CA1088634A

CA1088634A - Heart rate monitor

Info

Publication number: CA1088634A
Application number: CA284,693A
Authority: CA
Inventors: John M. Adams
Original assignee: Medtronic Inc
Current assignee: Medtronic Inc
Priority date: 1976-08-16
Filing date: 1977-08-15
Publication date: 1980-10-28
Also published as: DE2736751A1; AU510831B2; AU2786177A; GB1579690A; DE2736751B2; DE2736751C3; SE7709186L; SE424138B; JPS5323183A; FR2361857A1; FR2361857B1; CH615582A5; JPS5711211B2

Abstract

HEART RATE MONITOR

Abstract A wrist watch size heart rate monitor coupled with a flexible metal expansion band detects a electrocardiac signal on one arm and applies that detected signal to elec-tronic circuitry within the monitor. Means are also included for receiving a second electrocardiac signal from the other arm to obtain an electrocardiac lead I signal. The receiv-ing means may be a contact member adapted to being placed in physical contact with a second flexible metal expansion band around the wrist of the other arm and having a like contact member associated therewith or the receiving means may be metal members adapted to having the thumb and a finger of the hand of the other arm squeezed thereagainst.

Description

10~863 ...:
This invention relates to cardiac monitoring and more particularly to apparatus for detecting and providing cardiac signals to processing circuitry.
In the prior art, many devices exist which are adapted to be used in measuring the heart rate Or a sub~ect.
The devices vary from sophisticated, computer-controlled apparatus capable of monitoring a plurality of patients simultaneously to small, individual patient units designed to monitor only the heart rate. Of these latter type devices, the maJority accomplish the task of measuring the heart rate by measuring the pulse rate and are implemented in the form of a wrist band with pressure sensitive or sound sensitive transducers affixed to the band and positioned on the wrist adJacent to the palm of the hand.
In addition to the pulse monitoring devices, some small individual devices measure the electrocardiac signal which may be detected from the skin of the subject. This has been done by wires running from the device to electrodes affixed by conventional means to the chest area of the ~. ~
sub~ect. In this manner, a conventional electrocardiac ., .
signal is detected by the electrodes and applied through the ~ ~
wires to the device. The device may be adapted for use by ~ -the subject at several different places of the body. For instance, the device may include a small box-like configur-ation adapted to be worn on the belt of the subject or the device may be in the form of a wrist band adapted for being worn around the wrist. A problem, however, is that the wires leading -from the chest area to the device, wherever it may be placed, are prone to disconnection and breakage as the subject moves, thereby terminating the ability of the device to monitor.
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; The reason that wires have been utilized in measuring the elec-.~ trocardiac activity in the past is that it has been thought that it is nec-, essary to continuously monitor the patient. However, in many instances, it ` is only necessary to take an instantaneous reading of the heart rate. For instance, if a subject has previously suffered a coronary attack and has re-covered sufficiently to lead a normal, or near normal, life with the exception i~ that his heart rate should be limited to a certain maximum, it would only be ~.
necessary to take a measurement of the heart rate during strenuous activity, such as exercising. In this case, a continual monitoring of the cardiac lQ activity of that subject's heart would be wasteful utilization of the power source (battery) powering the device. Of course the prior art apparatus could be modified by placing a simple switch on the device to render it active ,! only when desired by the subject. However, the problem of wires running from the patient's chest area to the area where the device is located, still remains.In accordance with the present invention there is provided a cardiac signal detecter comprising: circuitry for processing a pair of elec-'~ trical signals which manifest the electrocardiac signals taken between dif-.~
:, ferent limbs of a person; housing means containing said circuitry and includ-:
ing electrically conductive material means capable of detecting electric sig-.: .
nals on the skin of such person, said conductive means being adapted to be ,....
positioned in firm contact with one of said limbs, and further being in electri-- cal communication with said circuitry to provide the signals at said one limb . .~
;~ as one of said pair of signals; and receiving means mechanically affixed to ~ said housing means and electrically isolated from said conductive means, said .
receiving means further being in electrical communication with said circuitry - and adapted to receive by physical contact an electric signal from said other - limb and to provide such received signal to said circuitry as the other one of .
said pair of signals.
4 A preferred embodiment of this invention is hereafter described with specific reference being made to the following Figures, in which:

