GB2153084A - External noninvasive cardiac stimulation and monitoring system - Google Patents

Abstract

An external cardiac stimulation and monitoring system has means for sensing electrical signals from the heart, means for sealing an external electric stimulation pulse, means for nullifying the interfering effects when a stimulating pulse occurs and means quickly thereafter causing display of the patient's ECG signal during the immediately following heart beat, thus permitting determining if the stimulation has been effective. ECG signals from electrodes 18 are amplified at 34 and 36 and fed to a pulse width modulated oscillator 38 connected to output devices by switch 52 except when artifacts are sensed. If a pacemaker pulse is detected at 40 or an overload voltage at 42, a protection switch diver 54 is caused to disconnect the incoming signal from a charging circuit in amplifier 36. The signal is re-connected after a small delay to allow the baseline to be re-established, or after a shorter delay if the charging ircuit is arranged to be discharged. <IMAGE>

Description

SPECIFICATION External noninvasive cardiac stimulation and monitoring system The invention relates to an external noninvasive cardiac stimulation and monitoring system.

External electric stimulation is a well-established noninvasive technique that provides effective heart beats in emergency resuscitation of patients from ventricular standstill. U.S. Patent No.

4,349,030 describes a noninvasive pacemaker that provides temporary stimulation of conscious patients without the pain associated with prior systems providing external cardiac stimulation, by providing stimulation pulses of constant current, between 5 and 40 ms or more in duration and at about 25-300 V and by using electrodes with skin-contacting members providing low current density.

Use of commercially available ECG monitors to evaluate the effectiveness of these external stimulation pulses is usually difficult and unsatisfactory because the differential voltages of about 1-2 V or more at the ECG sensing electrodes that result from the stimulation pulses disrupt the operation of the monitors in displaying ECG potential of ventricular excitation in the range of about 0.5-4.0 mV. The disruption typically causes a large and prolonged deflection that goes off the scale and does not return to baseline for a significant time interval, usually over 1 second, thus preventing one from determining whether or not the pulse was effective in stimulating the patient's heart.

ECG monitors have been provided with circuits to protect the monitor against defibrillation shocks, which impress several thousand volts on the body and result in high voltages at the sensing electrodes of the cardiac monitor. The time before any ECG potential can be displayed after stimulation may be as long as eight seconds.

Some cardiac monitors are provided with circuits to detect and not to count the short but small rectilinear pulses from an inplantable pacemaker. The April 1983 revision of the Proposed Standard for Cardiac Monitors, Heart Rate Meters and Alarms by the Association for the Advancement of Medical Instrumentation describes standards for recovery from disruption caused by defibrillation shocks and for rejecting implanted pacemaker pulses.

According to this invention there is provided an external noninvasive cardiat stimulation and monitoring system comprising means for providing external noninvasive stimulation pulses to a patient in appropriate timing with the patient's electrocardiographic activity, first sensing means for sensing electrical signals of said patient's heart, display means responsive to said first sensing means for dispaying said electrical signals of said patient's heart, second sensing means for sensing the occurrence of said external noninvasive stimulation pulses, and means responsive to said second sensing means for permitting display by said display means of said electrical signals of said patient's heart during at least part of the time interval immediately following said stimulation pulse.

We have discovered that a cardiac monitor can be effectively used to monitor external electric stimulation by providing the monitor with means for sensing an external electric stimulation pulse and means for quickly thereafter permitting display of the patient's ECG signal. Thus the determination can be clearly made whether or not the external stimulus has produced a cardiac response.

In preferred embodiments a switch is used to prevent the large signals from the pacemaker from interfering with the display of the cardiac signals; and the monitor circuitry includes an energy storage component, and the switch is either opened to disconnect the electrodes from the energy storage component or closed to cause the signals sensed at the electrodes to be passed to ground. In one embodiment a voltage sensor is used to sense the stimulation pulse and to activate the switch when the sensed voltage is higher than a threshold value or is changing quickly; and a second switch is used to mullify the effects of non zero turn-on and turn-off times of the first switch and the threshold voltage of the overload detector. In another embodiment a control signal from the pacemaker is used to sense the stimulation pulse.

