Patent Document:

the method of the invention uses non - invasive sensors placed at one or more sites on a patient &# 39 ; s body to monitor primary and reference parameters , as follows : a non - invasive sensor array that includes sensors for emg , amg ( in the below - described study ), and peripheral temperature ( fig1 a ) is placed at each monitor site ; ekg and reference sensors are applied where and / or as appropriate ( fig1 b ). splash - proof coverings can be used to protect each monitor site . to discriminate the cephalo - caudal spatial effect of the neural block it is preferable to use multiple monitor sites as shown in fig1 . in the clinical tests described below , three specific sites were identified for use : the fourth thoracic dermatome ( nipple , t4 ) and tenth thoracic dermatome ( umbilicus , t10 ) levels on the anterior axillary line and the anterior of the thigh , representing the second lumbar dermatome ( l2 ). for muscle innervation the level and density of the neural blockade is quantified by , respectively , the placement of the sensor array and a change in the signal amplitude of the surface emg , wherein the density is inversely proportional to the signal amplitude . for skin temperature , the level and density of the neural blockade is quantified by , respectively , the placement of the sensor array and a change in skin temperature , wherein the density of the neural blockade is directly proportional to skin temperature . finally , for heart rate , the level and density of the neural blockade are quantified by a change in heart rate , wherein the density of the neural blockade is inversely proportional to heart rate . the invention was demonstrated in a study involving seven men ( table 1 ) who were all undergoing elective radical retropubic prostatectomy using lumbar epidural blockade . table 1______________________________________subject demographics epi - time - to - sub - age weight asa dural incisionject ( years ) ( kg ) status site ( min ) notes______________________________________1 55 85 . 7 2 l4 - 5 n / a combined ga and regional due to history of apnea ; epidural dosed after incision2 42 83 2 l4 - 5 17 . 03 47 78 2 l2 - 3 17 . 74 63 104 2 l2 - 3 12 . 55 55 103 2 l3 - 4 15 . 06 65 110 2 l3 - 4 29 . 3 converted to combined ga and regional7 60 88 2 l - 5 15 . 5______________________________________ time from main epidural dose to start of incision spontaneous surface electromyogram ( emg ), acoustomyogram ( amg ) and temperature ( t ) measurements were made along the anterior axillary line at t4 , t10 , and l2 dermatomal levels along with lead - ii electrocardiogram ( ekg ). reference measurements included tympanic and ambient temperature , and ambient sound . as discussed below , time - series data were acquired before epidural dosing and at predefined intervals after dosing . a dedicated pc - based system provided system control and data storage . based on the predicted temporal response to epidural dosing , data epochs were collected immediately prior to the main epidural dose ( baseline ) and at 2 , 5 , 10 , 15 , 20 , 25 , 30 , 45 , 60 , 75 , and 90 minutes after dosing . the epochs were designed to have a minimum duration of 20 seconds . temperature data records were time - continuous , starting immediately after the thermocouples were placed and ending approximately 2 minutes after the last emg / amg / ekg sampling epoch . table 2 below summarizes the data acquisition conditions for the parameters of interest . table 2______________________________________data acquisition conditions for monitored parameters sampling rate no . ofparameter passband ( hz ) ( sec . sup .- 1 ) channels______________________________________amg 0 . 5 - 100 1000 4ekg 2 - 100 1000 1emg 10 - 500 2000 3temperature dc - 0 . 02 0 . 2 6______________________________________ as discussed below , root - mean - square ( rms ) of emg and amg , average t , and average heart rate ( r - r interval ) were assessed for the levels as a function of time relative to epidural dosing . changes in objectively monitored variables were compared to qualitative assessment ( e . g ., pinch test ) of the block effectiveness . both emg and amg are broadband signals that contain information in the time and frequency domains . what is needed is a single derived value that is descriptive of the instantaneous physiological condition of the patient . typically , the power in the signal provides such an indication in the time domain . power is proportional to the square of the amplitude of the signal . and , for a signal that has a non - constant ( e . g ., alternating current ) component , it is necessary to average over a finite period of the signal in order to generate a meaningful value . emg assessments have traditionally computed the average rectified emg ( aremg ) or the rectified integrated emg ( riemg ), given by : ## equ1 ## the historical computation methods are clearly not power indicators , so the invention uses the root - mean - square ( rms ) estimator : ## equ2 ## ( note , other methods , e . g ., spectral analysis , bi - spectral analysis , etc ., may also be useful . the gradient of the signals may also provide useful information .) regardless of the method used , selection of the integration interval ( i . e ., the value of n ) is an important factor . the value of n is inversely proportional to the upper frequency response characterized by the computation . an integration interval was selected that retained as much of the passband information as is practical . emg are integrated over 50 msec ( n = 100 ) and amg signals are integrated over 250 msec ( n = 250 ). many discrete rms computations are possible within 20 - second sampling epochs . running rms values are computed . the rms value at t = 0 includes the first n data points in a series . subsequent rms values incrementally delete the earliest data point and add the next latest point of the series . the minimum value of the computed rms sequence is selected as representative of the muscle - only condition for any pre - defined sampling epoch . the time - series data from the study subjects demonstrated that emg and amg signals contained noise artifacts that correlated to the use of the electrocautery , suction , and other electronic systems . the slope and intercept of the frequency spectra of the data were used as acceptance criteria to validate that the rms data values were devoid of noise . the absolute value of emg signals can be influenced by conditions such as skin conductance , skin temperature , electrode displacement , and site preparation . to mitigate discrepancies , the emg data were normalized , which involved applying a gain factor that resulted in an expansion or compression of the histogram of the raw data sets . based on a common time epoch