Mutually noninterfering transcutaneous nerve stimulation and patient monitoring

Transcutaneous electrical nerve stimulation is applied to the patient, as though electrocardiograph monitoring is not being practiced. The EKG signals are preprocessed through a selective sample and hold module, which performs amplification, comparison with selective frequency and amplitude standards (as by differentiation) and temporary holding of EKG signals at such time as the transcutaneous nerve stimulating pulses are occurring, as sensed against the frequency and amplitude criteria. In the absence of transcutaneous stimulation signals, EKG signals are coupled directly through for conventional EKG processing.

FIELD OF THE INVENTION 
This invention relates both to the monitoring of patient vital signals, 
such as by electrocardiograph, and to the application of transcutaneous 
electrical nerve stimulation to a patient for purposes of pain control. 
More particularly, it relates to simultaneous, mutually noninterfering 
conduct of such monitoring and such stimulation. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
Increasingly, transcutaneous electrical nerve stimulation (TENS), also 
often referred to as electronic pain control, is finding acceptance as an 
aid or supplement to anesthesia and/or analgesia. It has been known for 
some time that transcutaneous stimulation is a useful and effective 
moderator of post-operative pain (see, for example, U.S. Pat. No. 
3,911,930 to Hagfors et al.). More recent investigations indicate that 
TENS may also have a synergistic interaction with oft used anesthetics and 
analgesics in a fashion which substantially alleviates pain and discomfort 
of the patient, while substantially reducing the amount and character of 
drugs to which the patient must be subjected. In U.S. application Ser. No. 
133,211 to Bussey, filed Mar. 24, 1980, there is disclosed a method for 
utilizing the combination of transcutaneous nerve stimulation with 
anesthetics and analgesics during surgery. That application also features 
the utilization of electronic pain control apparatus in such a manner and 
of such a character as the electrodes which are positioned on the patient 
before and during surgery, may be left in place for utilization for 
alleviating pain during the post-operative period. 
Quite commonly, however, both during the operative procedure and during the 
recovery period thereafter, the surgeon, the anesthesiologist, and other 
attending physicians and nurses have need continuously to monitor certain 
vital signs of the patient, such as cardiac function. The conventional 
mode of monitoring cardiac function is utilization of the 
electrocardiograph (EKG) through the positioning of several sensing 
electrodes on the patient, in known fashion, thereby to detect induced 
electrical variations proportional to and directly representative of heart 
activity (i.e. the well-known cardiac PQRST complex). 
Problems arise in the simultaneous conduct of transcutaneous electrical 
nerve stimulation and the likes of EKG monitoring, however, because of the 
character and frequency spectrum of the EKG signal, in both healthy and 
ailing patients, and the character and frequency spectrum of electrical 
signals which are known to be effective for purposes of electronic pain 
control. Unless some appropriate precaution is taken, the EKG trace during 
and immediately after each pulsed application of TENS at best will be 
meaningless, and at worst will provide a false or erroneous indication of 
patient conditions which in fact are not occurring. 
It is accordingly a primary object of the present invention to provide 
apparatus and methods whereby transcutaneous electrical nerve stimulation 
may be applied to a patient, either alone or in combination with select 
anesthetic and analgesic agents, during times in which patient vital 
signs, such as cardiac function, are being continuously or intermittently 
monitored. 
Problems attendant to mutually interfering signals both generated in 
treating the patient and sensed in the patient through monitoring, are not 
new. Indeed, a number of patents purport to deal with such mutual 
interference problems in the operating theatre, in the intensive care 
unit, and elsewhere. For example, U.S. Pat. No. 4,117,848 to Naylor 
discloses the use of a follow and hold circuit to suppress pacer pulses in 
an EKG monitor. Likewise, U.S. Pat. No. 3,897,774 to Burdick et al., U.S. 
Pat. No. 4,137,908 to Degonde et al., and U.S. Pat. No. 3,534,282 to Day 
disclose alternative approaches to mutually noninterfering utilization of 
heart pacing and EKG monitoring. U.S. Pat. No. 3,716,059 to Welborn et al. 
describes a cardiac resuscitator including a disposable clamp controlled 
to inhibit transmission of EKG signals at times when electrical 
stimulation is being delivered to the patient. Fletcher U.S. Pat. No. 