This invention will now be described by way of example with reference to the drawings, in which :- Figure 1 is a block diagram of an external noninvasive cardiac stimulation and monitoring system according to the invention: Figure 2 is a schematic of a portion of a preamplifier of a monitor of the Fig. 1 apparatus; Figure 3 is a schematic of a portion of a preamplifier of another embodiment of the monitor of the Fig. 1 apparatus; Figure 4 shows ECG potential versus time displayed by an ECG monitor that does not have means to provide display of the ECG signal during the heart beat immediately following the application of a stimulation pulse: Figures 5 and 6 show ECG potential versus time displayed by ECG monitors having means to provide display of the ECG signal during the heart beat immediately following the application of a stimulation pulse; and Figure 7 is a timing diagram showing the relationship of a stimulation pulse and a control signal used in the Fig. 3 embodiment.

Referring to Fig. 1, there is shown pacemaker 10 (described in detail in U.S. Patent 4,349,030, hereby incorporated by reference), connected to patient 14 via external stimulation electrodes 16, and ECG monitor 12 connected to patient 14 by left and right ECG electrodes 18 and by ground or common mode voltage electrodes 20. Monitor 12 is an S 8 W monitor (Type 2010, Diascope 2, available from Simonsen 8 Weel, Denmark) that has been modified as described below. It includes preamplifier 22, post-amplifier 24, X,Y memory 26, and cathode ray tube (CRT) 28. Also shown on Fig. 1 is strip chart recorder 30 connected to post-amplifier 24 of monitor 12.

Preamplifier 24 includes low pass filter 32, instrumentation amplifier 34 (to amplify the differential voltage between right and left electrodes 18), single-ended amplifier 36 (to amplify the signal of amplifier 34), and pulse width modulated oscillator 38. Pacemaker detector 40 and overload detector 42 are both connected to trigger artifact generator controller 44. Post amplifier 24 includes first amplifier 46, second amplifier 48, artifact generator 50 and switch 52, making second amplifier 48 selectably connectable to either first amplifier 46 or artifact generator 50.

Protection switch driver 54 includes one or more switches connected between components in single-ended amplifier 36 and operated in a manner to avoid disturbances that would otherwise be caused by application of external pacemaker stimulation pulses and would prevent display of the actual ECG signal for the heart beat following the stimulation. The structure of protection switch driver 54 and its connections to pacemaker 10 and artifact generator controller 44 are different in the Figs. 2 and 3 embodiments, as is discussed below and is indicated by different dashed lines on Fig. 1.

Referring to Fig. 2, the first embodiment of preamplifier 24, except for low pass filter 32, is shown in detail. Instrumentation amplifier 34 includes amplifiers 55, 56 (TL072), connected to receive input voltages from the respective left and right electrodes 18 through low pass filters 32 (Fig. 1) and to provide output voltages to the inputs of amplifier 58 (TL072).

Single-ended amplifier 30 includes amplifiers 60, 62 (TL074). Normally closed MOS bilateral analog switch 64 (he5043) is connected between resistor R38 and the junction of capacitors C16 and Cl 7, and capacitor 65 (0.01 uF) is connected between the junction of switch 64 and resistor R38 and ground. Normally open MOS bilateral analog switch 66 (Hl5043) is connected with 4.7 kohm resistor 67 between the + input to amplifier 62 and ground. Both switches 64, 66 are controlled by the output on line 68 of artifact generator controller 44.Line 68 is connected to edge triggered monostable multivibrator 70 (4047), in turn directly connected to the control input of switch 64 and connected to the control input of switch 66 through fast turnon, slow turn-off time delay circuit 72, having 0.033 uF capacitor 74, 1 Mohm resistor 75 and diode 77 (1 N41 48), to provide the desired turn-off time delay.

Pacemaker detector 40 includes amplifier 76 (TL074), connected to detect when the output of amplifier 60 is quickly changing and to provide an output to trigger amplifier 78 in artifact generator controller 44. Overload detector 42 includes amplifier 80 (TL072), connected to detect when the output of amplifier 58 exceeds about i 8 V and to provide an output to trigger amplifier 78 in artifact generator controller 44.

Pulse width modulated oscillator 38 includes amplifier 82 (709C) and is connected to receive the output of amplifier 62 and to provide a width modulated rectangular wave to opto-isolator 84 (OPI 1 264B), isolating preamplifier 22 from post-amplifier 24. Artifact generator controller 44 similarly includes opto-isolator 86 (OPI 1264B) to isolate it from post-amplifier 24.

