there are a great many possible implementations of the invention , too many to describe herein . some possible implementations that are presently preferred are described below . it cannot be emphasized too strongly , however , that these are descriptions of implementations of the invention , and not descriptions of the invention , which is not limited to the detailed implementations described in this section but is described in broader terms in the claims . fig1 shows a schematic of a preferred implementation . this implementation includes an accelerometer ( and accelerometer housing ), force sensor built into a pressure pad , and an aed which is electrically connected to the accelerometer and force sensor and contains a display and / or speaker for user feedback . the pressure pad provides the structural member on which the accelerometer ( and housing ) is supported . neither the accelerometer nor force sensor of the pad are essential to detecting chest release , as other sensors can be used . the force sensor can measure force or pressure . the accelerometer housing can be shaped similar to a hockey puck and can rest either directly on the patient &# 39 ; s sternum or on the pad or other structural member . preferably the accelerometer is positioned to be over the victim &# 39 ; s sternum in the position recommended for chest compressions . a force sensor can be placed below ( as shown ) or above the accelerometer housing . the rescuer presses on the accelerometer housing ( or pressure pad ) to perform chest compressions . the accelerometer senses the motion of the chest during cpr and the force sensor measures the force or pressure applied . the aed supplies power to the sensors and digitizes the electrical signals coming from the accelerometer and force sensor . based on previous calibrations of the sensors , the accelerometer signal is integrated to determine the housing displacement , and the output of the force sensor is converted to standard pressure or force units . fig2 shows a sample drawing of the signals recorded during cpr using the implementation of fig1 . the acceleration signal is band pass filtered and integrated to derive displacement information ( e . g ., a displacement signal ). compressions ( c 1 - c 5 ) can be detected from the displacement signal . the compression rate is calculated from the interval between compressions ( e . g . ( time of c 2 − time of c 1 )), and compression depth is measured from the compression onset to peak displacement ( e . g . ( d 1 − d 0 )). the onset and peak compression values are saved for each compression . the pressures at the compression onset and offset are used to determine the force used to achieve a given compression depth . the compliance of the chest can be estimated from the compression displacement and the related compression pressure . the pressure “ p 0 ” is the reference pressure prior to the start of cpr and is related to the resting chest displacement “ d 0 ”. the pressure “ p 1 ” is the pressure required to achieve the displacement “ d 1 ”. the chest compliance is estimated from the following equation : where d 1 is the displacement at the peak of the compression , d 0 is the displacement at the onset of the compression , p 1 is the pressure at the peak of the compression , and p 0 is the pressure at the onset of the compression . the chest compliance can be calculated for each compression and averaged to improve the measurement accuracy . once the patient specific chest compliance is known , it can be used to estimate the absolute displacement of the puck when combined with the instantaneous puck pressure measure from the following equation : where p is the pressure measured from the puck at a point in time , p 0 is the resting puck pressure when there is no compressions or handling by the rescuer . therefore , the chest release displacement can be estimated by the following equation : where compliance is determined as described above , p 3 is the chest release pressure ( estimated as the onset pressure of the next compression ), and p 0 is the resting pressure . the chest release pressure can alternately be measured as the minimum pressure point between the two compressions . the chest release displacement point is compared to a pre - defined threshold level to determine if the chest was substantially released between two compressions ( i . e ., released sufficiently to create a pressure in the chest that facilitates venous filling of the heart ). a combination of voice prompts and display messages can be used to tell the rescuer to more completely release the chest between compressions if the chest release displacement point does not return below the set threshold . the chest release displacement value can be averaged to improve the estimate accuracy . the comparison to the threshold level could also be done via “ voting ” logic such as the last x out of y values exceed the set threshold and trigger the release of chest feedback . the cpr release of chest algorithm continually runs while the rescuer performs cpr and provides immediate feedback if the rescuer does not release the chest at any time