Abstract:
Method and apparatus for measuring the heart rate in a human body. A first and second derivative of an electrical signal from the heart known as the QRS complex or R-wave is measured, and the values of the peak of the R-wave, the peak of the first derivative of the R-wave, and the peak of the second derivative of the R-wave are determined. The three peak values are then multiplied together to provide an output responsive to the occurrence of the R-wave which is greater than either the value of the peak of the R-wave, the value of the peak of the first derivative of the R-wave, or the value of the peak of the second derivative of the R-wave.

Description:
FIELD OF THE INVENTION 
     The present invention relates to implantable cardiac stimulation devices, such as pacemakers, defibrillators and cardiaverters. More particularly, the present invention relates to an enhanced cardiac signal sensing system for sensing the occurrence of an R-wave. 
     BACKGROUND OF THE INVENTION 
     The field of implantable cardiac devices, such as pacemakers, defibrillators, and cardioverters, is well known. These devices typically monitor cardiac response under a variety of conditions. Sudden cardiac death presently claims an estimated 400,000 lives annually in the United States. To prevent sudden death, rapid treatment of cardiac conditions is required. Rapid treatment can be provided by implantable cardiac devices only if the heart rate can be accurately and reliably sensed. One potentially catastrophic cardiac event is fibrillation, wherein the heart ceases to function as a blood pump. Unless a functional heart rate is quickly reestablished, death can occur within minutes. Once the abnormal heart rate is sensed, a normal heart rate or sinus rhythm may be reestablished by application of a large electrical shock on the order of 10 joules or more, to defibrillate the heart. 
     Another serious condition is tachycardia or tachyrhythmia, which is a rapid heart rate which may eventually lead to fibrillation. Once the rapid heart rate is sensed, a normal heart rate or sinus rhythm may be reestablished by application of a slightly more rapid rate of very low level pacing shocks of less than 100 micro joules to capture the heart rate and slow the heart to a normal rate. The normal sinus rhythm may also be reestablished by application of electrical shocks on the order of 1 joule to perform cardioversion on the heart. 
     The necessity of identifying persons likely to suffer tachyrhythmia or fibrillation has led to the preventative step of implanting a device known as an Implanted Cardioverter Defibrillation (ICD) device. The ICD device is electrically attached to the patient&#39;s heart. Rather than relying on the patient or on attending medical personnel to identify a cardiac fibrillation event, the ICD device uses automatic triggering. With automatic triggering, accurate monitoring and detection of the heart rate is required to detect the cardiac fibrillation event and trigger the ICD device to defibrillate the heart. 
     Monitoring and detection of cardiac function typically involves electrical sensing of muscle and nerve cell depolarizations which can be correlated with cardiac muscle contractions. Electrodes are implanted in the heart which sense an electrical voltage which is measured over time to produce an electrocardiogram wave form. The electrocardiogram wave form under normal conditions includes a P wave, followed by a complex three-part wave form called the QRS complex, and then a T wave. Of these various components, the QRS complex or R-wave has a dominant amplitude feature and is therefore most typically used to sense the heart rate. The R-wave is the portion of the electrocardiogram wave form having the steepest slopes and the sharpest peaks. The heart rate is the interval between R-waves, and is sometimes termed the R—R interval. R-waves typically have a peak amplitude in the range of about 5 to 15 mv during a normal sinus rhythm. T waves typically have a peak amplitude of about half of the R-wave amplitude. Noise and extraneous muscle movements typically have peak amplitudes in the range of about 0.1-1 mv. During fibrillation, the R-wave amplitude may diminish to as little as 20% of normal amplitude (e.g., 1.0 to 3.0 mv), thus making the R-wave amplitude indistinguishable from noise levels. 
     In one approach, detection systems sense the occurrence of R-wave electrical events and signals exceeding a preset constant voltage, where the constant voltage is fixed at a preset amplitude between 3.4 to 10 mv. Such triggering levels start at approximately 67% of the amplitude of a normal R-wave, which is higher than typical noise or T-wave amplitudes. During fibrillation, the R-wave amplitudes decrease to a range of 0.5 to 2.0 mv. Unfortunately, this type of prior art detection system having the preset and fixed amplitude detection threshold is incapable of distinguishing or sensing diminished or degenerating R-wave events during fibrillation. With this approach, R-wave information on heart rate is completely unreliable during fibrillation. 
