DE2349624A1 - IMPEDANCE PLETHYSMOGRAPH - Google Patents

Description

Manfred Asrican, Instrumentation for "Medicine, Inc. 309 Greenwich Avenue, Greenwich, Connecticut U.S.A.

Impedance plethysmograph

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The invention relates to a plethysmograph _9, in particular an impedance plethysmograph. The invention is particularly suitable for determining the heart rate.

i '. The invention relates to a type of pl ^ thysmograph,

as described in U.S. Patent 3,340,867. In Compliance with this patent specification is in

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Chest of a human or mammal distributes electricity

created by placing electrodes on the neck and lower chest. The electrodes have the shape by ligaments that encircle the neck and rib cage over them An alternating excitation current is applied to the electrodes. This is done either using the same or different electrodes the impedance of the chest is measured to provide information about the heart's activity and especially the heart's output to obtain. In accordance with the above patent, the greater part of the excitation current flows through the '/ lung tissue and not lower by the area specific resistance of the larger thoracic arteries, veins and heart. Accordingly, it is possible to measure the change in blood volume in the lungs and derive the cardiac output from the impedance changes. The impedance plethysmo obtained between the electrodes Graphical waveforms monitor the pulmonary flow as it translates into changes in impedance in the pulmonary Reflecting vascular bed.

In the context of the above patent, the measurement of the electrical impedance changes in the chest takes place during the application of an alternating current (e.g. a 100 kHz current with an effective value of 5.0 mA) between the outside of the Electrodes attached to the chest. This procedure has the advantage of minimal constriction and preparation of the Examined.

In order to determine the cardiac output from Irrroedanzmes- · To recognize sings as correct and possible, one has to understand the responsible for the measured fluctuations.

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reach physiological phenomena. In particular, the course of the output current between the electrodes in the chest.

It has been found that the only convincing theoretical current distribution is that in which the majority of the current goes from the ribbon electrodes into the lung volume and the cardiac blood areas largely avoids. It follows from this current distribution that the measured impedance is an indication for the total movement of blood in the pulmonary Geffe'ßbett is.

• The shape of the curve that you get with two tape electrodes, one on the neck and the other about Attached 2 cm below the attachment of the sword process reflects the pulmonary blood pulsation in the lungs and not the direct ventricular volume change. Equipotential surfaces sketched after chest potential measurements indicate a current flow depending on the blood volume. The greatest density one of one in the midsection The electrode placed on the chest is found at the base of the lung posterior rib cage. These results indicate that in an electrode arrangement with one electrode around

. the neck and a second around the rib cage most of the time of the current flows through the lungs, so that most of the observed impedance curve is caused by the pulsating pulmon ^ lsn volume changes are determined. the comparatively precise determination of cardiac output with the help of these impedance measurements is based on the fact that the

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Indirectly reflects stroke volume of the right ventricle in the pulmonary vascular bed.

In order to eliminate the influence of respiratory and other effects when applying the above arrangement, is to calculate generally the first derivative of the change in impedance of the chest during a Cardiac cycle used. The patient's normal breathing namely produces a periodic fluctuation in the impedance, but its effect - by calculating on the basis the first derivative of the impedance can be minimized. Even when using the first time derivative has however, it has been shown that the best results are obtained if the examined person dies during the time of kessung Holding your breath. For example, the patient may be required to hold his breath for approximately a period of time six heartbeats stops to drift the measurement of the impedance change and the first time derivative of the measured impedance. Of course, this technique presupposes that the patient is conscious' is or somehow otherwise able to follow the instruction to hold his breath during the measurement. is the patient is unable to hold his breath, so great skill is required in interpretation of the measurement curves, one wants to determine the meaning of the measurement curves sensibly. In case of irregular fluctuation In the base or output impedance, the interpretation of the measurement curves can be extremely difficult, if not impossible. It can be extremely difficult to figure out which parts of the measurement curves contain valid results and which parts are to be disregarded as meaningless.