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Figure l shows a pair of wrist band detectors adapted for use with circuitry associated with one of the wrist bands;
Figure 2 shows the face of the housing containing the circuitry for processing the signals detected by the detectors of Figure l;
Figure 3 shows an alternate embodiment of the housing containing the circuitry shown in Figure l; and Figure 4 shows the orientation of Figures 4a and 4b which in turn show, in block format, a circuit diagram for processing the signals detected by the apparatus shown in Figure 1 or Figure 3.
Referring now to Figure 1~ a pair of wrist bands lO and 12 each adapted to be worn on a different wrist of a subject are shown. Wrist band 10 includes the housing 14, a contact member 16 and a pair of flexible metal expansion members 18 and 20, conne~ting housing 14 with contact 16.

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Housing 14 contains the electronic circuitry used in processing the pair of signals representing a lead I electrocardiac lead signal. This circuitry will be described '''`',''''' ~ ' '.',:" :
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'~7` ~08~634 in detail hereafter with respect to Figure 4. As is well-- known in the art, a lead I electrocardiac signal is derived from the left and right hand wrists of the sub~ect and measures the electrical activity of the heart along a plane .;
running generally parallel to the ground as a patient is in an upright position.
Contact member 16 includes a conductive portion 22 and an insulator portion 24 arranged so that conductive portion 22 is not in electrical communication with expansion members lô or 20. A uire 26 or other electrical conducting member connects conductive portion 22 to the circuitry within housing 14.
Expansion members 18 and 20 may be similar to " conventional metal expansion watch bands. However, members , . .
18 and 20 should be of a material capable of detecting electrical signals appearing on the skin when in firm contact with the skin. In addition, members 18 and 20 should be :
constructed to conduct the detected electric signal to the electric circuitry within housing 14, with which circuitry members 18 and 20 are in electrical communication. Expansion members 18 and 20 should also be constructed to be ad~ustable '~ by either deleting or adding links therein so that the entire urist band 10 can be sized to fit firmly around the wrist of the subJect thereby allouing a firm contact between bands 18 and 20 and the skin to achieve good electrical ~
detection of the skin æignals. Further, bands 18 and 20 should be of an adequate width to allow sufficient surface ,:
area for good contact.
Band 12 i6 similar to band 10 with the exception ::
that no housing for electrical circuitry is included and no ,.~
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r- ~0~8634 insulator portion, such as portion 24, is included in contact member 28.
More specifically, band 12 consists of conductive contact member 28 and a flexible metal expansion member 30 of similar design to members 18 and 20.
If desired, member 30 may be divided in half and a conventional wrist watch can be inserted in a manner similar to the inclusion of housing 14 in band 10 .
Band 12 is adapted to be worn on the wrist opposite of band 10.
When it is desired to take the subject's heart rate, contact member 28 is placed in firm contact with contact member 16. In this manner, the electro-cardiac skin signal detected by expansion member 30 from the wrist upon . which band 12 is worn are applied electrically through contact member 28 to the conductive portion 22 of contact member 16. From portion 22, the elec--~ trical signal is applied through wire 26 to the electronic circuitry in housing 14. At the same time, the signal from the other wrist is detected by expansion members 18 and 20 and applied directly to the electric cir-cuitry within housing 14.
: .: : , ~ At this point it should be noted that the exposed surface areas of ~, ~
contact members 16 and 28 are designed to provide a eood electrical connec-tion when physically placed in contact with one another. Such design may include similar size and a mirror image non-flat surfaces.