     More recently, detection systems have attempted to address the problem of sensing diminishing R-wave amplitudes during fibrillation by employing a sensitivity threshold which starts at a preset amplitude and then subsequently becomes more sensitive until a floor threshold is reached. The preset amplitude is typically 67% of the preceding R-wave amplitude, and the floor threshold is typically set at 0.3 to 0.5 mv. This floor threshold prevents the sensitivity threshold from dropping to such low levels that the increased sensitivity begins to incorrectly detect noise as R-waves. This reduction in threshold amplitude and increase in sensitivity occurs in the form of an exponential decay with a time constant on the order of 1 to 1.5 seconds. This time constant is reset after each R-wave event. In one example, the “SENTINEL”™ brand ICD device, developed by Angeion Corporation, employs a detection mechanism in which the initial threshold is a preset percentage of the most recent R-wave peak amplitude, and the decay is a standard exponential. The threshold used in the “SENTINEL”™ device is lowered from the initial level until a constant floor threshold is reached. In another example, an initial threshold reset may be used which decays with a reverse exponential time constant to the floor threshold, where the floor threshold is set to a constant level which is greater than the noise level. 
     One problem with these approaches is temporary supersensitivity when a low amplitude R-wave occurs during normal sinus rhythm or tachycardia. This problem results from the decay being based upon a fixed time constant, or from the detection mechanisms resetting the initial threshold after each R-wave detection. Once the detection mechanisms observe a low amplitude R-wave, for example, of less than 3 mv, the initial sensitivity threshold is set to a low level of about 2 mv or less, and then proceeds to decline to the floor level of about 0.3 to 0.5 mv. At this level, the sensitivity threshold may allow noise to be falsely detected as R-waves. This problem is particularly severe in situations where the floor threshold is quite sensitive (e.g., when the floor setting is 0.4 mv and the noise level is 0.5 or 0.6 mv). This incorrect or false detection of noise as R-waves resets the initial threshold sensitivity to inappropriately low levels resulting in possible adverse effects to the patient. These adverse effects may include the initiation of electrical shock therapy to the heart based upon noise timing rather than R-wave timing. The electrical shock therapy would continue until a true R-wave was detected and the system could correct itself. 
     More recently, in another approach, an ICD detection method for sensing the occurrence of an R-wave attempts to distinguish R-waves from noise through the use of variable declining sensitivity thresholds. This approach, disclosed in U.S. Pat. No. 5,709,215 and developed by Angeion Corporation, considers the amplitude of at least the previous most recent R-wave, and determines a declining threshold of sensitivity which is used to recognize a subsequent electrical signal as an R-wave. With this approach, the amplitude of the previous R-wave may be classified, based on amplitude. Based upon the classification, a desirable time constant for the declining threshold of sensitivity is provided as either an exponential or a reverse exponential decay. Alternatively, piece wise use of various decay formulas may be combined and used. 
     This approach, while an improvement over previous approaches, is still dependent solely upon R-wave amplitude, and requires the setting of an appropriate sensitivity threshold to recognize the occurrence of an R-wave. Furthermore, this approach is still dependent upon distinguishing R-wave signal amplitudes from noise level amplitudes, and thus requires the R-wave to have a greater amplitude than the noise level amplitude, in order to avoid false recognition of noise as R-waves. 
     SUMMARY OF THE INVENTION 
     An R-wave detection method and apparatus for sensing the occurrence of an R-wave by utilizing characteristics of the R-wave which distinguish the R-wave from other portions of the electrocardiogram waveform is disclosed. With this approach, a first and second derivative is taken from an electrical signal measured from the heart. The electrical signal, known as the QRS complex or R-wave, is unique from other portions of the electrocardiogram waveform in that the product of the maximum value of the amplitude of the R-wave, the maximum value of the first derivative or slope of the R-wave, and the maximum value of the second derivative or slope transition of the R-wave, is greater for the R-wave than for any other portion of the electrocardiogram waveform. Once the values of the peak of the R-wave, the peak of the first derivative of the R-wave, and the peak of the second derivative of the R-wave are determined, the three peak values are multiplied together to provide an output product value. The output product value is greater in amplitude than either the value of the peak of the R-wave, the value of the peak of the first derivative of the R-wave, or the value of the peak of the second derivative of the R-wave , and is also higher than any other portion of the electrocardiogram waveform including background noise. 
