by the variations of cardiac impedance associated with each contraction is recovered between two further electrodes or exploratory electrodes C, and C2 which are also adhered to the chest of the patient under surveillance between the feeding electrodes I, and 12. Figure 2 shows the best position which has been determined for the four electrodes, namely along a line merging with the electrical axis of the heart. The high frequency current supplied by the generator 1 is delivered by the electrodes I, and 12, the electrode I, being placed on the right half of the chest, level with the seventh rib, while the other feeding electrode I2 is placed on the left half of the chest, level with the tenth adjacent rib. The modulated signal is recovered by the exploratory electrodes C, and C2, the electrode C, being located on the sternum at the point of intersection with the axis I" 12, which the electrode C2 is located next to the electrode C, on the left half of the chest level with the ninth rib.

Moreover, a further electrode of mass M is used, the emplacement of which is not critical.

The cardiac frequency meter according to the invention uses a rheographic process with four electrodes because this process has the advantage of attenuating the effects of the impedance of tissue as well as the variations in impedance owing to the absence of contact between the electrodes and the skin, for example during movement of the patient.

The two explanatory electrodes C, and C2 are connected respectively to two inputs of a differential amplifier 2 which is connected to the input of a filter-cell 3 which eliminates the wave of the electrocardiogram, indicated briefly as ECG, as well as the stimulation pulse S of the cardiac stimulator. The signal thus modulated and filtered is fed to a detection stage 4, comprising a diode, this stage supplying the cardiac rheograph modulating the high frequency signal.

The high frequency signal collected by the exploratory electrodes C, and C2 is also modulated by the variations in the pulmonary impedance linked to the respiration of the subject.

The equivalent circuit diagram shown in Figure 3 shows that between the feeding electrodes I, and I2 is located, in series, an impedance P, of the skin tissue and of that between the skin and the contacting electrode 11, a total thoracic impedance of Zt, and an impedance P2, of skin tissue and of that between the contacting electrode I2 and the skin. The total thoracic impedance Zt, is altered by a cardiac impedance Z2 which is found between the exploratory electrodes C, and C2 with the impedance P2 and P4 corresponding to the skin tissue and to the contact impedance between the electrodes and the skin, and one or other side of the impedance Z2 by the impedances Z,/2, Z, being defined by Z1=Zt7Z2 The pulmonary variation in impedance, essentially represented by Z1, have risetimes comparable to those variations of the cardiac impedance. Also, it is difficult to eliminate them by using a simple electronic filter.

The variations of pulmonary impedance are eliminated by means of a channel, designated in this assembly by 5, this channel comprises a differential amplifier with an input 6 of high input impedance (IM). The two inputs of this amplifer are connected respectively to the electrodes I, and 12, collecting the rheographic pulmonary signal. The amplifier input 6 is connected to a filter 7 eliminating the electrocardiogram and the stimulation pulse, the output of this latter being in turn connected to a detection stage 8 for the rheographic pulmonary signal RP. The channel comprises an attenuator 9, for bringing the signal thus detected to an identical level to that which is collected between the exploratory electrodes C, and C2. The output signal of the attenuator 9 is fed to an inverter 10, and transmitted across a high frequency filter 11 to an input of a recombination stage 12. This stage receives, at another input, the signal supplied by the detection stage 4, after its passage across another high frequency filter 13.

The recombination stage 12 assures the elimination, by subtraction, of the pulmonary rheographic signal RP, so that RCG signal corresponding to the cardiac rheogram leaves the recombination stage 12 with the pulmonary signal removed. It is transmitted to a high pass filter 14, with a Butterworth response, in order to eliminate any final residue of the pulmonary signal.

The movements of the patient, also coughing, cause displacement of the electrodes, displacement which are transformed into variations in aleatory impedances, of high amplitude and with rapid rise times.

In order to attenuate these, the cardiac rheographic signal outputting from the filter 14 is fed through a low-pass filter 15.

It also passes across a filter 16 designed to eliminate the 50Hz frequency. The cardiac rheographic signal thus filtered is then amplified by an amplifier 17 then taken in the form of a square wave into a stage 18 in order to make uniform the signals before effecting the counting of the cardiac rhythm.

In order not to count a cardiac contraction, an "artifact" or a parasitic signal which could appear in the stage 18, the cardiac frequency meter comprises an additional channel, designated in the drawings by 19, allowing the establishment of a correlation of the cardiac rheogram with the electrocardiogram ECG on a subject who does not use a cardiac simulator or with the stimulation pulse S on a subject using a cardiac stimulator.

It can be seen that the precision of counting of cardiac rhythm by the cardiac rheogram is improved if one only counts as valid a pulse from the stage 18 if this pulse is the response to a signal which is either the wave R of the electrocardiogram, or the stimulation pulse.

The electrocardiogram ECG is picked-up by the exploratory electrodes C, and C2.

The correlation channel 19 comprises a filter 20 which is connected to the output of the amplifier 2 and which is adapted to eliminate the rheocardiogram and allow to pass the electrocardiogram ECG as well as the stimulation pulse S. The output of this filter is connected to the input of an amplifier 21 which is itself connected to a shaping stage 22. A selection commutator 23 is connected in part, either to the output of the electrocardiogram ECG, or the output of the stimulation pulse S coming from the stage 22, and in part to the input of a correlation stage 24 receiving at its other input the square wave pulse resulting from the output of the stage 18. This correlation stage delivers to its output the validated pulses which are fed to a counter 25 effecting counting of cardiac rhythms in a known manner. This counter is connected to a conventional display apparatus 26 of either analogue or digital type, having the possibility of pre-display bradycardiac and tachycardiac alarms.