' Housing 14 also includes a pair of switches 32 and 34. Each of .
switches 32 and 34 includes a pair of push-buttons, one of which will always extend out further than the other. To cause either one of switches 32 or 34 to change states, it is necessary to depress the more exposed button thereof.
Switch 32 is an ON/OFF switch which should only be in the ON state when measuring the heart rate and switch 34 is a 6/60 second selector switch, that is, the rate can be selectively measured by counting R-waves for six seconds and reporting ten times that count as beats per minute or the rate can be measured by counting R-waves for sixty seconds and reporting that - -30 count directly ;B beats per m1n~te. 30th s~itchea 32 ;nd 34 affect the ', ' .. : ' ':
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electronic circuitry within housing 14 in the manner described with respect to Figure 4.
With respect to ON/OFF switch 32, it should be noted that a pres-sure sensitive switch in contact member 16 of Figure 1 or contacts 40 and 42 of Figure 3 could serve the same function with the advantage that the sub-., , ~ect would not forget to turn the unit on or off.
Referring now to Figure 2, the face of housing 14 is shown and in-cludes a three-digit display 36 with each digit consisting of seven illum-inable segments. Selected ones of each of the segments can be illuminated . .
to cause any of the digits O-9 to be displayed in each digit of display 36.
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Display 36 may be any conventional digital display.