     In a preferred embodiment of the present invention, an R-wave detector is provided to measure the heart rate from a tip-ring signal provided from sensing electrodes implanted in the heart. The tip-ring signal is an electrical voltage signal which is provided to a buffer which may optionally perform signal preconditioning. This signal preconditioning may include filtering or automatic gain control to bring the signal amplitude up to or down to any desired level. The output of the buffer is provided to a first self-clearing peak detector which provides an output proportional to the peak of the electrical signal, which is also the zeroth derivative of the electrical signal. The output of the buffer is also provided to two cascade differentiators which determine the first and second derivatives of the electrical signal. The output of the first differentiator is provided to a second self-clearing peak detector which provides an output proportional to the peak of the first derivative of the electrical input signal. The output of the second differentiator is provided to a third self-clearing peak detector which provides an output proportional to the peak of the second derivative of the electrical signal. The peak detectors are self-clearing as they maintain the output for a predetermined time before clearing. The outputs of the first, second and third self-clearing peak detectors are provided to a multiplier which multiplies the outputs together to provide a product output. The multiplier product output is proportional to the product of the peaks of the zeroth, first and second derivative of the electrical input wave form. The multiplier product output is provided to a self-clearing fraction of peak detector which provides an output response once the product output from the multiplier is received. The self-clearing fraction of peak detector may optionally couple to a one-shot device which provides a digital pulse output having Complimentary Metal Oxide Semiconductor (CMOS) voltage levels. 
     In an alternative embodiment of the present invention, self-clearing peak to peak detectors are used rather than peak detectors to provide a greater amplitude output to the multiplier. In the alternative embodiment, the outputs of the buffer, first differentiator and second differentiator provide the zeroth, first and second derivatives respectively of the input electrical signal wave form to the first self-clearing peak to peak detector, the second self-clearing peak to peak detector and the third self-clearing peak to peak detector. The zeroth, first and second derivative have both positive and negative peaks which are inherent in the R-wave signal, and which may be measured by the self-clearing peak to peak detector. The self-clearing peak to peak detectors provide an output proportional to the sum of the peak positive amplitude and the inverted peak negative amplitude of the input signal. Since this sum is greater than either the positive or negative peak alone, a higher level output response from the self-clearing peak to peak detector is provided to the multiplier, thus improving the ability to detect low amplitude electrical signals measured from the heart. 
     In another alternative embodiment of the present invention, a buffer is provided to receive the electrical input signal measured from the heart, and perform any desired signal conditioning before coupling the electrical input signal to a self-clearing peak to peak detector and an input of a comparator. The self-clearing peak to peak detector output couples to a resistor divider network, and an output of the resistor divider network couples to the other input of the comparator. The comparator compares the output of the resistor divider network, which is a fraction of the peak to peak value of the electrical input signal, to the electrical input signal, and provides an output when the R-wave is detected. Since the self-clearing peak to peak detector provides an output proportional to the sum of the peak positive amplitude and the inverted peak negative amplitude of the detected R-wave, which is greater than the positive or negative peak of the R-wave signal provided from the buffer, the R-wave can be easily detected by the comparator. The output of the comparator may optionally couple to a one-shot device which provides a digital pulse output having CMOS voltage levels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a graphical representation of a heart rate electrical signal over time showing the R-wave; 
     FIG. 2 is a block diagram showing the preferred embodiment of the present invention; 
     FIG. 3 is a schematic diagram showing the input buffer of FIG. 2; 
     FIG. 4 is a schematic diagram showing the differentiator of FIG. 2; 
     FIG. 5 is a schematic diagram showing the self-clearing peak detector of FIG. 2; 
     FIG. 6 is a schematic diagram showing the multiplier of FIG. 2; 
     FIG. 7 is a schematic diagram showing the self-clearing fraction of peak detector of FIG. 2; 
     FIG. 8 is a block diagram showing an alternative embodiment of the present invention; 
     FIG. 9 is a block diagram showing the self-clearing peak to peak detector of FIG. 8; and 
     FIG. 10 is a block diagram showing an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a graphical representation of a heart rate electrical signal over time showing the R-wave. The electrical signal is shown generally at  10 , and illustrates R-waves occurring at  14  and  30 . The electrical signal  10  is typically generated from sensing electrodes implanted in the heart, and is shown being displayed on the ordinate and plotted over the course of time, along the abscissa. R-waves have typical peak amplitudes in the range of 5-15 mv during normal sinus rhythm. The peak of R-wave  14  is indicated at  18 . 
