Cardiac Pacer Monitoring Means With Rate And Pulse Discrimination

Abstract

In a cardiac pacer monitoring system for use with pacer patients, apparatus for deriving reliable pacer trigger signals from electrocardiac activity including, processing the electrocardiac signal to exclude QRS and 60 Hz problems, integrating the processed signal to act as a threshold control, comparing the processed signal with the threshold and discriminating the comparator output with pulse width discriminator and repetition rate discriminator means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention -- The present invention relates to the field of monitoring cardiac activity and more particularly to a technique for developing pacer trigger signals in cardiac pacer monitoring.

2. Description of the Prior Art -- One of the major uses of the Coronary Care Units (CCU) is the monitoring of the cardiac pacer function for the reason that a large number of CCU patients have cardiac pacer implanted for a variety of reasons. These patients must be monitored to determine if the cardiac pacer is functioning properly and if an appropriate benefit is being received. In such instances, it is important to know how often and why a pacer is activated, whether it is firing at appropriate times, whether it is, in fact, capturing (driving) the heart, or if it is having deleterious effects. Inappropriate firing may result in mechanically ineffective beats or in certain circumstances may lead to ventricular fibrillation. Unfortunately, very few, if any, systems are now available for automatically monitoring the commonly used battery powered cardiac pacers. A pacer rhythm monitor which could provide the essential information and alarms for evaluating cardiac pacer efficiency would be a tool of major clinical usefulness. In any such system, a pacer trigger circuit which operates reliably, is essential to monitor the medical performance of cardiac pacers in the critically ill.

SUMMARY OF THE INVENTION

The purpose of the present invention is to reliably and accurately identify a pacer spike to provide a reliable, noise rejecting cardiac pacer trigger circuit. The latter is accomplished by provision of a pacer identification circuit which processes an EKG signal within a bandwidth falling outside of the QRS complex for selectively passing pacer pulse information. The filtered processed data is compared with an integral of itself which is employed as a threshold level. The comparison output is further discriminated from a pulse width standpoint and thence from a repetition rate standpoint to result in a reliable cardiac pacer trigger for monitoring purposes.

A further embodiment of the invention allows for readily modifying a repetition rate discrimination circuitry to adapt the unit for use with either continuous asynchronous cardiac pacers or demand cardiac pacers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrative of the circuit of the invention.

FIG. 2 is a schematic circuit diagram of the repetition rate descriminator 27 shown in FIG. 1.

FIG. 3 shows a plurality of time related waveforms for explaining operation of the unit shown in FIG. 2.

FIG. 4 shows a plurality of time related waveforms for explaining operation of the system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a general outline of a system encompassing an embodiment of the present invention, wherein pickup electrodes 11 attached to the patient 12 provide an electrocardiac signal via a connecting cable to a front-end EKG preamplifier 13 which is connected to an AGC unit 14 for holding down a dynamic range of the patient derived signals to a steady level. It should be noted that conventional AGC units of the type represented by 14 are normally not fast enough to catch some EKG fadings during an EKG patient run and do not react to sudden increase in EKG amplitude.

The output of AGC unit 14 is connected to a narrow bandpass filter (BPF) 15 with a 3db bandwidth of approximately 100 to 500 Hz per second with at least 18db/octave roll-offs to effectively eliminate the QRS complex and 60 Hz problems. This bandwidth is sufficient to cover the pacer pulsing frequencies which normally lie within a frequency range of about 80 Hz to 500 Hz. A foldover unit 16 is connected from BPF 15, serving as a full wave rectifier to accommodate bipolar signals and fold them to the positive side only.

Foldover unit 16 is separately connected via a pair of legs to a comparator 24, and is additionally coupled to ground via zener diode 17. The first of the two legs includes a variable amplifier 18. The second leg includes a low leakage diode 19 connected to an integrator 21 to provide for integration of positive signal levels only. Diode 19 prevents leakage back to foldover unit 16. Connected from integrator 21 is a variable amplifier 22 with an output denoted as 54, which is coupled to the comparator 24. The lead intermediate variable amplifier 22 and comparator 24 is provided with a DC offset from a B+ source via resistor 23 as will be explained hereinafter.