Referring now to Figure 3, a face plate 38 is shown which consti-tutes an alternate embodiment to the apparatus shown in Figure 1. The only , difference between housing 38 shown in Figure 3 and housing 14 shown in Fig-,::
~; ures 1 and 2 i s the addition of contact members 40 and 42. ~embers 40 and ~i. .
42 are positioned on the periphery of housing 38 in a position approximately ., ~, 90 degrees from the axis through the flexible members 18 and 20. In utiliz-} ing housing 38, the second watch band 12 apparatus shown in Figure 1 is : eliminated. Rather, the thumb and one of the fingers of the hand remote from th~ limb wearing the monitoring apparatus are placed firmly in contact with contact members 40 and 42. In this manner, the electrocardiac signal present on the skin at the fingertips is transmitted through members 40 and 42 to the circuitry within housing 38. A word of caution with regard to the ::
utilization of the embodiment shown in Figure 3 is that some individuals may have calloused fingertips that could prevent adequate signal transmission :
from the skin to contacts 40 and 42.
Referring now to Figure 4, a block diagram of the circuitry 50 ~- contained within housing 14 is shown. The two signals from the wrists of the sub~ect wearing the apparatus of either Figure 1 or Figure 3 are applied 3 respectively to terminals 52 and 54. Terminal 54 is connected to a point of ., , :' reference potential, such as system ground and terminal 52 is connected to the amplifying input of amplifier 56. The amplified signal from amplifier 56 is passed through a low-pass filter, which may filter out all signals having a frequency above, for instance, 20 hertz to thereby eliminate 60 cycle and muscle noise.
The output of low-pass filter 58 is connected to the input of a voltage threshold sensitive circuit 60, such as a Schmitt trigger. Circuit 60 provides a pulse signal each time the voltage applied thereto exceeds the .
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set threshold voltage there. The threshold voltage of circuit 60 is ad~ust-ed to be responsive only to the R-wave of the electrocardiac signal so as to provide a pulse each time an R-wave occurs. It should be noted that the R-wave of an electrocardiac signal is a positive excursion of the signal to a . .~
: ~ point of maximum amplitude of the signal. The R-wave is normally proceeded . by a smaller magnitude positive excursion known as the P-wave and followed by ~nother smaller magnitude positive excursion known as the T-wave. Al-, though not shown in the Figures herein, a potentiometer ad~ustment to Schmitt .
trigger 60 may be provided on housing 14 to allow the threshold magnitude to be ad~usted so that Schmitt trigger 60 is only triggered by the R-wave and ;~ not the P- or T-waves for the individual patient utilizing the rate monitor-20 ing apparatus.
The output pulse from Schmitt trigger 60 is provided to a "one-shot", or monostable multivibrator, 62, which provides a controlled pulse signal each time an R-wave i8 detected by Schmitt trigger 60. The output from "one-shot" 62 is provided through a refractory circuit 64 back to an - inhibit input of Schmitt trigger 60. Refractory circuit 64 may be simply another "one-shot" circuit which inhibits Schmitt triBger 60 from providing another pulse for a certain time (e.g., the width of the pulse provided therefrom) after the detection of the initial R-wave. Refractory circuit 64 is provided to prevent a large magnitude T-wave from being detected as a iecond W-w~ve or, on the other hand, ln t~e event that a P-wave is detected ,,' "
' - 1~8~634 as an R-wave, refractory circuit 64 prevents the R-wave shortly following the P-wave from being detected as a second R-wave. Again, a potentiometer adjustment (not shown) may be incorporated in housing 14 to allow the re-fractory time to be ad~usted, depending upon the patient's normal heart rate and the maximum rate to which the patient's heart should be allowed to be.
The signal processing circuitry consisting of amplifier 56, filter 58, Schmitt trigger 60, one-shot 62 and refractory circuit 64 is all under the operability control of power supply 66. For clarity, the application of power +V to each of these components is not shown. However, it should be ,;j. 10 understood that this is the case as well as power being supplied to each of the other components described with respect to the block diagram shown in Figure 4. As previously mentioned, the output from power supply 66 is under the control of switch 32 such that when switch 32 is closed power ~V can be applied. However, when switch 32 is open, the power to each of the compon-ents in circuit 50 is cut off and thus the components are non-operative.
. The output from one-shot 62 is a pulse substantially coincident in time with each R-wave of the electrocardiac signal. This pulse is processed by the remainder of the logic circuitry in the manner hereafter described.
The heart of the logic circuitry is the six second timer 68 which provides a pulse every six seconds. 6/60 switch 34 actually is a three-bank switch consisting of switches 34a, 34b and 34c with each switch having a switching arm and a pair of output poles shown schematically in the upper or lower positions in which the switch arm may be placed in. The switch arms of switches 34a and 34b are both connected to the output of the six second timer 68. The upper contact of switch 34a and the lower contact of switch 34b are both disconnected from any other component within the system. The lower contact of switch 34a is connected to a conventional divide by 10 cir-cuit which provides an output pulse after ten input pulses have been pro-vided thereto, or, in the case of circuit 50, every sixty seconds. The up-; 30 per terminal of switch 34b is connected to the output of the divide by ten ., _ 9 _ :`

108~634 circuit 70. Thus, when switch 34 is in the "up" position, divide by ten circuit 70 is bypassed and, a pulse appears at the output of divide by ten . . .
circuit 70 every six seconds and when switch 34 is in the "down" position, a pulse appears at the output of divide by ten circuit 70 every sixty sec-onds.
The upper terminal of switch 34c is connected to +V voltage uhich - herein is equivalent to a logic "1" signal. Connected to the lower terminal of switch 34c is reference, or ground, voltage which herein is equivalent to , , ~, a logic "0" signal.
~ 10 The timing pulse from the output of divide by ten circuit 70, when ,~ switch 34 i8 in the "down" position or from the upper terminal of switch 34b when switch 34 is in the "up" position, iB provided as the input to one-shot circuit 72 to cause a desired uidth pulse to be provided therefrom. The output pulse from one-shot 72 is provided to the enable input to each of twenty-one AND gates 74, which are divided into three groups of seven gates each. The output from one-shot 72 is also connected to the reset input o~
each of three decade counters 76, 78, and 80, with the counter 76 constitut-. ing the least significant digit and counter 80 constituting the most sig-nificant digit. Counters 76, 78 and 80 are designed to respond to the trail-ing edge of the pulse from one-shot 72 to be reset to a count of zero upon its occurrence.
When switch 34c is in the "up" position, +V voltage is applied to the enabled input of A~D gate 82 so that the pulses corresponding to the : .
detected R-waves of the electrocardiac signal appearing at the output of one-shot 62 are applied through AND gate 82 to the input of decade counter 78. When switch 34c is in the "down" position, ground voltage is applied :
through inverter 86, and from there as a ~V voltage, or logic "1" signal, to the enable input of A~D gate 84, so that the pulses corresponding to the detected R-waves are applied through AND gate 84 to the input of decade counter 76.