     R-waves have several attributes which are unique. R-waves generally have steeper transitional slopes (indicated at  16  and  20 ), and higher peak amplitudes (indicated at  18 ), than other portions of the electrocardiogram waveform. The product of the peak amplitude of the R-wave, indicated at  18 , the peak value of the slopes or first derivative of the R-wave, indicated at  16  and  20 , and the peak value of the slope transition or second derivative of the R-wave, indicated at  18 , is greater for the R-wave than for any other portion of the electrocardiogram waveform. 
     The R-wave shown in  14  further has a negative peak amplitude indicated at  22 , and has a third slope indicated at  24 , which returns to the beginning of a T-wave shown at  26 . The peak to peak amplitude of the R-wave is measured between the peak at  18  and the peak at  22 . The peak to peak value of the first derivative of the R-wave is equal to the addition of positive values of the peak positive first derivative and peak negative first derivative measured at  16 ,  20  and  24 . The peak to peak value of the second derivative of the R-wave is equal to the addition of positive values of the peak positive second derivative and peak negative second derivative measured at  18  and  22 . The product of the peak to peak amplitude of the R-wave, the peak to peak value of the first derivative of the R-wave, and the peak to peak value of the second derivative of the R-wave, is greater for the R-wave than for any other portion of the electrocardiogram waveform. 
     FIG. 2 is a block diagram showing a preferred embodiment of the present invention. FIG. 2 shows an R-wave detector which differentiates the QRS complex or R-wave from electrical signal  10  by using attributes other than R-wave amplitude. The R-wave generally has several attributes unique in the overall waveform, including the highest amplitude, the steepest slopes or first derivatives, and the sharpest peaks or second derivatives, all occurring within a short period of time. While other portions of electrical signal  10  may occasionally be greater, only the R-wave has the near simultaneous occurrence of a high number of these attributes. The R-wave detector shown generally at  40  thus detects the R-wave as the signal having the highest amplitude, first and second derivatives occurring nearly simultaneously. 
     With the R-wave detector shown at  40 , electrical signal  10  is input to buffer  44  via interface  42 . The input at  42  may be the tip-ring signal from a lead, such as a typical intra cardiac bipolar electrode, to sense the waveform or electrical signal  10  generated by the passage of a depolarization wave through a region of the heart. Buffer  44  is optional in the preferred embodiment and may be used to perform any required signal conditioning or filtering. The filtering may be used to remove noise in the signal. Buffer  44  may also perform preconditioning such as automatic gain control to bring the amplitude of electrical signal  10  up or down to a desired level. 
     The output of buffer  44  couples to differentiator  48  and self-clearing peak detector  66  via interface  46 . Differentiator  48  and differentiator  52  perform as two cascade differentiators to provide a first and second derivative of the buffer  44  output. The output of buffer  44  at interface  46  is the zeroth derivative, while the output of differentiator  48  at interface  50  is the first derivative, and the output of differentiator  52  at interface  54  is the second derivative. The output of buffer  44  is directly proportional to the peak of R-wave  14 , shown at  18  in FIG.  1 . The output of differentiator  48  is proportional to the slope or first derivative of R-wave  14 , while the output of differentiator  52  is proportional to the second derivative of R-wave  14 . 
     The outputs of differentiator  52  at interface  54 , differentiator  48  at interface  50  and buffer  44  at interface  46 , are input to self-clearing peak detector  56 , self-clearing peak detector  62  and self-clearing peak detector  66 , respectively. The outputs of buffer  44 , differentiator  48  and differentiator  52  are nearly instantaneous, and depend only on the band width, slew rate or phase delay of the particular operational amplifiers used. The self-clearing peak detectors are used to look for high values of the zeroth, first and second derivatives obtained from electrical signal  10 . Self-clearing peak detectors  56 ,  62  and  66  look for high values of the outputs of differentiator  52 , differentiator  48  and buffer  44 , respectively, and provide an output proportional to the high value for a predetermined time until clearing. The delay until clearing or resetting of the outputs of self-clearing peak detectors  56 ,  62  and  66  can be preset depending on how much delay is needed. For example, the delay in electrical signal  10  may determined by the time delay between the beginning upward transition of slope  16  of R-wave  14 , and the end of the negative going transition at  20  for R-wave  14 . 