Integrator 21 has a relatively short attack time and relatively long release time. One example of integrator 21 would be to have an attack time of 22 ms and a release time of 1.5 seconds that would include typical impedance values of an attack resistance of 4.7K, a capacitance of 4.7 microfarads and a release resistance of 320K. It should, of course, be understood, however, that other combinations of attack times and release times would be suitable in operation of the present invention. For example, one combination might include a ratio tolerance of 2:1 for each attack and release time constant.

The output of comparator 24 is connected to a pulse width discriminator for selectively passing pulse widths between 0.5 to 5 ms to essentially reject anything outside of this window whether longer or smaller in duration for the reason that pacer pulses (spikes) have a width of 1.0 to 4.0 ms. The one shot output unit 26 of the pulse width discriminator generates a 20 ms pulse width to provide output pulses of uniform width and normalized digital amplitude.

Connected from the one shot output 26 is a repetition rate discriminator 27. With reference to FIG. 2, the repetition rate discriminator is shown to include a retriggerable one shot 500 ms unit 28 which in turn is connected to a retriggerable one shot 750 ms unit 29. NAND gate 31 is connected from the negative output of one shot unit 28, from one shot unit 29, and also from one shot 26. The lead connection intermediate one shot 29 and NAND gate 31, however, is interrupted by a switch 32 which is controlled by a relay 34 or any other suitable switching device (such as a digital gate) via a manually operable switch 33.

The position of switch 33 will depend on whether a continuous asynchronous cardiac pacer (fixed rate) or demand type pacer unit is being monitored. In the fixed rate position relay 34 is activated to close switch 32 and thus provide an output from one shot unit 29 to the NAND gate 31. With switch 33 in the demand position relay 34 is deactivated to open switch 32 and relay 35 is activated to close the switch 36, to supply a fixed potential to the same input terminal of NAND gate 31. The output NAND gate 31 provides from one shot 26 a normalized 20 ms pacer trigger signal.

The one shot 28, 500 ms period is equivalent to a repetition rate of 120 beats per minute (equivalent to 2 beats per second) and serves to exclude pulse information which exceeds this rate. A lower end demarcation rate is formed by the combined one shot units 28 and 29 which have a total period equivalent to 48 beats per minute below which rate, pulses are rejected, to in effect, provide an overall window of 48 b/m to 120 b/m for passing pacer pulses. Pacers do not normally run at a rate lower than 50 b/m or higher than 120 b/m. If desired, however, a programming switch could provide manual variation for higher or lower beat/minute rejection.

Operation of the repetition rate discriminator may best be shown in reference to FIG. 3 wherein output pacer pulses p and noise pulses n are shown to be triggered by the trailing edge of the 20 ms one shot 26. The first three waveforms are generated with the switch 33 in a demand position and where the demand pacer has a pulse rate interval of 750 ms. The next four waveforms are generated with switch 33 in a fixed position and where the fixed pacer has a pulse rate interval of 1,000 ms. As illustrated, retriggerable one shot 28 is triggered by pulses p and retriggered by n when n occurs less than 500 ms. after p, the inverted output of which is fed to NAND gate 31 and the leading edge of the noninverted output of which is used for triggering the retriggerable 750 ms one shot 29.

In the demand operational state the noise pulses n, as is depicted, will be eliminated at the NAND gate 31 output by the 500 ms retriggerable O.S. when these noise pulses effectively increase the rate to above 120 b/m. In the fixed operational state, noise pulses n are rejected if occuring above or below the 48-120 ms window. Use of the lower end rejection gate (below 48 b/m), will however, cause either a first pulse p and/or a pulse p subsequent to a missing cardiac pacer pulse, to be lost. The former will be of no practical significance, nor will the latter as any missing cardiac pacer pulse is normally sufficient to actuate an alarm.

In turning back to FIG. 1, it may be desirable to blank out or inhibit the cardiac pacer trigger signal upon detection of noise via suitable noise detector circuitry 51 and NAND gate 52. The detected noise would be that detected in the QRS complex for blanking the QRS pulse in a QRS trigger detection circuit with which the present invention would be employed. A typical example of a noise detector might be that disclosed in applicant's co-pending U.S. Pat. Application Ser. No. 195,396 filed on Nov. 3, 1971 for Arrhythmia Detection Technique.