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10~8634 , The four outputs from each stage of the decade counters 76, 78, and 80 are respectively applied to four-bit to seven-se Bent convertor cir-cuits 88, 90, and 92 which convert a four line binary coded decimal (BCD) signal from coun~ers 76, 78, or 80 to an appropriate seven line signal which .~; will cause a seven se B ent display to display the value of the BCD signal.

Each of the seven outputs from each of the convertor circuits 88, 90, and 92 .:.
~i are applied through the associated AND gate 74 at the time the gate is en-.. . .
abled by the pulse from one-shot 72, to a latch circuit, represented in the -block diagram as one of the latches in the seven latch circuits 96, 98, and 100. In addition, the pulse signal from the output of divide by ten circuit 70 or the upper terminal of switch 34b is applied to the reset inpu~ of each latch in latch circuits 96, 98, and 100. The output signals from each of the latches 96, 98, and 100 are applied respectively to seven-segment dis-play devices 102, 104, and 106.
The operation of circuit 50 will hereinafter be described. First, assuming that switch 34 is in the "up" or six second position, as shown in Figure 4, a pulse will be provided to trigger one-shot 72 every six seconds.
Coincident with each latch in latch circuits 96, 98, and 100 is reset and :
. after one-shot 72 is triggered, decade counters 76, 78, and 80 are reset at the trailing edge of the one-shot 72 pulse. When switch 34 is in the "up"
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position, a +V voltage, or logic "1", is applied to the enable input of AND
gate 82 and a negative voltage, or logic "0", is applied through inverter , 86 to the enable input of A~D gate 84. Thus, A~ gate 82 is enabled and '''' passes the detected R-wave pulses from one-shot 62 to the second stage de-cade counter 78. Each time a pulse is applied to decade counter 78 it is incremented by one. After six seconds, a pulse is applied to clear the latches in latch circuits 96, 98, and 100 and to cause a pulse to be pro-vided from one-shot 72. This pulse enables each of the AND gates 74 to provide the signals stored by counters 76, 78, and 80, as converted by con-vertors 88, 90, and 92, to latches 96, 98, and 100. It should be noted that :