     The outputs of self-clearing peak detector  56 , self-clearing peak detector  62  and self-clearing peak detector  66  are input to multiplier  60  via interfaces  58 ,  64  and  68 , respectively. Multiplier  60  performs a multiply or logical AND function on the input signals. Multiplier  60  provides an output which is proportional to the product of the inputs at interface  70 . 
     Self-clearing fraction of peak detector  72  has an input at interface  70  and an output at interface  74 . Self-clearing fraction of peak detector  72  can be set to detect any desired output level or threshold of multiplier  60  at interface  70 , and provide an output at interface  74 . Self-clearing fraction of peak detector  72  is coupled to one shot  76  via interface  74 . One shot  76  may optionally be used to provide a digital pulse output at interface  78  where the digital pulse output has Complimentary Metal Oxide Semiconductor (CMOS) voltage levels. 
     FIG. 3 is a schematic diagram of the input buffer of FIG.  2 . The input buffer is shown generally at  90 , and is buffer  44  of FIG.  2 . Input buffer  90  has an input at interface  92 . Interface  92  corresponds to interface  42  of FIG.  2 . Input buffer  90  has an output at interface  102 . Interface  102  corresponds to interface  46  of FIG.  2 . Operational amplifier  100  may be any number of operational amplifiers well known in the art. For example, operational amplifier  100  may be a Harris 4741 Quad Operational Amplifier, a National LF 347 Quad Operational Amplifier, or a National LM 348 Quad Operational Amplifier. Operational amplifier  100  has a positive input which is tied to ground through interface  98 . The negative input of operational amplifier  100  is coupled to resistor  94  via interface  96 . Resistor  94  also is coupled to interface  92 . 
     It is appreciated by those skilled in the art that input buffer  90  may perform any number of functions including amplification of an input at interface  92 , or provide a positive or negative gain to the input at interface  92 . Furthermore, it is appreciated that the values of the resistors and capacitors utilized within input buffer  90  are well known in the art, and may be dependent upon the particular manufacturers operational amplifier used, as well as the specific function being performed. 
     FIG. 4 is a schematic diagram showing the differentiator of FIG.  2 . The differentiator is shown generally at  110 , and is the circuit diagram for either differentiator  48  or differentiator  52  in FIG.  2 . The input to differentiator  110  is at interface  112 . Operational amplifier  120  is equivalent to operational amplifier  100  discussed in FIG.  3 . Capacitor  114  and resistor  116  are coupled in series between interface  112  and the negative input of operational amplifier  120  at interface  118 . Capacitor  124  and resistor  126  are coupled in parallel between interface  118  and the output of operational amplifier  120  at interface  122 . The positive input of operational amplifier  120  is coupled to ground via interface  124 . It is appreciated that the values of the resistors and capacitors utilized within differentiator  110  are well known in the art, and may be dependent upon the particular manufacturers operational amplifier used. 
     FIG. 5 is a schematic diagram showing the self-clearing peak detector of FIG.  2 . The self-clearing peak detector is shown generally at  130 , and is representative of self-clearing peak detector  56 , self-clearing peak detector  62  or self-clearing peak detector  66  shown in FIG.  2 . Self-clearing peak detector  130  provides an output at interface  168  which is proportional to the peak input level at interface  132 . Self-clearing peak detector  130  also provides the level representing detection of the peak for a period of time before clearing. This period of time is at least as great as the time response of electrical signal  10  of FIG. 1 between positive slope  16  and negative slope  20 . The time delay must also account for any inherent delay occurring through buffer  44 , differentiator  48  and differentiator  52 , to provide multiplier  60  with enough time to perform the multiplication. Once each output of self-clearing peak detector  56 ,  62  and  66  are held respectively at interfaces  58 ,  64  and  68 , multiplier  60  can output a product of the three inputs at interface  70 . After the predetermined time delay, the output of self-clearing peak detector  130  returns to a pre-peak detection level. In the preferred embodiment, this level is zero volts. 