The operation of the present invention might best be explained with reference to FIGS. 1 and 4, wherein it is illustrated that the raw EKG signals are passed by way of electrodes 11 from the patient 12 and processed through an EKG amplifier 13, AGC unit 14, bandpass filter 15 and foldover unit 16, to provide a filtered absolute EKG signal as is illustrated in FIG. 4 at waveform a, the bandpass filter 15 range being such as to eliminate the QRS complex and 60 Hz problems. The output of variable amplifier 18 is depicted at 53 in FIG. 4 at waveform b and the integrated amplified threshold output from variable amplifier 22 is indicated at 54 in FIG. 4 at waveform b.

The integrated threshold signal represented by waveform 54 is designed to have a relatively short attack time and a relatively long release time, so as to depend from or be influenced by previous pacer spikes, to accurately adapt to the actual pacer amplitude output thereby providing an on-line real time threshold signal for the pacer spikes 53. The relatively long release time of integrator 21 is slow enough to support a meaningful threshold level above most noise signals or ground clutter to virtually prevent triggering by noise spikes exceeding threshold 54. On the other hand, the release time is fast enough to respond to sudden pacer fadings (e.g., two or four successive missing pulses) to indicate an alarm condition. Comparator 24 is provided with the DC offset through resistor 23 of approximately +0.5 volts represented at 56 to establish a minimum fixed threshold level to which 54 would descend to prevent the automatic threshold level from sinking to ground level in which case it would respond to any small amount of noise. The signals 53 and 54 are weighed in the comparator 24 whereby a signal is emitted from comparator 24 upon penetration of the threshold 54 by spike signal 53 ad depicted at FIG. 4 at waveform C.

The pulse width discriminator 25 is adapted to discriminate against pulses not falling within the 0.5 to 5 ms window thereby rejecting pulses lying above or below this upper and lower limit criteria. As earlier explained 0.5 to 5 ms pulse width is selected to cover a pulse width range adapted by the pulse cardiac pacer industry although other widths could be easily arranged. As illustrated at FIG. 4 at waveform c the cardiac pacer pulse 55' generated by the comparator output is indicated lying outside this selected pulse width range and, therefore, is emitted normalized at FIG. 4 at waveform d, representing the pulse width discriminator output from the one shot unit 26.

The repetition rate discriminator, on the other hand, analyzes the rate or frequency of the output of the one shot unit 26 on a beat-to-beat basis to analyze which of these pulses falling within the pacer frequency spectrum are indicative of functions such as muscle tremors that lie outside the pacer rate and, therefore, can be eliminated. As earlier stated the pulse rate accepting window has lower and upper limits of 48 and 120 b/m which rates are set by one shot units 28 and 29.

Claims

I claim:

1. Apparatus for use with pacer patients, for deriving pacer signals from electrocardiac signals comprising:

means for deriving and processing the electrocardiac signals to produce a filtered, absolute output signal;

means for integrating said output signal;

comparator means for comparing signal levels of the output of said integrating means with said processing means output, to produce a comparator output signal representative of pacer activity when the processing means output signal attains a predetermined relationship with respect to the integrating means output signal;

pulse width discriminator means for rejecting comparator output signals lying outside a preselected pulse width range; and

repetition rate discriminator means for rejecting comparator output signals which lie outside a preselected range of pulse rates.

2. In a system according to claim 1 wherein said integrating means includes impedance means to provide a relatively short attack time and relatively long release time to depend from the signal level of a prior signal, to generate a threshold level which quickly adapts to the intensity of the derived pacer signals.

3. In a system according to claim 2 wherein said processing means includes filter means adapted to pass pacer signals in a range from approximately 100 to 500 Hz and means for providing full wave rectification of the signals passed by the filter means.

4. In a system according to claim 1 wherein said pulse rate discriminator means is provided with a comparator output pulse acceptance range of approximately 0.5 to 5 ms.

5. In a system according to claim 1 wherein said repetition rate discriminator means defines a rate acceptance window of approximately 48 to 120 beats per minute for accepting pacer signals.

6. In a system according to claim 1 wherein for monitoring cardiac pacers of the fixed type, said repetition rate discriminator means includes:

first means for rejecting comparator output signals which lie above said preselected range of pulse rates; and

second means for rejecting comparator output signals which lie below said preselected range of pulse rates.