during the resuscitation . although not necessary , the threshold level is preferably adjusted dynamically as a function of the calculated chest compliance . for patients with a lower compliance , the threshold can be increased since increased force will have little or no effect on displacement . for patients with higher compliance , the threshold may need to be decreased . the calculated estimate of chest compliance can also be used with the output of the force sensor to estimate the depth of chest compression . thus , for example , the output of the accelerometer could be used with the output of the force sensor during an initial time interval to calculate an estimate of chest compliance . thereafter , the estimated chest compliance could be used to convert the force measurement into an estimated depth of chest compression . fig3 shows another implementation wherein the force sensor is replaced with a mechanical or electrical switch . the rescuer performs cpr by pressing on the switch / housing assembly . the switch is activated based on the forces used with cpr compressions and deactivated when a compression is released . the switch may provide for bistable positional states such as in a domed switch that when depressed would provide tactile feedback to the hand of the rescuer upon the start of the compression ( dome collapse ) and at the end of compression ( dome return ). the switch vibration associated with the transition between the two states may also be sufficient to provide an audible feedback to the rescuer as well . if the compression release vibration is heard and / or felt , the chest can be considered by the rescuer to have been released . fig4 shows the acceleration , derived displacement , and switch output signals during a sample of cpr . each compression is identified at the top of the diagram ( c 1 - c 5 ). the compression interval , rate , and depth are measured from the acceleration signal . the dashed line overlaying the switch output curve indicated the force on the puck assembly and is drawn to show the actuation of the switch when the force curve exceeds that activation threshold ( solid straight line ). time t 1 shows the actuation of the switch and time t 2 shows the release of the switch . on the third compression ( c 3 ), the compression switches ( on ) at time t 3 , but does not switch off at time t 4 because the force on the chest does not go below the trigger threshold . the acceleration signal shows that chest compressions are continuing , but the switch indicates that the chest is not being substantially released . when chest release is not occurring , the aed can audibly and / or visually prompt the user to release the chest . in another implementation , the acceleration waveform alone is analyzed without the use of a switch or force sensor . fig5 depicts the acceleration , velocity and displacement for a single compression / decompression cycle . the input signal from the acceleration sensor , as shown in the block diagram in fig6 , is conditioned and filtered to minimized artifact and noise and is input to an a / d converter . the a / d converter is then read by the microprocessor . in fig5 , the points of interest in the acceleration waveform are as follows : 1 . a 0 is the point of maximum acceleration during the compression downstroke . 2 . a - 2 is the compensatory small upstroke that rescuers often do just prior to the initiation of the compression downstroke and marks the initiation point of the compression downstroke . 3 . a - 1 is the point of maximum acceleration of the compression downstroke . 4 . a 1 is the point of maximum deceleration on the decompression upstroke . 5 . a 2 is a small upward release when the rescuer &# 39 ; s hands are slightly lifted from the patient &# 39 ; s sternum during an optimum compression cycle . 6 . a - 3 and a 3 are inflection points where the signal deviates from baseline . 7 . sa 0 and sa 1 are the slopes of the acceleration of the line segments on each side of a 0 . 8 . sv 0 andsv 1 are the slopes of the line segments (˜ acceleration ) as shown on the velocity curve . many algorithms can be used for determination of substantial release of the chest . one algorithm is as follows : 1 . determine fiducial point a 0 . completion of the compression determination should approach real time in order to provide maximum benefit to the rescuer . delays of approximately 1 - 4 seconds are acceptable and will limit the types of ‘ search forward ’ algorithms that can be implemented . a 0 can be detected by a number of means . one method is to band pass filter the acceleration signal to produce maximum output signal amplitude of signals having a slope most closely approximating those observed in real compression signals . the band pass output is then input to a threshold detection function . if the signal amplitude is larger than the threshold , then sa 0 has been detected . the threshold itself may be dynamically adjusted in amplitude to minimize susceptibility to noise and interference . for instance , if out of band noise such as 60 hz interference is detected , then the threshold may be increased . the threshold may also be gradually lowered following an sa 0 detection such that the probability of detection is increased for signals that occur at the expected interval and is decreasing for false signals that may occur immediately subsequent to the detection . once sa 0 has been detected , the algorithm can search forward until it finds the peak amplitude , a 0 . 