     The input section of self-clearing peak detector  130  consists of operational amplifier  134  coupled to an anode of diode  136  and a cathode of diode  138 . Resistors  140  and  142  are coupled in series across diodes  136  and  138 . The connection between resistors  140  and  142  is coupled to the output of self-clearing peak detector  130  at interface  168 . The connection between diode  136  and resistor  142  is coupled to the emitter of transistor  148  through resistor  144 , and the emitter of transistor  150  through resistor  146 . The base of transistors  148  and  150  are coupled to the output of self-clearing peak detector  130  at interface  168 . The collector of transistor  148  is coupled to the positive input of operational amplifier  166 , while the negative input of operational amplifier  166  is coupled to interface  168 . The output of operational amplifier  166  is also coupled to interface  168 . 
     The time delay and peak detection function of self-clearing peak detector  130  is performed as follows. The collector of transistor  150  is coupled through resistor  152  to a V-voltage supply, which has a negative potential. The collector of transistor  150  also couples to the base of transistor  154 , and the emitter of transistor  154  couples to the V-voltage supply. The delay time constant is set by the values of resistor  156 , resistor  158 , and capacitor  160 . The peak value sensed by self-clearing peak detector  130  is stored in capacitor  164 , which is coupled to the positive input of operational amplifier  166 . It is understood that once the time constant of resistor  156 , resistor  158  and capacitor  160  has expired, transistor  162  turns on and shorts capacitor  164  to a near ground potential, to reset self-clearing peak detector  130 . 
     FIG. 6 is a schematic diagram showing multiplier  60  of FIG.  2 . The multiplier is shown generally at  180 . Multiplier  180  has inputs at A, B, and C which correspond respectively to interfaces  58 ,  64  and  68  of multiplier  60  in FIG.  2 . Multiplier  180  has an output labeled OUTPUT, which corresponds to interface  70  of multiplier  60  in FIG.  2 . The inputs at D and F are tied to a reference potential. Operational amplifiers  184  and  192 , resistors  182 ,  190  and  194 , and transistors  186  and  188  provide the functionality for input A. Operational amplifiers  198  and  206 , resistors  196 ,  204  and  208 , and transistors  200  and  202  provide the functionality for input B, and also provide the product output at the OUTPUT interface. Operational amplifiers  212  and  220 , resistors  210 ,  218  and  222 , and transistors  214  and  216  provide the functionality for the C input. Multiplier  180  is a one-quadrant log-antilog multiplier, where the three numerator input are A, B and C, and the three denominator inputs are D, OUTPUT and E. In operation, once the peak values of the zeroth, first and second derivatives are received at inputs at A, B and C, multiplier  180  performs a multiplication and provides a product output at the OUTPUT terminal, where the product output is proportional to the product of the inputs at A, B, and C. 
     FIG. 7 is a schematic diagram showing the self-clearing fraction of peak detector of FIG.  2 . The self-clearing fraction of peak detector is shown generally at  240  and has an input at interface  242  which correspond to interface  70  of FIG. 2, and an output at interface  268  which corresponds to interface  74  of FIG.  2 . Self-clearing fraction of peak detector  240  has an input section comprised of operational amplifier  244 , diodes  246  and  248 , resistors  250  and  252 , and transistor  254 . The output of operational amplifier  266  at interface  268  couples back to resistors  250  and  252  and provides a base input to transistor  254 . The emitter of transistor  254  is coupled to diode  246  and resistor  252 , and the collector of transistor  254  is coupled to the positive input of operational amplifier  266 . The negative input of operational amplifier  266  is coupled to interface  268 . Capacitor  264  couples between the positive input of operational amplifier  266  and a ground potential to perform a charge storage function to store the fraction of peak detection level for a predetermined period. 
     The fraction of the peak input at interface  242  which is detected is set by the ratio of resistors  256  and  258 . Resistors  256  and  258  comprise a voltage divider which couples between the output of self-clearing fraction of peak detector  240  at interface  268  and a V- potential. The voltage divider connection point between resistors  256  and  258  couples to both the minus input of operational amplifier  260  and the emitter of transistor  262 . Operational amplifier  260  has a positive input which is coupled to ground and has an output coupled to the base of transistor  262 . Operational amplifier  260  performs the self clearing function after the predetermined period by turning on transistor  262  to discharge the charge stored across capacitor  264 . 
     FIG. 8 is a block diagram showing an alternative embodiment of the present invention. In the alternative embodiment, shown generally at  280 , self-clearing peak to peak detector  282 , self-clearing peak to peak detector  284 , and self-clearing peak to peak detector  286  replace self-clearing peak detector  56 , self-clearing peak detector  62  and self-clearing peak detector  66 , respectively, as shown in FIG.  2 . The self-clearing peak to peak detector provides additional signal sensing capability by providing an output which is proportional to the peak to peak value of the input voltage, not just the peak value as with the self-clearing peak detector shown in FIG.  5 . 