All components shown on Fig. 2 except switches 64. 66, capacitor 65, resistor 67, line 68, monostable multivibrator 70, and time delay circuit 72 are provided in the standard preamplifier of the S 8 W monitor mentioned above. The elements just noted thus make up protection switch driver 54 and its connections to preexisting components of the standard S 8 W preamplifier. The following table includes descriptions of the remaining resistors, capacitors and diodes shown on Fig. 2.

Resistors Ohms R28, 29, 30, 31, 32, 33, 34, 35 22 k R20 2.2 k R16, 17, 36 100 k R14 1.8 k R1, 15 4.7 k R18, 19, 25, 39 1.5 M R37 470 k R38 1.5 k R21, 22. 68 k R13 20 k R5 10 k R4, 27 100 k R6, 12, 26 1.0 k R7 7.5 k R8 3.3 k R9, 24 3.9 k R23 33 k Capacitors Farad C7, 10 3.3 n C8, 9, 13 100 n C12 10 n C17 330 n Cli 470 p C16 1 u C3 150 p Diodes Description D1, 8-16 1N4148 D 17, 18 FD300 Z 1, 2 1N821 Referring to Fig. 3, the second embodiment of monitor preamplifier 22, except for low pass filter 32, is shown in detail.This embodiment is the same as the Fig. 2 embodiment except that switch 66, line 68, multivibrator 70 and time delay circuit 72 of the Fig. 2 embodiment are not used, and the control input of switch 64 is connected via opto-isolator 88 (OPI 1264B) and opto-isolator driver 90 (a 2N3904 transistor and two 1 kohm resistors) to pulse former 92 (for example a fixed pulse width monostable multivibrator), connected via line 93 to the trigger amplifier (28) of the pacemaker described in the above-mentioned patent, which pacemaker is also modified to include a delay component between its trigger amplifier and its monostable multivibrator (30). The output of opto-isolator 88 is also connected via diode 94 (1N4148) and line 96 to the input of artifact generator controller 44.The input of opto-isolator 88 is connected by 1 kohm resistor 89 to the indicated voltage source, and the output of opto-isolator 88 is connected by 100 kohm resistor 91 to the indicated voltage source. All components shown on Fig. 3 except switch 64, capacitor 65, opto-isolator 88 (and its associated resistors), driver 90, pulse former 92, diode 94 and line 96 are provided in the standard preamplifier of the S 8 W monitor mentioned above. The elements just noted thus make up protection switch driver 54 and its connections to existing components in the S a W preamplifier.

Except as noted below in regard to the Fig. 3 embodiment, the operation of external pacemaker 10 is as described in the above-mentioned patent.

The general operation of an unmodified monitor 12 without protection switch driver 54 will first be described, and this will be followed by discussion of the operation of the preamplifiers modified according to the invention and shown in Figs. 2 and 3.

The differential voltage between signals from electrodes 18 pass through low pass filter 32, which reduces noise, to instrumentation amplifier 34, the output of which is a voltage related to the magnitude of the differential voltage bwetween electrodes 18. This output voltage is amplified by single-ended amplifier 36, in which the DC common mode voltage is removed by blocking capacitor C17; frequencies below about 1/2 Hz are cut off by capacitor C17 and resistor R39; high frequencies are cut off by capacitor C13 and resistor R25; capacitor C16 limits the high frequency response, and diodes D18, D17 clamp the input to amplifier 62 to gound if the voltage is above + 3 6 V or below - 0.6 V.

The output of amplifier 36 is provided to pulse width modulated oscillator 38, providing rectangular wave pulses, the widths of the positive and negative portions of which are related to the value of the incoming signal. The rectangular wave pulses pass through opto-isolator 84 to post-amplifier 24, which rectifies the received rectangular wave pulses, and amplifies the signal (assuming that amplifier 46 is connected to amplifier 48 via switch 52) and provides it to strip chart recorder 30, for recording, and to X, Y memory 26, which is continuously updated and read to provide the image displayed on CRT 28.

If an overloaded voltage or a pacemaker pulse are detected by overload detector 42 or pacemaker detector 40, artifact generator controller 44 provides an output through opto-isolator 86 to artifact generator 50, which is then connected via switch 52 to amplifier 48 and provides a 25 ms constant voltage pulse that produces artifact 97 (Fig. 4) on CRT 28 and the chart of recorder 30.