WHAT WE CLAIM IS: 1. A cardiac frequency meter, comprising four electrodes for placement on the chest of the subject to be surveyed, namely two electrodes connected to a high frequency signal generator, for feeding a high frequency current and for placement between these feed electrodes two exploratory electrodes connected to a channel for detecting, filtering and forming the rheocardiographic signal, this channel being connected to a pulse counter, there being provided a channel for determining variations of pulmonary impedance connected to the two feed electrodes and delivering a pulmonary rheographic signal, and, in the detecting channel, a combining stage in which the pulmonary rheographic signal is subtracted from the rheographic signal of the detector channel.

2. A meter according to claim 1, in which the determining channel comprises a differential amplifier with a high input impedance, whereof the two inlets are respectively connected to the two feed electrodes, a filter for eliminating the electrocardiographic signal and any stimulating pulse, a stage for detecting the pulmonary rheographic signal, an attenuator, an inverter, and a high frequency filter connected to an inlet of the combining stage.

3. A cardiac frequency meter, comprising four electrodes for placement on the chest of the subject to be surveyed, namely two electrodes connected to a high frequency signal generator for feeding a high frequency current, and, for placement between these electrodes, two exploratory electrodes connected to a channel for detecting, filtering and forming a rheocardiographic signal, this channel being connected to a pulse counter, there being provided a channel establishing a correlation between the rheographic signal and the electrocardiographic signal or the stimulating pulse, this channel including a filter connected to the outlet of a differential amplifier forming part of the detecting channel and whereof the two inputs are respectively connected to the two exploratory electrodes, this filter eliminating the rheocardiographic signal, an amplifier, a shaping stage and a selection commutator connected on the one hand, either to the electrocardiographic signal output, or the stimulating pulse output of the shaping stage, and, on the other hand, to the input of a correlation stage forming part of the detecting channel.

4. A cardiac frequency meter, substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. effecting the counting of the cardiac rhythm. In order not to count a cardiac contraction, an "artifact" or a parasitic signal which could appear in the stage 18, the cardiac frequency meter comprises an additional channel, designated in the drawings by 19, allowing the establishment of a correlation of the cardiac rheogram with the electrocardiogram ECG on a subject who does not use a cardiac simulator or with the stimulation pulse S on a subject using a cardiac stimulator. It can be seen that the precision of counting of cardiac rhythm by the cardiac rheogram is improved if one only counts as valid a pulse from the stage 18 if this pulse is the response to a signal which is either the wave R of the electrocardiogram, or the stimulation pulse. The electrocardiogram ECG is picked-up by the exploratory electrodes C, and C2. The correlation channel 19 comprises a filter 20 which is connected to the output of the amplifier 2 and which is adapted to eliminate the rheocardiogram and allow to pass the electrocardiogram ECG as well as the stimulation pulse S. The output of this filter is connected to the input of an amplifier 21 which is itself connected to a shaping stage 22. A selection commutator 23 is connected in part, either to the output of the electrocardiogram ECG, or the output of the stimulation pulse S coming from the stage 22, and in part to the input of a correlation stage 24 receiving at its other input the square wave pulse resulting from the output of the stage 18. This correlation stage delivers to its output the validated pulses which are fed to a counter 25 effecting counting of cardiac rhythms in a known manner. This counter is connected to a conventional display apparatus 26 of either analogue or digital type, having the possibility of pre-display bradycardiac and tachycardiac alarms. WHAT WE CLAIM IS:

1. A cardiac frequency meter, comprising four electrodes for placement on the chest of the subject to be surveyed, namely two electrodes connected to a high frequency signal generator, for feeding a high frequency current and for placement between these feed electrodes two exploratory electrodes connected to a channel for detecting, filtering and forming the rheocardiographic signal, this channel being connected to a pulse counter, there being provided a channel for determining variations of pulmonary impedance connected to the two feed electrodes and delivering a pulmonary rheographic signal, and, in the detecting channel, a combining stage in which the pulmonary rheographic signal is subtracted from the rheographic signal of the detector channel.

2. A meter according to claim 1, in which the determining channel comprises a differential amplifier with a high input impedance, whereof the two inlets are respectively connected to the two feed electrodes, a filter for eliminating the electrocardiographic signal and any stimulating pulse, a stage for detecting the pulmonary rheographic signal, an attenuator, an inverter, and a high frequency filter connected to an inlet of the combining stage.

3. A cardiac frequency meter, comprising four electrodes for placement on the chest of the subject to be surveyed, namely two electrodes connected to a high frequency signal generator for feeding a high frequency current, and, for placement between these electrodes, two exploratory electrodes connected to a channel for detecting, filtering and forming a rheocardiographic signal, this channel being connected to a pulse counter, there being provided a channel establishing a correlation between the rheographic signal and the electrocardiographic signal or the stimulating pulse, this channel including a filter connected to the outlet of a differential amplifier forming part of the detecting channel and whereof the two inputs are respectively connected to the two exploratory electrodes, this filter eliminating the rheocardiographic signal, an amplifier, a shaping stage and a selection commutator connected on the one hand, either to the electrocardiographic signal output, or the stimulating pulse output of the shaping stage, and, on the other hand, to the input of a correlation stage forming part of the detecting channel.

4. A cardiac frequency meter, substantially as hereinbefore described with reference to the accompanying drawings.