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The invention now provides a device with which these difficulties are overcome by the display an instrument of this type is blocked if the display, for example as a result of breathing., is meaningless is.

In the following the invention will be described in detail with reference to the accompanying drawings. On this is

Fig. 1 is a block diagram showing the basic elements shows an impedance plethysmograph and how it is connected to a human;

Figo 2 is a block diagram of an inventive Plethysmograph;

3 is a diagram showing a section of a measured with an impedance plethysmograph Curve for the change in impedance and its first time derivative for a normal test object shows; and

4 is a diagram of part of a measurement curve on a strip showing the change in impedance and shows its first time derivative for a patient in whom the Fluctuation in the basic impedance interferes with the interpretation of the measurement curve.

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Like Pig. 1 shows, the basic system or the impedance plethysmograph has a current-stabilized oscillator which sB supplies a sinusoidal alternating current of U mA rms current strength and 100 kHz that is kept constant. The output of the constant current oscillator 10 lies between an electrode attached to the upper part of the patient's neck and an electrode 12 further down the body. The system also has a voltage pick-up and detection circuit 130, the input of which is between an electrode 14 on the lower part of the patient's neck and an electrode 15 located slightly below the appendage of the sword process of the patient. The electrodes can. be applied around the patient's body, for example.

The stabilized current coming from the oscillator Io passes through the patient's chest in the longitudinal direction between the electrodes 11 and 12. The product of this current and the chest impedance generates a voltage E = IZ _Q between the electrodes 13 and Ik. and detector circuit 130 was added, which has a linear amplifier with high input impedance in its input stage. The circuit 130 also has detection, balancing and calibration circuits and supplies the output _{signals Z Q} , ΔΖ, dz / dt. Z _Q is the basic chest impedance and gives a direct numerical measurement of fluid changes in the chest. ΔΖ represents the change in impedance over the course of a cardiac cycle and is a parameter that is used when measuring peripheral circulation. The time derivative dz / dt is the first derivative of

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ΔΖ, and is the parameter that is used when measuring stroke volume and cardiac output finds.

2 shows a block diagram of a circuit arrangement according to the invention. The circuit comprises a current stabilized sine wave oscillator 10 of conventional type connected to electrodes 11 and 12 as described above. The basic impedance Z of the chest appears, as described above, between the electrodes 13 and 14. The electrode 14 can be grounded, the electrode 13 is connected to an AC voltage amplifier 15 which has a high input resistance. The output of the amplifier 15 is connected to a detector 16. The output of the detector 16 is in turn connected to a digital output unit 17 which, after suitable filtering, supplies a digital measured value corresponding to the base impedance Z. The unit 17 can, for example, have filter circuits for filtering off the DC voltage component in the output signal of the detector 16, followed by an analog-digital converter and a digital display. For a normal subject, the base level displayed by the digital output unit 17 is approximately 25 ohms. The output of the detector 16 can also be applied to a terminal Z _Q for further external use.

The output signal of the detector 16 is also above a switch 21 "to an amplifier 20 and via a gate 23 and a switch 24 to a differentiating and filter circuit 22. The output of amplifier EO

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is applied to a terminal A Z, where an output signal appears which corresponds to the change in impedance Z measured from its base level,

The amplifier 20 can be, for example, a differential amplifier whose second input connected to the oscillator 10 via a potentiometer 30, an amplifier 31, a detector 32 and a switch 33 is. With this arrangement, the impedance of the potentiometer 30 can be set to a value which corresponds to the The basic value corresponds to the impedance of the chest, so that the output of the amplifier 20 reflects the changes in the Represents chest impedance based on this level. An example of an arrangement of this general type is in U.S. Patent 3,3,0,867. According to In another solution, the amplifier 20 can have a sample and hold circuit with automatic reset have to match the desired output signal to deliver the impedance difference. The differentiating and filtering circuit 22 is of conventional design and provides an output signal at terminal dz / dt, which the corresponds to the first derivative according to the time of the impedance.