2 . searching backwards and forwards from point a 0 , the points a - 3 , a - 2 , a - 1 , a 0 , a 1 , a 2 and a 3 can be determined . 3 . the acceleration signal can then be decomposed into constituent triangles formed from these fiducial points . trianglea 0 refers to the triangle formed by the a - 1 , a 0 and a 1 fiducial points ( in gray in fig5 .). 4 . the triangles are then parameterized using such morphological characteristics as width , amplitude , area , center of mass , skewness , height / width ratio , etc . 5 . area ratios are then calculated for the various triangle pairs . for example the ratio of the areas of trianglea 0 and trianglea 1 , acceleration triangular area ratio ( 0 , 1 ) [ tara ( 0 , 1 )] 6 . amplitude ratios are then calculated for the various triangle pairs . degree of skew is incorporated into the amplitude calculation by incorporating either skewness or center of mass into the calculation for each triangle . for example the ratio of the areas of trianglea 0 and trianglea 1 , triangular amplitude ratioa ( 0 , 1 ) ( tampra ( 0 , 1 )) 7 . the same process is repeated for the triangular width ratio ( twr ). 8 . a rescuer who applies too much downward force during the decompression upstroke will cause incomplete decompression . this downward force opposes the natural elastic force of the thoracic cage and as a result causes a decreased amplitude and elongation of trianglea 1 and trianglea 2 as shown in fig7 . 9 . the acceleration signal is integrated beginning at inflection point a - 3 and ending just subsequent to a 3 in order to calculate the velocity . the same analysis is used to calculate the fiducial points v - 2 , vmax , v 0 and v 1 , as well as tar , tampr and twrs for the velocity curve . 10 . the velocity curve segment is integrated a second time to calculate displacement . displacement values d - 3 and d 3 and dmax are calculated . differential displacement , δd = d - 3 − d 3 is calculated . 11 . based on dmax , the device can prompt the rescuer if the depth of compressions are not sufficient . 12 . based on vmax , the user can be prompted to deliver a ‘ sharper ’ more rapid pulse to improve hemodynamics . 13 . end tidal carbon dioxide ( etco2 ) measurements are taken during the course of cpr . visual and / or audible prompting from the device can encourage the rescuer to increase rate and depth of compressions to improve hemodynamics . 14 . the calculated parameters of width , amplitude , area , center of mass , skewness , height / width ratio , tar , tampr and twr for both the acceleration and the velocity as well as δd are used to make a decision on whether the chest was released . the methods used may be standard decision logic ( if - then - else ) or may involve methods such as fuzzy - logic decision methodology or statistical estimation such as bayesian methods . in general , δd alone would not be used to determine chest release , but nonetheless the signal processing methods have made it possible with this method to be able to measure δd without the use of switches or force sensors . 15 . final determination of compression release can be withheld for a number of compression cycles to measure longer term trending of the parameters . for example , the rescuer may have momentarily had to pause to wipe their brow . alternatively , other signal detection and classification methods known to those skilled in the art may be used to determine the relevant morphological features such as those shown in fig7 ( cpr with substantial chest release is shown by solid lines ; inadequate chest release , by dashed line ). in another implementation , a velocity sensor is used to determine the motion parameters . one of many possible techniques for sensing velocity is to use magnetic induction to generate a voltage proportional to the relative velocity between a magnet and coil . the configuration is shown in fig8 . the magnet may take the form of a permanent magnet , but preferably it is an electromagnet . as shown in fig9 a and 9b , a defibrillation pad is placed on the left thorax and another defibrillation pad is placed on the victim &# 39 ; s back in the left scapular area . these are optimal locations for defibrillation and provide a good placement to generate magnetic flux changes proportional to sternal displacement . the coils are incorporated directly into the outer edge of each of the defibrillation electrodes . alternatively , if the desired electrode position is anterior / anterior with both electrodes on the front of the chest , a separate backboard panel may be supplied which is placed under the patient and contains the receiving coil . the use of an electromagnet serves two main purposes : it can be used to calibrate the setup after the electrodes have been applied to the patient and they can be used to provide a synchronous modulation / demodulation of the signal to improve accuracy and minimize susceptibility to noise and artifact . the defibrillation electrodes can be constructed with a conventional configuration . an electrically conductive sheet of material that delivers defibrillation current to the patient is backed with an insulating thin foam material , and a slightly adhesive conductive gel coupling agent adheres the conductive sheet to the patient &# 39 ; s skin . the foam backing also forms an approximately 0 . 