Pacemaker detector 40 is designed to detect spiked pulses (typically less than 2 ms in duration) of implanted pacemakers. Amplifier 76 of pacemaker detector 40 receives only quickly changing signals from the output of amplifier 60, because slowly changing signals are blocked by capacitor C12, owing to the time constant for capacitor C12 and resistors R15 and R14.

When the value of a quickly changing signal (e.g., a pacemaker spiked pulse) is greater than + 0.6 V or less than - 0.6 V at the input to amplifier 76, the signal passes diode D9 or diode D8, and amplifier 76 provides an output to trigger artifact generator controller 44.

When the output of instrumentation amplifier 34 is greater than about + 8 V or less than about - 8 V, amplifier 80 of overload detector 42 provides an output to trigger artifact generator controller 44.

In a typical overload situation when using the unmodified preamplifier with external pacemaker 10, instrumentation amplifier 34 and amplifier 60 have fast recovery, as there is no significant storage of energy in them. Overload energy is however stored between amplifier 60 and amplifier 62 in capacitor C16 and in blocking capacitor C17, which discharges slowly after the 40 ms pacemaker pulse has ended, owing to the large time constant of capacitor C17 and resistor R39.Fig. 4 illustrates an ECG recording on a strip chart for a time period during which an external stimulation pulse from pacemaker 10 has been applied; it is seen that the recording goes off of the negative scale and slowly recovers and returns to the baseline, preventing viewing of the ECG signal during the immediately following heart beat, thus preventing one from determining whether the stimulation pulse was effective.

In the Fig. 2 embodiment, to reduce the time required for the monitor to recover after the application of an external pulse, switch 64 is opened to limit charging of capacitors C16 and C17 as soon as a pulse has been detected by by overload detector 42 or pacemaker detector 40, and switch 66 is activated to quickly remove the charge placed on capacitor C17. If switch 66 were not used, the signal of the immediately following heart beat could be seen on the strip chart recording. but it would take some time for the baseline to return to zero, as illustrated in Fig. 5. Capacitor C16 is discharged quickly through amplifier 60, but the right hand side of capacitor C17 must return to zero before the baseline returns to zero. Capacitor C17 is discharged through resistor 67 (which is much smaller in value than R39 and could even be a short circuit).In order to make sure that switch 66 remains closed a short period (i.e., a few milliseconds) after switch 64 has been closed, a time delay is provided for switch 66 by time delay circuit 72. An illustration of a strip chart recording with both switches 64, 66 in use is shown in Fig. 6. from which it is seen that the recorded ECG returns to baseline quickly after artifact 97 has ended. The external pacing pulse is 40 ms long, but its effect in the patientmonitor system may last for some time afterward. Multivibrator 70 provides a pulse to keep normally closed switch 64 open for the appropriate time period.

In the Fig. 3 embodiment, switch 64 is opened prior to, throughout and after an external pacemaker pulse by a switching control pulse that is provided by switching pulse former 92, amplified by opto-driver 90, and transmitted through opto-isolator 88 to the control input of switch 64. The relationship of the timing of pacemaker pulse 98 and switching control pulse 100 provided by pulse former 92 is shown in Fig. 7.

Time tI, the delay between the beginning of switching control pulse 100 and the beginning of pacemaker pulse 98. is about 1-5 ms, and merely guarantees that switch 64 is opened prior to application of the pacemaker pulse to patient 14. This delay is provided by the delay means added between the trigger amplifier and pulse width monostable multivibrator in the pacemaker.

Time t2, the delay between the ending of pacer pulse 96 and the ending of the switching pulse 98, is between 5 and 20 ms. and is used to delay the closing of switch 64 until after the disturbance to the differential voltage at electrodes 18 that remains after the ending of the pacer pulse has dissipated sufficiently to reduce baseline shift. This delay is provided by making the width of pulse 100 the proper amount, by the components used in switching pulse former 92.

The resulting ECG recording is as is illustrated in Fig. 6.

The Fig. 2 embodiment is preferred over the Fig. 3 embodiment in terms of ease of physically modifying the S s W preamplifier, but the Fig. 3 embodiment is preferred over the Fig. 2 embodiment in terms of direct and simple sensing of externally applied pacemaker pulses.

Modifications of the embodiments described above are possible.