Furthermore, the circuit shown in Fig. 2 has a calibration generator ^ O, which calibration signals for the Outputs ΔΖ and dz / dt generated. The type of calibration signal is described in more detail in the following sections. An output signal from the calibration generator ilO is applied to the amplifier 20 via switches 21 and 33, while a second output signal of the calibration generator

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is input into the differentiating and filtering circuit 22 via a switch 2k.

The circuitry described above, with the exception of gate 23, is of conventional design and has already been implemented used in the past to measure chest impedance,

FIG. 3 shows part of the measurement curves on a strip for the output variables dz / dt and & Z of the arrangement from FIG. 2 for a normal test person. _{Curve 50, which speaks of the change in impedance Z Q} calculated from a base value, is shown with a constant base line, ie without the influence of respiratory or other effects. The vertically extending sections 51 of this curve correspond to the maximum drop in impedance during systole (contraction of the heart chambers). The pulmonary flow rate is a function of the rate of change of the impedance and thus the slope of the sections 51 of the curve 50. This slope is determined by differentiating the impedance Z, for example in the circuit 22 of FIG. 2, the curve shape 53 being obtained. The height of the extremes 5 ¹ I, measured from a calibration base line 55, thus corresponds to the pulmonary flow rate.

The curves 50 and 53 are typical curves obtained when using the instrument. These curves are shown to the left of a vertical line 56, which marks an arbitrary point in time at which the switches 21, 24 and 33 from their working position shown in FIG.

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position can be switched to the output of the calibration generator 40. As shown to the right of line 56, if the calibration signal for the Δ Z amplifier exists, which generally comes unchanged in form from its exit, from a series of impulses 57, which have rising edges with a calibrated slope. Since the normal variation in impedance is on the order of 0.1 ohms, the pulses 57 Amplitudes with corresponding values. The calibration signal determines a baseline 59, which is the baseline impedance is equivalent to. Similar pulses as the pulses 57 are applied to a differentiating and filtering element 22, the output of this circuit having a waveform 6l with pulses 62 whose Height of the slope of the leading edges of the pulses 57 is equivalent to. For example, the height of the impulses can change the rate of change of the impedance by 1 ohm per second. The curve 61 also determines the base line 55 of the curve 53.

As stated above, when taking measurements with an impedance plethystnograph, the Patient asked to take his breath, for example six heartbeats to stop so that fluctuations in the output impedance, for example as a result of breathing, do not interfere with the measurement result. In the presence of breathing, that is corresponding to the change in impedance Curve 50 superimposed on a periodically fluctuating base, which impedance changes may correspond to the greater are than those resulting from pulmonary flow. As a result, the slope corresponds to Change in impedance during each systole, and thus the level of

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Pulses 5 ¹ J ₉ do not exactly reflect the pulmonary flow rate. Although this effect can be minimized if the patient holds his breath, a certain degree of interpretation of the curve is still necessary in order to determine which of the pulses 5 * 1 exactly indicates the flow rate; This technique can of course only be carried out if the patient is conscious and can follow the instructions to hold his breath; however, it is always not possible if the patient is unable to do so. In these cases it was necessary in the past to interpret the measurement results with great skill in order to obtain a correct statement about the flow rate. In some cases, especially with irregular fluctuations in the baseline, it was impossible to obtain a statement about the pulmonary flow rate with sufficient accuracy. A measuring strip with a curve for an abnormal patient is shown in FIG. Here, the curve 50, which corresponds to the change in impedance, is superimposed on a baseline of unknown and irregular fluctuation, so that it is difficult to visually pick out those portions of the curve which correspond to a systole. Because of the fluctuations in the slopes of curve 50 resulting from the fluctuation of the baseline, curve 53 therefore shows peaks 5 ¹ J with widely differing heights. The selection of tips 54 that correspond to the actual pulmonary flow rate is consequently difficult.