5 to 1 . 0 inch border around the active conductive area . the magnetic coil element can be added onto the foam backing and becomes part of the border area , as shown in fig1 . the device ( e . g ., aed ) can be provided with circuitry for determining whether or not the electrodes have been properly applied to the patient . the method currently employed by most manufacturers of defibrillators is to use a small amplitude , high frequency signal (˜ 2 microamps , 60 khz ) to measure impedance . the electrodes are determined to be applied when the impedance falls into the physiologic range . when the device has detected the application of the electrodes , the device can prompt the rescuer to stand clear . at this time , the device will perform calibration of the velocity sensor . a time - varying signal , typically a ramp or sine wave of several frequencies of interest , such as the modulation frequency , is applied to the electromagnet and the signal is measured at the receiving coil . from this , gain and offset coefficients can be calculated for use during the cpr event . this calibration step allows for improved accuracy with patients of varying chest sizes and in the presence of any nearby magnetically conductive surfaces or objects . preferably , a synchronous detector can be employed to minimize susceptibility to noise and artifact as shown in the block diagram in fig1 . a sine wave carrier frequency of 500 hz or more is supplied to the electromagnet coil to generate an oscillating magnetic field that , in turn , induces a voltage on the receiving coil . chest compressions vary the field intensity at the receiving coil , thus causing an amplitude modulation of the carrier . as can be seen in fig1 , a band pass filter immediately subsequent to signal reception reduces interference outside the range of the carrier frequency such as ac magnetic interference . the phase lock loop ( pll ) is used for carrier regeneration , but since the transmitter and receiver are in the same device , the transmission carrier can be used for detection as well , as long as circuitry is provided for phase adjustment of the demodulation signal . multiplexer s 1 , combined with the demodulation signal , causes rectification of the signal , which can then be low pass filtered to recover the compression velocity waveform . alternatively , a synchronous am demodulator can be employed with an analog multiplier stage . in another implementation , the velocity signal may then be used to reduce artifacts in the ecg signal . this is accomplished by first time - aligning the ecg and velocity signal by such methods as cross - correlation techniques known to those skilled in the art . this will provide alignment of the two signals relative to the compressions . then , preferably , adaptive filtering methods are used such as those involved in the minimization of the mean - squared error between the ecg and the velocity . in a further implementation , more sophisticated signal processing methods may be used to minimize ecg artifacts induced by cpr chest compressions . for example , methods known as feed forward active noise cancellation ( fanc ) may be used . fig1 shows a block diagram of the filtered - x least mean squares ( fxlms anc ) algorithm , as developed by widrow and burgess . p ( z ) represents the unknown plant through which the signal x ( n ) is filtered . digital filter w ( z ) is adaptively adjusted to minimize the error signal e ( n ). in one implementation , as depicted in fig1 , x ( n ) is the unfiltered ecg signal , p ( z ) is eliminated from the diagram , and d ( n ) is approximated with the chest compression velocity signal v ( n ). in the lms algorithm , assuming a mean square cost function ζ ( n )= e [ e2 ( n )], the adaptive filter minimizes the instantaneous squared error , ζ ( n )= e2 ( n ), using the steepest descent algorithm , which updates the coefficient vector in the negative gradient direction with step size μ : where ñζ ( n ) is an instantaneous estimate of the mean square error ( mse ) gradient at time n equal to − 2v ( n ) e ( n ). stability and accuracy of the fxlms anc algorithm by adding a variable cutoff low pass filter h ( z ) to eliminate frequency components in the ecg not related to the chest compression artifact . in general , the spectral energy of the chest compression artifact is predominately lower than those of the ecg . a cutoff frequency of approximately 3 hz is adequate in many cases , but this may vary from patient to patient and among different rescuers performing chest compressions . to overcome this difficulty , an fft is performed on v ( n ) and input into a cutoff frequency estimation ( cfe ) procedure that determines the optimal cutoff frequency , fc , for the lowpass filter . in a preferred implementation , the decision is based on calculating the frequency , not to exceed 5 hz , below which 80 % of the waveform energy is present , but this percentage may vary and additional decision logic may be employed . for instance , an fft may also be calculated for x ( n ), also input to the cfe procedure . by first normalizing amplitude of the frequency spectra x ( z ) amplitude peak of the compression artifact and then subtracting the velocity spectra v ( z ) from the normalized input x ′( z ), the difference spectra is calculated δx ′( z )= x ′( z )− v ′( z ). frequencies are then determined for v ( z ) and δx ′( z ) at which most of the spectral energy is within , set in this embodiment to 97 %, and labeled fcv and fcx , respectively , and shown in fig1 . fc is then set to the lesser of fcv and fcx . alternatively , fc can be set to some intermediate frequency between fcv and fcx . a simpler though related implementation is shown in fig1 , in which the cfe procedure is used to calculate the cutoff frequency for a high pass filter . using the same methods as described in the previous paragraph , an fft is performed on v ( n ) and input into a cutoff frequency estimation ( cfe ) procedure that determines the optimal cutoff frequency , fc , for , in this case , a high pass filter . in the preferred embodiment , the decision is based on calculating the frequency , not to exceed 5 hz , below which 80 % of the waveform energy is present , but this percentage may vary and additional decision logic may be employed . an fft may also be calculated for x ( n ), and also input to the cfe procedure and the optimal high pass cutoff frequency can be determined by the methods described in the previous paragraph . for instances when the spectral energy of the compression artifact is distinct from the ecg signal , this method will have a performance equivalent to the fxlms just described ; its performance will be somewhat inferior when the spectra of the ecg and compression artifact overlap , however . one possible implementation is illustrated by a flow chart in fig1 . the front end of an aed acquires both the ecg signal and the cpr signal , which is the velocity of compression of the chest . if chest displacement or acceleration are measured instead of velocity , velocity can be mathematically acquired via one or more integration or differentiation operations from the measurement signal . the velocity signal undergoes pre - processing , and is then fed to an adaptive filter . in a preferred implementation , the pre - processing is a normalization of the velocity signal so that the signal supplied to the adaptive filter is limited to be within 0 and 1 . but normalization is not required . in another implementation , a time - aligning process is performed on the ecg and the reference signal by such methods as cross - correlation . this provide alignment of the two signals relative to the compressions so that the input signals of the adaptive filter are better aligned . but this aligning process is not required . other preprocessing can be applied to the velocity signal to improve the performance of the adaptive filter . in fig1 , x ( n ) and y ( n ) are the input and the output of the adaptive filter h , which can be an fir filter , an iir filter , or another type of filter . in a preferred implementation , the coefficients of the filter are dynamically controlled by the estimated ecg signal : where h ( n ) is a vector containing the filter coefficients , m is a vector containing the step sizes for each filter coefficients , e ( n ) is the estimated ecg signal , and x ( n ) is a vector containing the input data . the estimated ecg signal is computed by subtracting the filter output y ( n ) from the measured ecg signal ( containing artifact ). in some implementations , there is an automated resetting mechanism . when the difference between the filter output y ( n ) and the measured ecg s ( n ) is beyond a threshold , the adaptive filter will reset its coefficients so that the system will not become unstable . other filter structures than the one shown in fig1 , as well as other mathematical representations of the filtering , are possible . fig1 shows samples of the performance of the adaptive filter of fig1 in response to a normal sinus rhythm . the signal in ( a ) is the ecg signal with cpr artifact . the signal in ( b ) is the compression velocity used as the reference signal . the signal in ( c ) is the output of the adaptive filter . fig1 shows samples of the performance of the adaptive filter of fig1 during ventricular fibrillation . the signal in ( a ) is the ecg signal with cpr artifact . the signal in ( b ) is the compression velocity used as the reference signal . the signal in ( c ) is the output of the adaptive filter . as shown in both fig1 and fig1 , the implementation of fig1 is able to suppress the cpr artifacts embedded in the measured ecg signals ( a ). the cpr