for all study subjects , emg data were normalized such that the basis histogram contained 50 % of all data values between ± 5 mv . other epochs were scaled by the same gain factor calculated for the basis . it was apparent that amg signal artifacts were correlated to motions directly induced by the surgeons or indirectly induced by movements near the patient . amg data are compensated , on a sample - by - sample basis , for the presence of acoustically transmitted noise by subtraction of the ambient signal level . changes in skin temperature are adversely influenced by changes in both body core temperature and ambient air temperature . to accommodate these conditions , the raw ( t dl , si ) dermatome temperature value is standardized ( t dl , si ) by the tympanic and ambient temperatures as in the following : where dl is the dermatome level and si is the sampling interval . si = 0 correlates to the data epoch before the main epidural dose . standardized temperature data were then parsed into 20 - second segments that correlated with the emg / amg / ekg sampling epochs . the average of each of the normalized epochs was computed as a simple arithmetic mean of the data . each study subject &# 39 ; s average heart rate was determined from the lead - ii ekg data sets . the average for the 20 - second data intervals was computed as a simple arithmetic mean of the related r - to - r intervals . the occurrence of the leading edge of the r peak was determined by an automated algorithm . the data revealed three notable aspects of the emg signals . first , emg signal amplitude decreased at a rate inversely related to the number of dermatomal levels separating the monitor site and the epidural catheter . that is , the signals at the more cephalad dermatomal levels decreased later than the more caudal levels . second , after a period of time ( between 15 and 30 minutes ), all emg signal amplitudes assumed a generally constant and lower level than at initiation of the primary dose . third , the constant level at the l2 dermatomal level was approximately 30 % lower than that of the t4 and t10 dermatomal levels , perhaps indicating that effective block was created at this discrete level simply by the test dose . average temperature trends for the tympanic - and ambient - compensated data all showed a gradual increase as a function of time ; longer onset was noted at more rostral dermatomal levels . heart rate increased by an average of 7 . 8 ± 9 . 2 bpm to 10 minutes following initiation of the primary dose , then decreased an average of 19 . 0 ± 10 . 8 bpm in the following 15 minutes . in sum , temporal changes in emg , skin surface temperature , and heart rate were associated with adequate blockade and correlated to the clinical assessment of the level and density of the block . during block onset the rms emg signal level decreased & gt ; 15 mv , temperature increased & gt ; 1 ° c ., and ekg decreased & gt ; 7 beats per minute ( bpm ). amg showed primary correlation to external influences ; response to the block was not evident in the presence of noise . fig2 shows a set of curves of the rms emg signal amplitudes for all seven subjects . the data are fifth - order polynomial curves fit to the normalized signal amplitudes for the seven study subjects at each of three monitor levels . the plot also indicates the typical dermatomal level of blockage achieved , according to pinch tests administered by the attending anesthesiologist . time is referenced to the administration of the main epidural dose ( t = 0 ) and the baseline data set is plotted at t =- 1 minute . the traces are normalized to the baseline by subtracting the respective baseline value from each successive data point . the time between the main dose and the surgical incision was 17 ± 6 minutes . lower amplitude limits of the l2 and t10 signals are reached sooner ( t = 15 minutes ) than the t4 signal ( t = 30 minutes ). this suggests that the l2 and t10 levels achieve onset before the t4 level , as expected . fig3 presents average temperature data for the dermatomal levels for all seven subjects . all levels exhibit the same upward trend as a function of time ; they approach a relative increase of approximately 3 . 5 ° c . over the 90 - minute data collection period . the changes in temperature reflect the effect of sympathectomy upon administration of local anesthetic . these changes also suggest differences in the contribution of blood volume changes and vascular relaxation at each dermatomal level . the average heart rate for all seven subjects is shown in fig4 . these data conform to the previously reported effect of local anesthetic on the cardiac accelerator fibers in the t1 - t4 spinal segments . the use of passive and non - invasive monitors to objectively distinguish the level and density of neural blockade as a function of time , and as related to the administration of local anesthetic , has been examined . the data from the emg , temperature , and ekg monitors , combined with the clinical assessment provided by the anesthesiologist and the conditions of the operative procedure , confirm that an objective measure of block level and density can be performed in the clinical setting . a universal neural blockade monitor must be fully functional regardless of when it is applied , relative to the administration of the anesthetic agent . the rate of change , or gradient , of the absolute signals presents salient information . the most compelling indicator is the change in emg signal level between the time of zero minutes and 10 minutes , as shown in fig2 . level l2 decreases approximately 1 . 5 times that of t10 and 2 . 5 times that of t4 . using the monitored parameters in combination rather than singly will enhance the utility of a universal monitor . the weighted summation of emg and temperature will likely satisfy the basic requirement for a level - discriminating determination of block density . the method of the invention can be integrated into an automated system for controlling drug delivery to the patient or simply notifying the physician or nurse about patient status , e . g ., during post - op recovery . the invention is also applicable to use with animal patients . the onset of neural blockade can be objectively monitored by placement of one or more sensors and quantifying a decrease in signal amplitude of a surface emg ; an increase in skin temperature ; and changes in heart rate . moreover , the blockade density determined by these objective means appears to compare favorably with the traditional subjective method of pinch - tests . the invention provides the anesthesiologist with a passive , objective tool for real - time , non - invasive monitoring of the level and density of neural blockade .

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