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with switch arm 34c in the "up" position, the least significant stage de-cade counter 76 is bypassed and thus maintains a zero count therein. In ~ this situation, the seven latches in latches 96 are set to cause display i 102 to display a numerical zero as the least significflnt aigit of the dis-play 36, whereby the rate displayed is ten times the number of pulses count-ed during the six second interval. Thus, if six pulses had been provided ; from one-shot 62 during the six second interval, display 104 would have a six appearing thereon and displays 102 and 106 would each have a zero dis-i played thereon or in other words, a heart rate of 60 would be displayed.
On the other hand, if twelve pulses had been counted during the six second interval, decade counter 78 would contain a two and decade counter 80 a one count. There would be applied through in the manner previously described to displays 104 and 106 with display 102 beine zero, whereby a count of 120 would be displayed.
The trailing edge of the pulse from one-shot 72 would then reset each of the counters 76, 78, and 80 and the above process would be repeated ; again until such time as another pulse appeared at the input of one-shot 72 which pulse would clear the latches in latch circuits 96, 98, and 100 and ... .
cause one-shot 72 to provide a pulse to enable gate 74.
20 When switch 34 is placed in the "down" position, or sixty second ; position, it requires ten pulses from the six second timer 68 to be applied through divide by ten circuit 70 before a pulse is applied to one-shot 72.
Between one-shot 72 pulses, pulses provided at the output of one-shot 62 are applied through enabled AND gate 84 to decade counter 76. In this instance, ground voltage from switch 34c is inverted by invertor 86 and then applied to enable A~D gate 84, whereby least significant counter 76 is not bypassed and the count therein is displayed on display 102 in the manner previously described.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cardiac signal detector comprising: circuitry for processing a pair of electrical signals which manifest the electrocardiac signals taken between different limbs of a person; housing means containing said circuitry and including electrically conductive material means capable of detecting electric signals on the skin of such person, said conductive means being adapted to be positioned in firm contact with one of said limbs, and further being in electrical communication with said circuitry to provide the signals at said one limb as one of said pair of signals; and receiving means mechani-cally affixed to said housing means and electrically isolated from said conductive means, said receiving means further being in electrical communica-tion with said circuitry and adapted to receive by physical contact an electric signal from said other limb and to provide such received signal to said circuitry as the other one of said pair of signals.

2. The invention according to claim 1 wherein said receiving means is adapted to receive said electrical signal from said other limb only when a portion of said other limb is physically placed thereon.

3. The invention according to claim 2 wherein said limbs are the arms, and said portion of said other limb is at least one finger thereof.

4. A cardiac signal detector comprising: a housing containing cir-cuitry for processing a pair of electric signals, each one of which manifests the electric signal at a different limb of a person; a band member of an electrically conductive material selected to be capable of detecting electric signals on the skin of such person, said band being affixed to said housing and of a size to allow said housing and band combination to firmly contact one of said limbs, said band further being in electric communication with said circuitry to provide the signal at said one limb as one of said pair of signals; and receiving means mechanically affixed to said housing and band combination and electrically isolated therefrom, said receiving means further being in electrical communication with said circuitry and adapted to receive by physical contact an electric signal from said other limb and to provide such received signal to said circuitry as the other one of said pair of signals.

5. The invention according to claim 4 wherein said receiving means is adapted to have a similar type receiving means placed in physical and electrical contact therewith, said similar type receiving means being affixed to a band firmly contacting said other limb and in electrical contact with said band contacting said other limb.

6. The invention according to claim 5 wherein said receiving means is affixed to said band member.

7. The invention according to claim 4 wherein said receiving means is adapted to have a portion of said other limb placed in contact therewith.

8. The invention according to claim 7 wherein said receiving means is affixed to said case and adapted to have at least one of the thumb and fingers of said other limb placed in contact therewith.

9. The invention according to claim 4 wherein said housing further includes display means in electrical communication with said circuitry for displaying the heart rate of said person.

10. The invention according to claim 9 wherein said circuitry detects and counts individual heartbeats of said person as said heartbeats are mani-fested by the signals applied to said circuitry, and provides a signal to said display means to cause said heart rate to be displayed.

11. The invention according to claim 10: wherein said detector further comprises switch means for selecting the time during which said detected beats are counted; wherein said circuitry includes timing means responsive to the state of said switch means for providing timing signals to cause said detected beats to be counted for said selected time; and wherein said circuitry further includes means responsive to said timing signals and the counted beats for providing signals to said display means manifesting the heart rate of said person.

12. The invention according to claim 11: wherein said timing signals can be selected to occur at intervals of six or sixty seconds; wherein in the event a sixty second timing signal is selected, the number of detected and counted heartbeats is caused to be displayed; and wherein in the event a six second timing signal is selected, ten times the number of detected and counted heartbeats is caused to be displayed.

13. The invention according to claim 12 wherein in the event said six second timing signal is selected, the number of detected and counted heartbeats is displayed on "10's" and higher significant digits of said dis-play.