artifact is nearly , if not completely , removed in the estimated ecg signal ( c ). the velocity signal ( b ) used as a reference signal is clearly correlated with the cpr artifacts in the measured ecg signals ( a ). the adaptive filter assumes that the artifact in the signal is correlated with the reference signal and uncorrelated with the desired signal ( estimated ecg ). it thus adaptively estimates the artifact using the reference signal and subtracts the estimated artifact from the measured ecg signal . the results shown in fig1 are based on a 0th - order fir filter , which simply scales the current sample of the ecg signal adaptively . the cpr artifact was significantly reduced , if not completely removed . this implementation thus combines simplicity and efficiency in its performance . in the applications of adaptive filters , the speed of adaptation convergence is usually controlled by a step - size variable . a faster convergence requires a larger step size , which usually tends to make the filter less stable . the automatic resetting mechanism of some implementations can dynamically change the step size and thus improve the relation of convergence and stability . the coefficients of the filter are updated in a sample - by - sample manner . the changes of the coefficients , i . e ., h ( n )− h ( n − 1 ) is proportional to the product of the step size and the reference signal . the amplitude of the reference signal can thus affect the stability and convergence of the filter . the pre - processing of the reference signal can therefore enhance the performance of the filter by adjusting the reference signal . in another implementation , a time - aligning process is performed on the ecg and velocity signals by such methods as cross - correlation . this provide alignment of the two signals relative to the compressions . then , preferably , adaptive filtering methods are used such as those involved in the minimization of the mean - squared error between the ecg and the velocity . a processing unit could be provided for detecting when compressions are being applied and automatically turning on the adaptive filter . the output of the adaptive filter ( i . e ., the ecg signal with artifact reduced ) could be supplied to a ventricular fibrillation ( vf ) detection algorithm ( e . g ., a shock advisory algorithm ) of an automatic external defibrillator ( aed ) an error signal could be produced that is representative of the difference between the ecg input and ecg output of the adaptive filter . this error signal would give a measure of the amount of cpr artifact in the signal , and it would be useful as a means of modifying the subsequent processing of the ecg . for instance , if the artifact level gets high enough ( e . g ., higher than a first threshold ), the vf detection algorithm thresholds could be increased to make it more resistant to any cpr artifact that still remained in the ecg signal . if the level got even higher ( e . g ., higher than a second threshold higher than the first threshold ), the vf detection could be shut off entirely . in preferred implementation , the filter output is presented graphically on the display of a defibrillator or other medical device incorporating an electro - cardiographic function . the filter output may also be printed on a strip - chart recorder in the medical device . alternatively , the filter output may provide the input signal for subsequent signal processing performed by the processing means . the purpose of such signal processing may take the form of qrs detection , paced beat detection during pacing , arrhythmia analysis , and detection of ventricular fibrillation or other shockable rhythms . spectral analysis could be performed on the error signal , and based on the major bands of frequency content of the error signal , the pre - filtering of the ecg signal prior to the vf detection can be adjusted . for instance , if the error signal is found to reside primarily in the 3 - 5 hz band , additional filtering can be provided in that band prior to input into the vf detection ( or other ecg processing ) algorithm . many other implementations of the invention other than those described above are within the invention , which is defined by the following claims . for example , methods of adaptive channel equalization may be employed to ameliorate both synchronization and phase errors in the velocity waveform . kalman filtering techniques may also be employed to improve performance of the filter when rescuer performance of chest compressions changes over time and is better modeled as a non - stationary process . time alignment of the ecg and velocity signal may also be accomplished by such methods as cross - correlation techniques known to those skilled in the art . this will provide alignment of the two signals relative to the compressions . then , preferably , adaptive filtering methods are used such as those involved in the minimization of the mean - squared error between the ecg and the velocity . in a further implementation , more sophisticated signal processing methods may be used to minimize ecg artifacts induced by cpr chest