3,910,257 discloses a sample and hold approach to EKG signals, for 
purposes of monitoring and subsequent coupling to a data acquisition unit. 
Each such prior art approach, is less than preferred for the electronic 
pain control situation, not to mention for their own purported situation, 
for a variety of reasons. Not the least of these is the need externally to 
interconnect the respective signal generating and signal sensing units; 
also they tend to sense signals in a fashion which is counterproductive to 
overriding TENS objectives, or which involves specialized and elaborate 
synchronization and signal processing requirements, or which is so 
especially adapted to the application disclosed in the patents that there 
results little of relevance to the TENS-EKG situation. 
SUMMARY OF THE INVENTION 
The present invention is based on the proposition that, due to the inherent 
character of effective transcutaneous electrical nerve stimulation signals 
and their transit through the body, and the inherent character of cardiac 
function as represented by electrocardiograph signals, suitable monitoring 
of the EKG signals, and suitable application of select, predetermined 
amplitude and frequency criteria thereto, will permit the detection of the 
occurrence of TENS signals directly from the EKG signals. During such 
periods of TENS "interference", the EKG signals which occurred just prior 
to such sensing are sampled and held, extending also for a desired period 
after the occurrence of the stimulation pulse. 
Thus, in accordance with the principles of the present invention, 
transcutaneous electrical nerve stimulation is applied to the patient, 
utilizing placement and pulsing of TENS stimulating electrodes in such 
fashion as may be safe and effective, virtually irrespective of the 
conduct of EKG monitoring. No interconnection is required between the TENS 
unit and the EKG monitoring. Likewise, the EKG electrodes are positioned 
as desired, and the EKG leads are coupled to a unit embodying the 
principles of the present invention and situated intermediate the leads 
and the EKG processing unit. 
In a unit embodying the principles of the present invention in preferred 
fashion, EKG patient leads are first coupled to an input differential 
amplifier featuring appropriate amplification, filtering, and common mode 
rejection functions. The signal path then is bifurcated, one arm of which 
includes a delay circuit followed by a sample and hold circuit. The other 
path of the bifurcation first includes a high frequency differential 
amplifier which in preferred form performs a differentiation function, 
whereby amplitude and frequency criteria may be applied simultaneously 
based on time rate of change of the EKG signal. In this fashion, it is 
possible to discriminate between the stimulating pulse and certain 
similarly composed aspects of the electrocardiograph signal, such as the 
R-wave. For such signals which exceed the joint frequency/amplitude 
criteria, indicating occurrence of a stimulating pulse, a "hold pulse" 
generator is activated, typically a one shot timer circuit which operates 
a sample and hold switch located at a terminating junction of the 
bifurcated paths. An EKG signal which occurred just prior to initiation of 
the stimulating pulse thereby is held for an appropriate time, determined 
by the duration of the hold pulse, whereafter the normal EKG channel 
passes subsequent signals. In a preferred form of the present invention, 
the sample and hold operation extends not only for the actual duration of 
the associated transcutaneous electrical nerve stimulation pulse, but 
furthermore for a time thereafter appropriate to insure that "clean" EKG 
signals will once more be sensed. Finally, appropriate attenuation is 
utilized to scale the remaining signal (i.e. the EKG signal) for 
introduction into a conventional EKG amplifier and monitor. 
It is a foremost feature of the present invention that the occurrence of 
the interference (i.e. TENS) signal is detected in the true (i.e. EKG) 
signal, with no additional connection required from the TENS unit for 
synchronization of sampling/holding operations. This provides considerable 
advantage and freedom in selection of TENS units with disparate 
operational characteristics, and in application of any given such unit to 
the patient free of inteconnection and mechanical or electrical interface 
connections or constraints.

BEST MODE FOR CARRYING OUT THE INVENTION 
In the ensuing discussion, little attention is accorded the actual 
mechanism of application of transcutaneous stimulating electrodes to a 
patient, or to any precise apparatus for generating the TENS signals and 
transmitting them to the electrodes. It is to be understood that the art 
of transcutaneous electrical nerve stimulation for pain control is by now 
reasonably well developed, and that numerous units are commercially 
available from a variety of sources, (including the assignee hereof), 
involving electronics and electrode construction of varying degrees of 
sophistication and expense. Indeed, the preponderance of commercial units 
operate within the constraints set forth in the aforementioned Hagfors et 
al. patent. To the extent necessary, that patent is incorporated by 
reference herein for purposes of completing the instant disclosure, 
including provision of illustrative circuitry, specification of TENS wave 
form parameters, and positioning of electrodes on the patient. 
In accordance with the principles of the present invention, the body of the 
patient serves as an interconnection between the TENS unit and the 
electrocardiograph, with the interaction of TENS and EKG being sensed by 
EKG electrodes, then being appropriately processed intermediate the 
electrocardiograph sensing electrodes and an electrocardiograph system 
which preferably, although not necessarily, is one of common commercial 
pedigree. It is also to be understood that the positioning of EKG 
electrodes on the patient is, in accordance with the principles of the 
present invention, the same as would be employed in accordance with 
conventional EKG methods absent the application of electronic pain 
control. 
Referring, then, to FIG. 1, patient leads are shown at 100, it being 
understood that such leads are appropriately connected, as is known in the 
art, to suitably positioned EKG sensing electrodes. The EKG signals are 
coupled, by the leads, to an input differential amplifier 101. Generally, 
the amplifier 101 employs a high input impedance, high common mode voltage 
capability, and very high common mode rejection. In this respect, the 
differential amplifier 101 involves high frequency suppression networks 
and pulse suppression networks, offset adjustment, and common mode 
rejection adjustment. At amplifier output node 102, the signal path 
bifurcates, and the signals from amplifier 101, which still include both 
EKG and TENS signals (when the latter occur) are coupled both to a delay 
unit 103 and to an amplifier-differentiator 106. The signal path through 
delay element 103 and buffer amplifier 104 is the principal signal path, 
and the delay 103 is provided to compensate for time taken by information 
processing in the alternative path, which includes a differentiator 106 
and a generator of holding pulses 107. Hence, EKG signals normally pass 
from amplifier 101, through delay unit 103 and amplifier 104, through a 
switch 105 and to output circuitry 108, so long as no TENS signals are 
superimposed over the EKG signals. Typically, the output circuit 108 
involves attenuation in order to scale the EKG signals into the range 
commonly acceptable to EKG monitors, designated at 110. The differentiator 
106 and generator 107 serve to detect the presence of TENS signals in the 
EKG waveform, and when such signals are detected, to issue a holding pulse 
at 107 which energizes the sample and hold switch 105 to interrupt the 
continuing coupling of EKG signals to the output circuit 108, such holding 
continuing an appropriate time after the TENS signal pulse has ceased. 
Such holding time is in essence established by the duration of the holding 
pulse issued by generator 107. 
In accordance with the principles of the present invention, the 
amplifier/differentiator 106 involves sections, generally parallel to one 
another, for respective processing of positive and negative transitions of 
signals from node 102, the subsequent combination thereof, and level 
shifting and/or amplification of signals above a certain level 
(corresponding to TENS signals but excluding any aspects of EKG signals). 
Thus, the evaluation of the derivative of combined EKG/TENS signals (i.e. 
utilization of the time rate of change, or slope) provides an effective 
joint amplitude and frequency criterion for discrimination of TENS signals 
from EKG signals. Level sensitive amplification of the derivative permits 
the hold pulse generator 107, embodied as a one shot/timer circuit, only 
to fire a hold pulse upon receipt of an input signal (i.e. amplitude 
processed time derivative of a signal from node 102) which is a TENS 
signal (or other spurious signals), but not any aspect of the cardiac 
function signal represented by the EKG trace. It will be understood that 
frequency and amplification criteria may be established in manners other 
than utilization of time derivatives and level sensitive amplification as 
described in conjunction with FIG. 1. 
The operation of the illustrative embodiment set forth in block 
diagrammatic form in FIG. 1 may perhaps be better understood upon 
consideration of the circuit schematic set forth in FIG. 2, which in fact 
exemplifies a preferred embodiment of the principles of the present 
invention. In FIG. 2, each of the functional blocks 101 through 108, 
inclusive, of FIG. 1 is provided as a phantom enclosure for detailed 
circuitry which performs the operations attributed to the corresponding 
functional element of the FIG. 1 embodiment. 
The electrocardiograph leads, designated 100 in FIG. 1, are shown in FIG. 2 
as respective negative, reference, positive, and ground connections. The 
former three are connected to respective RC high frequency suppression 
networks 201-202, 203-204, and 205-206. Also, each is coupled to a pulse 
suppression network defined by respective diode pairs 207, 208, and 209. 
Amplifiers 210, 211, and 212, considered together with their respective 
feedback, biasing, and interconnection components, jointly form a 
differential amplifier. Offset adjustment is provided by variable resistor 
214, and common mode rejection adjustment is provided by variable resistor 
215. As noted, the output signals from amplifiers 211 and 212 are coupled 
to the input of a differential amplifier 210, and amplifier 213 feeds 
common mode signal, in inverse phase, back to the patient (i.e. via pulse 
suppression network 208 and high frequency suppression network 203-204), 
for improved common mode rejection. 
Signals at node 102, at which the signal path divides, include EKG signals 
and, sometimes, TENS signals. The output of amplifier 101, along the 
principal signal path, is first coupled to the delay network 104, defined 
by resistor 216 and capacitor 217, and then to an amplifier 220, with 
associated feedback and biasing circuitry. The output of buffer amplifier 
104 is coupled to, and as appropriate, through, a junction FET switch 227 
and to a holding capacitor 231. The ability of JFET 227 to pass further 
signals from amplifier 220 on to capacitor 231 and therebeyond is 
dependent on the presence or absence of TENS signals in the EKG signals, 
as detected by high frequency amplifier 106 and coordinated operation of 
hold pulse generator 107. 
As shown, signals from node 102 are coupled to a pair of amplifier sections 
218 and 219, the former of which 218 processes positive transitions of any 
TENS signals present and the latter of which 219 processes negative 
transitions. Outputs of the amplifiers 218 and 219 are combined by diodes 
221 and 222 and resistor 223, and then delivered to a level sensitive 
amplifier 224 for appropriate level shifting and gain. The combined 
operation of amplifiers 218 and 219 and the combination of signals 
therefrom at diodes 221 and 222 and resistor 223 develops a signal 
proportional to the time derivative of the signal at node 102; the 
amplitude of that derivative, when adequate to energize transistor 224, 
corresponds to detection of a TENS signal in the EKG waveform. Appropriate 
biasing circuitry for transistor 224 establishes this precise level. As 
appropriate, transistor 224 energizes a one shot circuit preferably 
embodied by a timer of the type commonly known as a "555" timer, with 
appropriate interconnection as shown. The one shot 226 drives the switch 
227 through transistor 228 and diode 229. 
Thus, when TENS signals are detected at 106 and 107, the switch 227 ceases 
to conduct, and the voltage on capacitor 231 immediately prior to such 
cessation, continues to be held and coupled to the EKG monitor. The 
duration of the holding time, established by the one shot 226 together 
with its associated circuitry suitably embraces the TENS pulse plus a 
desired time thereafter. When JFET 227 is again rendered conductive, the 
EKG signals, again free of TENS interference, resume charging and 
discharging of capacitor 231, and thereby conveying the time varying EKG 
signal to the output. 
Output stage 108 is defined by a buffer amplifier 232 whose gain is 
adjusted by variation of resistor 250. EKG signals from amplifier 232 are 
fed to an RC filter 233 and 234, and then to an output resistor network 
235-238, inclusive, which are separated by output terminals 239-242 
inclusive. The network defined by resistors 235-238 serves first to 
attenuate the signal to such levels as EKG monitors are adapted to 
operate, and secondly, to provide an input configuration for the EKG 
monitor such that the EKG input amplifier's active feedback will operate. 
The foregoing has set forth illustrative and preferred embodiments of the 
principles of the present invention; it is to be understood that numerous 
alternative embodiments will occur to those of ordinary skill in the art 
without departing from the spirit or scope of the present invention.