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To overcome this problem are under the Invention means are provided which suppress the output of measured values for the signal corresponding to dz / dt, whenever this signal has a meaningless value «· Specifically, the signal is difficult to interpret, as soon as the baseline on which the impedance difference Λ Ζ is superimposed exceeds a certain value. Therefore, in accordance with the invention, a threshold circuit 65 (see FIG. 2) is provided which is connected to the output of the amplifier 20 and emits an output signal as soon as the output signal of the amplifier exceeds a certain value. The circuit 65 can, for example, be a Schmitt trigger be. Since it is desirable to suppress the dz / dt signal for the entire beat, the output signal can the threshold circuit 65 to a gate generator 66, for example a monostable multivibrator. The output signal of the gate generator 66 arrives as a control signal to the gate 23 and blocks this in the event of the occurrence of a specific one Vert exceeding signal at the input of the threshold value circuit. So every time the baseline has fluctuated by such a value that the output signal of the Differentiating and filtering circuit 22 or meaningless would be difficult to interpret, the output signal will be blocked by gate 23 so that the output signal only appears as a straight line. You then only get an output of measured values that are so accurate that that they can be subjected to a correct interpretation without difficulty.

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In summary, the invention provides an impedance plethysmograph with an oscillator through which a current is constantly applied to the patient's chest will. A detector circuit connected to the patient provides an output signal which the Corresponds to the impedance of the chest. Circuits that respond to the measured impedance provide a Output signal corresponding to the change in impedance and the first time derivative of the impedance, where Means are provided which block the signal corresponding to the derivative when it, for example as a result of breathing, is falsified and therefore without a statement.

- patent claims -

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Claims (6)

-Patent claims Plethysmograph with a device for outputting a first signal which corresponds to the changes in a measured impedance and a second device for outputting a second signal corresponding to the first time derivative of the measured impedance, thereby characterized in that it includes means responsive to the first signal and the second signal suppressed when the first signal exceeds a certain value. 2. Plethysmograph for measuring cardiac output with a Source for generating a stabilized alternating current, which is set up for use in a mammal or human, with an amplifier and detector, which to connect with the chest area of the subject to be examined and which an impedance signal corresponding to the chest impedance generate, further comprising a device which is responsive to this impedance signal and a first, the Change in the chest impedance corresponding signal generated, and with another device which is responsive to the impedance signal and a second signal corresponding to the first time derivative of the impedance , characterized in that it comprises a device responsive to the amplitude of the first signal to block the second signal (23.65 ^ 66). 3. Plethysmograph according to claim 2, characterized in that the device for generating the second signal a differentiating circuit (22) contains that it has a gate (23) via which the output of the 409846/0658 stronger (15) and detector (16) existing unit is placed on the differentiating circuit (22), and that the device (23,65,66) for / blocking the second signal comprises a device (65 »66) which is responsive to the first signal and for blocking des (Sates (23) is switched. 4. Plethysmograph according to claim 3, characterized in that that the device (65 »66) for blocking the gate switched one for receiving the first signal Threshold circuit and a device (66) for applying the output signal of the threshold circuit (65) to the gate (23). 5. Plethysmograph according to claim ¹ J, characterized in that the device (66) for applying the output signal of the threshold circuit (65) to the gate (23) defines the state of the gate over a predetermined period of time so that the second signal for this period is blocked when the first signal exceeds a certain value. 6. Plethysmograph according to claim M, characterized in that that the threshold value circuit (65) has an output signal emits when the first signal at its input exceeds a certain value that the duration of the output signal is equal to the length of time for which the first signal exceeds the certain value and that the device (66) for applying the output signal of the threshold value circuit to the gate (23) by an optionally adjustable Time interval longer applies a signal to the gate and this blocks than the first signal at the input the threshold value circuit has exceeded the specific value. October 2, 1973 / 955d 409846/0658