For example, in the Fig. 2 embodiment, switch 66 could be excluded if the shifting of the baseline resulting when using switch 64 alone were acceptable. Also, instead of preventing passing of signals sensed by the electrodes to common mode capacitor C17 by switch 64, other means of preventing storage of energy could be used; e.g., a switch causing the signals to pass to ground could be used.

In the Fig. 3 embodiment, switch 64 could be replaced with switches located at the + input to amplifier 62, at the + input to amplifier 60 or at both inputs (two switches would have to be used) to amplifiers 55, 56. Switches used at these locations would be normally open and would connect to ground when activated.

Claims (20)

1. An external noninvasive cardiac stimulation and monitoring system comprising means for providing external noninvasive stimulation pulses to a patient in appropriate timing with the patient's electrocardiographic activity, first sensing means for sensing electrical signals of said patient's heart, display means responsive to said first sensing means for displaying said electrical signals of said patient's heart, second sensing means for sensing the occurrence of said external noninvasive stimulation pulses, and means responsive to said second sensing means for permitting display by said display means of said electrical signals of said patient's heart during at least part of the time interval immediately following said stimulation pulse.

2. The system of claim 1 wherein said first sensing means includes electrodes for contacting said patient, and said means for permitting display includes switch means responsive to said second sensing means for preventing at least portions of disturbances to said electrical signals that result from application of said external stimulation pulses and are sensed by said electrodes from being displayed by said display means.

3. The system of claim 2 wherein said display means includes an energy storage component, and said switch means includes a switch that is responsive to said second means for sensing to prevent signals sensed by said electrodes from passing to said energy storage component during said stimulation pulses.

4. The system of claim 3 wherein said switch is a normally closed switch that disconnects said electrodes from said energy storage component when opened.

5. The system of claim 3 wherein said switch is a normally open switch, and said switch means includes means causing said signals sensed by said electrodes to pass to ground before reaching said energy storage component during said stimulation pulses.

6. The system of claim 2 wherein said second sensing means includes voltage sensing means for sensing the differential voltage sensed by said electrodes, and wherein said switch means is responsive to said voltage sensing means.

7. The system of claim 6 wherein said voltage sensing means includes means for sensing when said differential voltage is larger than a threshold value.

8. The system of claim 6 wherein said voltage sensing means includes means for sensing when said voltage is changing quickly.

9. The system of claim 6 wherein said display means includes an energy storage component, and said switch means includes a first switch that is responsive to said voltage sensing means to prevent signals sensed by said electrodes from passing to said energy storage component during said stimulation pulses.

10. The system of claim 9 wherein said first switch is a normally closed switch that disconnects said electrodes from said energy storage component when opened, and wherein said switch means also includes a normally open switch connected between said energy storage component and ground, said normally open switch being closable by said voltage sensing means to discharge charges caused by said disturbances and stored in said energy storage component prior to opening of said normally closed switch by said voltage sensing means.

11. The system of claim 2 wherein said display means includes means for displaying an artifact indicating application of a stimulation pulse.

12. The system of claim 3 wherein said switch means includes means to provide a first control pulse of a predetermined length of time for activating said switch a predetermined length of time.

13. The system of claim 10 wherein said switch means includes means responsive to said voltage sensing means to provide a first control pulse of a predetermined length of time to cause said normally closed switch to be open a predetermined length of time.

14. The system of claim 13 wherein said switch means includes means responsive to said voltage sensing means to provide a second control pulse that ends a predetermined length of time after said first control pulse and causes said normally open switch to be closed for a predetermined length of time after said normally closed switch has been closed.

15. The system of claim 2 wherein said second sensing means includes means for receiving a signal from said monitor indicating the occurrence of a stimulation pulse, and said switch means is responsive to said means for receiving a signal.

16. The system of claim 13 wherein said means for receiving a signal includes means for providing a control pulse to said switch means, said control pulse extending beyond said stimulation pulse.

17. The system of claim 16 wherein said display means includes means responsive to said control pulse for displaying an artifact indicating application of a stimulation pulse.

18. The system of any preceding claim wherein said means for permitting includes means for permitting electrical signals of a patient's heart to begin being displayed less than 400 ms after said stimulation pulse has ended.

19. The system of any preceding claim wherein said stimulation pulse is greater than 5 ms in duration.

20. An external noninvasive cardiac stimulation and monitoring system, substantially as hereinbefore described with reference to the drawings.