compressions . for example , methods known as feed forward active noise cancellation ( fanc ) may be used . fig1 shows a block diagram of the filtered - x least mean squares ( fxlms anc ) algorithm , as developed by widrow and burgess . p ( z ) represents the unknown plant through which the signal x ( n ) is filtered . digital filter w ( z ) is adaptively adjusted to minimize the error signal e ( n ). in one implementation , as depicted in fig2 , x ( n ) is the unfiltered ecg signal , p ( z ) is eliminated from the diagram , and d ( n ) is approximated with the chest compression velocity signal v ( n ). in the lms algorithm , assuming a mean square cost function ζ ( n )= e [ e2 ( n )], the adaptive filter minimizes the instantaneous squared error , ζ ( n )= e2 ( n ), using the steepest descent algorithm , which updates the coefficient vector in the negative gradient direction with step size μ : where ñζ ( n ) is an instantaneous estimate of the mean square error ( mse ) gradient at time n equal to − 2v ( n ) e ( n ). stability and accuracy of the fxlms anc algorithm can be improved by adding a variable cutoff low pass filter h ( z ) to eliminate frequency components in the ecg not related to the chest compression artifact . in general , the spectral energy of the chest compression artifact is predominately lower than those of the ecg . a cutoff frequency of approximately 3 hz is adequate in many cases , but this may vary from patient to patient and among different rescuers performing chest compressions . to overcome this difficulty , an fft is performed on v ( n ) and input into a cutoff frequency estimation ( cfe ) procedure that determines the optimal cutoff frequency , fc , for the lowpass filter . in a preferred implementation , the decision is based on calculating the frequency , not to exceed 5 hz , below which 80 % of the waveform energy is present , but this percentage may vary and additional decision logic may be employed . for instance , an fft may also be calculated for x ( n ), also input to the cfe procedure . by first normalizing amplitude of the frequency spectra x ( z ) amplitude peak of the compression artifact and then subtracting the velocity spectra v ( z ) from the normalized input x ′( z ), the difference spectra is calculated δx ′( z )= x ′( z )− v ′( z ). frequencies are then determined for v ( z ) and δx ′( z ) at which most of the spectral energy is within , set in this embodiment to 97 %, and labeled fcv and fcx , respectively , and shown in fig2 . fc is then set to the lesser of fcv and fcx . alternatively , fc can be set to some intermediate frequency between fcv and fcx . the quality of other physiological signals , such as impedance cardiographic ( icg ), impedance pneumographic ( ipg ), or pulse oximetry , known to those skilled in the art , may also be also be enhanced by the filter , particularly if the sensor is located on the thoracic cage in nearby proximity to the motion sensor from which the velocity signal is derived . minimization of compression artifact with impedance pneumography signals can be accomplished with any of the previously described methods . the adaptive filter can be used to minimize the cross - correlation of the adaptive - filter output with the reference signal or the cross - correlation of the adaptive - filter output with the measured ecg signal . many other implementations of the invention other than those described above are within the invention , which is defined by the following claims . for example , it is not necessary that the invention include an external defibrillator , as a device for assisting delivery of cpr could be provided without defibrillation capability . the cpr assistance device could even be a pocket device that is for assisting with manual delivery of cpr . features of the one aspect of the invention may not be required in implementations of other aspects of the invention . for example , it is not necessary in some implementations of the invention that chest compliance be determined , or that substantial release of the chest be determined , or that a particular type of sensor ( e . g ., accelerometer , force sensor , velocity sensor ), or combination of sensors , be used , or that there be analysis of features of a motion waveform , or that maximum velocity be estimated , or that artifacts in detected ecg signals be reduced .
an apparatus for assisting a rescuer in performing chest compressions during cpr on a victim , the apparatus comprising a pad or other structure configured to be applied to the chest near or at the location at which the rescuer applies force to produce the chest compressions , at least one sensor connected to the pad , the sensor being configured to sense movement of the chest or force applied to the chest , processing circuitry for processing the output of the sensor to determine whether the rescuer is substantially releasing the chest following chest compressions , and at least one prompting element connected to the processing circuitry for providing the rescuer with information as to whether the chest is being substantially released following chest compressions .