     Self-clearing peak to peak detector  286  is coupled to the output of buffer  44  and provides an output proportional to the difference between the positive peak of R-wave  14  at  18 , and the negative peak of R-wave  14  at  22  (see also, FIG.  1 ). Self-clearing peak to peak detector  284  is coupled to the output of differentiator  48  and provides an output proportional to the sum of the peak positive amplitude and the inverted peak negative amplitude of the first derivative output from differentiator  48 . The first derivative is maximized for R-wave  14  at the positive and negative slopes shown at  16 ,  20  and  24  in FIG.  1 . Self-clearing peak to peak detector  282  is coupled to the output of differentiator  52  and provides an output proportional to the sum of the peak positive slope transition and the inverted peak negative slope transition of the second derivative output from differentiator  52 . The second derivative is maximized for R-wave  14  at the peak positive and negative slope transitions shown at  18  and  22  in FIG.  1 . 
     FIG. 9 is a block diagram showing the self-clearing peak to peak detector of FIG.  8 . The self-clearing peak to peak detector is shown generally at  300 , and is representative of self-clearing peak to peak detector  282 ,  284  or  286 , as shown in FIG.  8 . The input to self-clearing peak to peak detector  300  is at interface  302 , and the output is at interface  318 . The input at  302  is coupled to buffer  310 , which is a unity gain buffer similar to buffer  44  shown in FIG.  8 . Buffer  310  also performs an inversion function to invert the signal input at interface  302 , and provides the inverted signal at interface  312  to self-clearing peak detector  314 . Self-clearing peak detector  304  is coupled to the input at interface  302 . Self-clearing peak detector  130  shown in FIG. 5 is representative of self-clearing peak detector  304  and self-clearing peak detector  314 . 
     Self-clearing peak detector  304  or  314  can sense either positive or negative going peaks. With the proper inversion provided by buffer  310 , the peak to peak detection function can be accomplished. With the self-clearing peak to peak detector  300 , self-clearing peak detector  304  and  314  detect positive going peaks. Since buffer  310  inverts the input provided at interface  302 , self-clearing peak detector  304  provides an output responsive to the positive going peak at  302 , and self-clearing peak detector  314  provides an output responsive to the negative going peaks at interface  302 . Adder  308  is coupled to self-clearing peak detector  304  via interface  306 , and self-clearing peak detector  314  via interface  316 . Adder  308  sums these two inputs and provides a sum output at  318 . 
     FIG. 10 is a block diagram showing an alternative embodiment of the present invention. The alternative embodiment is shown generally at  330  and includes a buffer  334 , a self-clearing peak to peak detector  338 , an operational amplifier  352 , and a one-shot  356 . Buffer  334  provides the same functionality as buffer  44  of FIG.  2 . Self-clearing peak to peak detector  338  is described in FIG.  9 . 
     Electrical signal  10  is input at interface  332 . Buffer  334  provides an output at  336  which is responsive to the input at  332 . Buffer  334  may be a unity gain amplifier or may provide other signal conditioning, as discussed in FIGS. 2 and 3. Self-clearing peak to peak detector  338  provides an output responsive to the sum of the positive and negative going peaks of the input at  336 . The output of self-clearing peak to peak detector  338  is provided at interface  340  to a voltage divider network consisting of resistor  346  and  348 . The positive input of operational amplifier  352  is coupled via interface  350  to resistor  346  and  348 . The negative input of operational amplifier  352  is coupled to the output of buffer  334  at interface  336 . Operational amplifier  352  performs a comparison function and compares the voltage divided peak to peak input from resistors  346  and  348  to the output of buffer  334 . Since the R-wave of electrical signal  10  will typically have a greater peak to peak amplitude than other portions of the electrocardiogram waveform, by setting the proper threshold detection level through the ratio of resistors  346  and  348 , operational amplifier  352  can sense the occurrence of an R-wave, and provide an output at interface  354 . 
     The output of operational amplifier  352  is coupled to one shot  356  via interface  354 . One shot  356  may optionally be used to provide a digital pulse output at interface  358 , where the digital pulse output has CMOS voltage levels. 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached.