US 3565058 A
Description (OCR text may contain errors)
United States Patent  Inventor Peter B. Mansfield 3,064,641 11/1962 Manenti et a1 128/2.1 Needharn, Mas. (Massachusetts General 3,199,508 8/1965 Roth 128/2.06 Hospital Boston, Mass. 02114) 3,426,150 2/1969 Tygart... 128/2.06X E 1 gr p;- 32 1 3 FOREIGN PATENTS 1e Patented Feb. 1971 1,008,027 10/1965 Great Britain l28/2.05
Primary ExaminerAnton O. Oechsle Attorney Charles Hieken  MONITORING APPARATUS WITH AUDIO OUTPUT FRE UENCY RESPONSIVE TO EKG SIGNAL AMPSITUDE ABSTRACT: An electrocardiograph tone output signal is pro- 4 Claims, 2 Drawing Figs vided so that the audible frequency of the tone is representative of the amplitude of the EKG signal. The EKG signal is -(2' 128/2-06 sensed by conventional electrodes, amplified and then modu-  5/04 lates the frequency of an audible signal centered about 1,500  Field of Search 128/2.05, cycles so h a i i EKG ig al increases the frequency of 2-06, the audio signal. This frequency-modulated tone is  References Cited reproduced by a loudspeaker so that the medical personnel may concentrate their eyes on the operat1on or other medical UNITED STATES PATENTS procedure while continuously monitoring the EKG with their 2,712,975 7/1955 Golseth et a1....; 128/2.06X ears. A typical unit provides a frequency deviation of from 2,815,748 12/1957 Boucke l28/2.05(T) 100 to 8,000 cycles.
AUDIO FREQUENCY SOU RCE 4 l2 ,L i V F EK6 REQUENCY MODULATING 1 u CIRC UIT I3 I VARIABLE VARIABLE GAIN (LOUDNESS) MONITORING APPARATUS WITH AUDIO OUTPUT FREQUENCY RESPONSIVE T EKG SIGNAL AMPLITUDE BACKGROUND OF THE INVENTION The present invention relates in general to monitoring electrocardiographic signals and more particularly concerns novel methods and means for aurally monitoring EKG signals to thereby enable medical personnel to concentrate their vision on the surgical or other medical procedure while continuously monitoring the cardiac state of the patient with their ears. A preferred embodiment of the invention is portable, of pocket size and powered by a battery that needs replacement about once a year despite. routine and daily use of the invention.
Prior-art techniques for monitoring electrocardiographic signals require visible inspection of recordings from devices which are not truly portable. It need hardly be stated that medical personnel concentrating on a delicate operation are unable to monitor an EKG signal while operating. As a result, the operating medical personnel must rely on another to report from time-to-timeon the state of the EKG signal being visually presented. The continuous exchange of commands and responses limits the time in which the monitor of the EKG visual signal can make reports.
Accordingly, it is an important object of this invention to provide techniques for continuously monitoring an EKG signal while allowing the interested person to continue to use his eyes in connection with a medical procedure on the patient whose EKG signal is being continuously monitored. 1
It is another object of the invention to achieve the preceding object with an audible signal.
It is a further object of the invention to achieve the preceding objects with a tone whose pitch is varied in accordance with the amplitude of the EKG signal.
SUMMARY OF THE INVENTION According to the invention, there is an input terminal for receiving an EKG signal, and a source of an audiofrequency signal whose frequency is modulated in accordance with the amplitude on the EKG input terminal to provide a frequencymodulated tone that is transduced into an audible signal of pitch representative of the amplitude of the EKG signal.
BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a block diagram illustrating the logical arrangement of a system according to the invention. An EKG amplifier I1, typically comprising a differential amplifier with high common mode rejection receives an EKG. signal developed between terminals 12 and 13 derived from corresponding electrodes attached to a patient. The amplified signal from EKG. amplifier ll modulates the frequency of the audiofrequency signal provided by audiofrequency signal source 14 in frequency-modulating circuit 15 to provide a frequency-modulated tone for amplification by audioamplifier 16 that is converted to an acoustical signal by loudspeaker 17 of tone representative of the instantaneous amplitude of the potential difference between input terminals 12 and 13. The invention takes advantage of the ability of the human ear to detect slight changes in-pitch so that the invention facilitates aural monitoring over an exceptionally wide dynamic range.- This monitoring is continuous so that a surgeon performing a delicate operation is instantaneously apprised of any change in the cardiac state of the patient. This allows the surgeon to take almost immediate corrective action when the EKG signal being monitored discloses onset of a dangerous condition.
Referring to FIG. 2, there is shown a schematic circuit diagram of a preferred embodiment of the invention. Since the schematic circuit diagram with specific parameter values set forth enables one skilled in the art to build a preferred embodiment of the invention, this circuit will not be discussed in detail. Transistors, Q1, Q2, Q3, Q4 and Q5 and associated circuit components comprise a high gain differential amplifier with high common mode rejection and high rejection of noise components for providing a signal that is further amplified by transistors Q7 and Q8 to providea modulating signal of amplitude controllable by the setting of potentiometer R12 for providing a modulating signal on the collector of transistor 08. Transistor Q9'and associated circuitry comprise frequen cy modulating circuit 15 with unijunction transistor Q10 and associated circuitry comprising a relaxation oscillator that oscillates at a frequency determined'by the potential on the emitter of transistor Q9 which in turn is related to the amplitude of the EKG signal.
Advantages of the invention will be better understood from the following discussion of functions of specific circuit elements:
Diodes CR1 and CR2 function as a limiter to prevent overloading and thereby allow prompt recovery of normal operation following an abnormally strong input signal. Transistors Q1 and 02 actually function as emitter followers that isolate the input terminals from the following amplification stages. Transistor Q6 functions as an emitter follower for energizing a monitor output terminal that may be connected to a graphic recorder or other device for providing a visual representation of the EKG signal bearing the usual EKG spectral components.
Transistor Q9 and associated circuitry comprises a constant-current source. The gain of this current source is determined by the resistive networks connected to the emitter by an associated conducting one variation diodes CR4-CR6. As the emitter becomes more negative, more of these diodes conduct. This arrangement allows the nominal center frequency to be placed at a convenient value and have tonal variations as a function of EKG signal best suited to facilitate aural discrimination. The human ear can detect a smaller absolute change in pitch at low frequencies. Thus, a change of frequency of 50 Hz. at a base frequency of 300 Hz. is much easier to discriminate than the same change at 8,000 Hz. It is therefore advantageous to alter the current variation as a nonlinear function of input signal amplitude provided by transistor 09 so that smaller absolute changes in pitch occur at lower frequencies for a given change in input signal amplitude. It has been found advantageous to choose a base frequency of 1,500 Hz. to correspond to the EKG signal of substantially zero amplitude.
NPN transistor Q11 functions as an emitter follower to isolate and prevent the output amplifier from influencing the oscillator rate. The frequency of oscillator comprising unijunction transistor Q10 and associated circuitry is related to the current provided by the transistor Q9 current source and capacitor C8. A tone is then developed across volume control potentiometer R29 of frequency representative of the EKG. between terminals 12 transistors 13 for amplification by the audioamplifier. This audioamplifier comprises transistors O12, Q13 and Q14 and associated circuit components.
The specific circuitry and component values shown in FIG.
2 have been chosen to heavily filter the EKG signal that is ap-' plied to the modulating circuit and conventionally filterthe signal applied to the monitor output to be used for visual readout. It has been determined through experimental use with the invention that certain frequency components of the EKG signal do not contribute to the aural signal analysis. First,
frequencies high enough to be near the audiofrequency range over which the emitted tone may vary has little time to infiuence the frequency of the tone. In fact, it has been discovered that frequencies greater than 2030 Hz. contribute negligible information to the aural analysis. By designing the amplifier so as to sharply attenuate higher spectral components normally the EKG. signal that is applied to the modulator, there is adequate information transmission while improving amplifier stability, increasing signal-to-noise ratio and rejecting undesired 60 cycle frequency components. At the same time the EKG-monitoring output receives all the usual spectral components present in the EKG signal normally directly recorded or otherwise visually displayed.
Although the QRS complex amplitude may not be quantitatively measurable by aural analysis alone with the invention at the high gains necessary to interpret P, ST and T wave changes, because the bandwidth limitations of the amplifier reduce the QRS complex amplitude to approximately 2-4 times the maximum T wave amplitude found in limb leads, the gain of the amplifier may be increased so that T wave or T wave complexes can swing the tone over its complete range of frequencies. At these high gain levels the QRS complex is clipped by the limiting circuit at the input. Although no amplitude component can be identified by ear, still the duration may be heard.
QRS amplitude can be determined with the invention at low EKG gains, although the P, ST and T wave amplitudes are reduced. The vector-cardiographic direction of the means QRS vector can be determined by using the two input leads to determine the null axis for the QRS signal (minimum QRS amplitude). The mean vector direction is then at right angles to this null axis. Subsequent movement of one lead determines positive and negative charge distribution in the frontal plane. A similar technique can be used to determine the AP plane QRS axis. At higher gains the T wave axis can be similarly determined.
The invention has been exceptionally useful during the period of experimental use. A common application has been for EKG monitoring during anesthesia for cases which would not normally have such monitoring with conventional EKG equipment. Multiple arrythmias, including potentially fatal ones, have been rapidly recognized, allowing immediate institution of appropriate therapy.
During long surgical procedures, it has been found advantageous to keep the volume low so as not to tire the anesthetist. Pattern changes changes were immediately recognized, and interpretation could then be made by increasing the volume.
The invention has also been useful during cardiac resuscitation efforts. In these and other emergency situations, physicians have found that pausing to visually assess EKG variations often reduced the effectiveness of their resuscitative measures. The invention allowed them to concentrate on the patient, could be left on during defibrillation pulses, gave them instantaneous information, and, since the device is small enough to be carried on their person, was always immediately available.
The invention also has a number of special uses. Because the unit is powered from its own battery power source, it avoids possible fibrillating currents when attached to electrodes in direct contact with the heart. Intraventricular electrode placement may be accomplished without fluoroscopy, and pericardiocentesis needles monitored by the physician placing the needle without diverting his attention or requiring relay of messages from someone else inspecting a visual AC- powered EKG readout. Arrythmias can be monitored directly by the cardiac catheter manipulator allowing rapid cessation of arrythmia inducing manipulations.
A preferred version of the invention also includes an earphone jack that preferably disables the loudspeaker when an earphone plug is inserted. A cardiac surgeon may use the earphone monitor when suturing in the region of the bundle of His, to avoid further suturing in areas which lead to QRS widening or complete heart block.
An actual embodiment of the invention conforming essentially to the schematic circuit diagram of FIG. 2, is preferably encased in a plastic container about the size of a package of cigarettes to produce a tone that may vary from Hz. to 8,000 Hz. centered about 1,500 Hz. Plastic is preferred to avoid inadvertent grounding or coupling to other power sources in the area, either directly or capacitively. The unit can be moved and handled while in use without interfering with the signals involved. Although the unit may contain a recessed jack for a ground connection, it is rarely needed because the battery-powered unit floats at the charge or voltage level of the patient. When other equipment which requires grounding is used on the patient, the ground lead may in some cases be needed.
The connections to the patient are leads which plug into terminals 12 and 13 and adapt to all the common types of electrodes used for EKG. monitoring. The input circuitry is so arranged that signals can be detected from even high-impedance sources, and the overpotentials generated from some of the less sophisticated pickup electrodes still do not interfere with the functioning of the unit.
Tum-on time for the unit is less than 2 seconds. Recovery from overload is less than 1 second, an important consideration when monitoring during cardiac resuscitation. The diodelimiting circuit at the input allows the units to be left on even during defibrillating pulses so that effectiveness of the pulse can be assessed within 1 second following the application of the pulse.
Still another advantage of the invention is that the frequency-modulated audio signal may be recorded on magnetic tape or transmitted via telephone line.
In addition to other uses mentioned above, the invention may be used for establishing legal death and for the diagnosis of syncope and the comotose patient. The portability of the invention makes it especially convenient for use at the scene of emergencies.
The specific embodiment described herein is by way of example only for disclosing the best mode now contemplated for practicing the invention. It is evident that those skilled in the art may use numerous different kinds of oscillators, modulating schemes and schemes for producing the frequency-modulated aural signal within the principles of the invention. It is evident that numerous uses and modifications of and departures from the specific embodiments described herein may now be practiced by those skilled in the art without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and to be limited solely by the spirit and scope of the appended claims.
l. Electrocardiographic apparatus comprising:
means defining an input for receiving an EKG signal from a patient whose condition is to be monitored;
a source of an audio frequency signal;
means responsive to the EKG signal on said input for modulating said audiofrequency signal so that the frequency thereof is representative of the amplitude of the EKG signal at said input; transducing means for providing an audible tone of frequency corresponding to that of said audiofrequency signal;
means for coupling the modulated audiofrequency signal to said transducing means to provide an audible tone of pitch representative of the EKG signal applied to said input; and means responsive to the EKG signal on said input for effecting an incremental change in said audiofrequency in response to a prescribed incremental change in EKG signal amplitude that is greater when changing from a first audiofrequency which than when changing from a second audiofrequency which first audiofrequency is greater by at least a predetermined amount than said second audiofrequency.
2. Electrocardiographic apparatus in accordance with claim 1 wherein said means for modulating includes means for varying said audiofrequency over the range from substantially 100 to 8,000 l-lz. about a quiescent frequency of substantially 1,500 Hz. 1
3. Electrocardiographic apparatus-in accordance with claim 1 wherein said means responsive .to the EKG signal on said 5 input includes a means for attenuating spectral components of the EKG signal above a predetermined frequency which predetermined frequency is below powerline frequency and high enough to pass spectral components meaningful for aural analysis of said EKG signal and said predetermined frequency is within the range of substantially -30 Hz.
4. Electrocardiographic apparatus in accordance with claim 1 wherein said source of an audiofrequency signal comprises:
oscillatory circuit means defining a relaxation oscillator;
said oscillatory circuit means including a capacitor whose rate of charging is related to said audiofrequency;
said means for modulating includes modulating circuit means defining a current source for providing a current of amplitude related by a gain factor to that of the EKG signal on said input for charging said capacitor at a rate related to the current amplitude; and
said modulating circuit means including means responsive to the EKG signal amplitude for effecting changes in said gain factor.