electrical stimulation is delivered via lead ( s ) or electrode ( s ). these leads can be epicardial ( external surface of the heart ) or endocardial ( internal surface of the heart ) or any combination of epicardial and endocardial . leads are well known to those skilled in the art . lead systems can be unipolar or bipolar . a unipolar lead has one electrode on the lead itself , the cathode . current flows from the cathode , stimulates the heart , and returns to the anode on the casing of the pulse generator to complete the circuit . a bipolar lead has two poles on the lead a short distance from each other at the distal end , and both electrodes lie within the heart . fig1 illustrates a plan view of implantable electronic stimulation device 102 and its associated lead and electrode system , in conjunction with human heart 104 . as illustrated , the device includes right atrial appendage lead 106 , right atrial septal lead 108 , first coronary sinus lead 110 and second coronary sinus lead 112 . each of these multiple small electrodes can be inserted intravenously and includes an independent generator . fig2 illustrates a plan view of implantable electronic stimulation device 102 illustrating an alternative location of leads and electrodes in relation to human heart 104 . as illustrated , the device includes right atrial appendage lead 106 , right atrial septal lead 108 , first coronary sinus lead 110 , second coronary sinus lead 112 and left free wall lead 204 . each of these multiple small electrodes can be inserted intravenously and includes an independent generator . because of the use of independent generators , each electrode can be timed differently . in a preferred embodiment , left free wall lead 204 is placed by piercing septum 206 and passing left free wall lead 204 through the septum to the left side of the heart . the aforementioned placement of leads is for illustration purposes only , and is not intended as a limitation . it is contemplated that multiple leads placed in a variety of locations could be used . each site ( area of lead placement ) can be entrained separately , and then brought to the same phase . in a preferred embodiment each site is gradually brought to the same phase ; however , certain situations could require that each site is quickly brought to the same phase . in an alternative embodiment , the sites can be sequenced to mimic a normal heart beat . in addition to allowing multi - site stimulation capability , the sensing circuits of each electrode also allow for multi - site sensing . through triangulation the multi - site sensing provides a means for determining the site ( s ) of any atrial ectopic focus . referring to fig2 a , a block diagram of the major functional components of the implanted pacemaker 102 is illustrated . pacing / control circuitry 500 , in conjunction with microprocessor 501 detects the occurrence of tachycardia ( and / or bradycardia ) and in response thereto controls the delivery of the various pacing therapies available via control bus 512 . the microprocessor 501 also detects the occurrence of atrial fibrillation . detection of atrial fibrillation may be accomplished by the microprocessor 501 using any of the various detection methodologies known to the art . generally , atrial fibrillation may be detected in response to an extended series of high rate atrial depolarizations . if greater specificity for atrial fibrillation is desired , analysis of regularity of rate waveform morphology may also be employed . termination of atrial fibrillation may be detected in response to a decrease in the rate of atrial depolarizations and / or an increase in their regularity . the operation of the microprocessor 501 is controlled by programming stored in a read only memory 505 and in a random access memory 503 . the operation of the device may be altered by the physician by altering the programming stored in the memory 503 , using control and telemetry circuitry conventional in implantable stimulators . communication to and from the microprocessor 501 , the memories 503 , 505 , and the control logic 500 is accomplished using an address / data bus 507 . the atrial sensing circuit 509 can be any conventional cardiac sense amplifier circuits equivalent to any atrial cardiac sensing circuits employed in previous devices known in the art . the implanted pacemaker 102 has a switch matrix 516 that allows selective delivery of pacing pulses from the atrial pacing driver 514 to the electrodes . the matrix 516 may be embodied as simply a collection of one or more fet and / or scr switches activated under control of the pacing / control circuitry 500 to selectively pacing circuitry 516 to electrodes 106 and 108 , or to electrodes 110 and 112 , or other combinations of the electrodes . thus , atrial anti - tachycardia ( or anti - bradycardia pacing ) is performed using any combination of the deployed pacing electrodes . in a preferred embodiment , stimulation is administered at threshold until capture has occurred , at which time stimulation is administered at a subthreshold level . in alternative embodiments , stimulation is : ( 1 ) initiated at threshold and remains at threshold ; ( 2 ) initiated subthreshold and remains subthreshold ; ( 3 ) conventional prior to capture and then biphasic ; ( 4 ) biphasic prior to capture and then conventional or ( 5 ) biphasic throughout . threshold refers to the minimum voltage level ( or pulse width using a fixed voltage ) which succeeds in stimulating ( capturing ) the myocardium . to capture is to produce a driven beat because of the stimulus given . thus , in the absence of the pulse , the beat would not have been produced . pulses which do not capture are subthreshold , ( even though they may be shown to perturb the membrane potential somewhat , and transiently ). subthreshold pulses thus may affect subsequent conduction , but not by the mechanism of initiating a driven beat . generally , to determine threshold , voltage ( or pulse width ) is varied ( upward or downward ) until capture is gained or lost . conventional stimulation is well known to those skilled in the art and comprises monophasic waveforms ( cathodal or anodal ) as well as multiphasic waveforms wherein the nonstimulating pulses are of a minimal magnitude and are used , for example , to dissipate a residual charge on an electrode . fig3 - 7 depict a range of biphasic stimulation protocols . these protocols have been disclosed in u . s . patent application ser no . 08 / 699 , 552 by the present inventor , which is incorporated by reference herein , in its entirety . fig3 depicts biphasic electrical stimulation wherein a first stimulation phase comprising anodal stimulus 302 is administered having amplitude 304 and duration 306 . this first stimulation phase is immediately followed by a second stimulation phase comprising cathodal stimulation 308 of equal intensity and duration . fig4 depicts biphasic electrical stimulation wherein a first stimulation phase comprising cathodal stimulation 402 having amplitude 404 and duration 406 is administered . this first stimulation phase is immediately followed by a second stimulation phase comprising anodal stimulation 408 of equal intensity and duration . fig5 depicts a preferred embodiment of biphasic stimulation wherein a first stimulation phase , comprising low level , long duration anodal stimulation 502 having amplitude 504 and duration 506 , is administered . this first stimulation phase is immediately followed by a second stimulation phase comprising cathodal stimulation 508 of conventional intensity and duration . in differing alternative embodiments , anodal stimulation 502 is : 1 ) at maximum subthreshold amplitude ; 2 ) less than three volts ; 3 ) of a duration of approximately two to eight milliseconds ; and / or 4 ) administered over 200 milliseconds post heart beat . maximum subthreshold amplitude is defined for purposes of this application as the maximum stimulation amplitude that can be administered without eliciting a contraction . in a preferred embodiment , anodal stimulation is approximately two volts for approximately three milliseconds duration . in differing alternative embodiments , cathodal stimulation 508 is : 1 ) of a short duration ; 2 ) approximately 0 . 3 to 1 . 5 milliseconds ; 3 ) of a high amplitude ; 4 ) in the approximate range of three to twenty volts ; and / or 5 ) of a duration less than 0 . 3 millisecond and at a voltage greater than twenty volts . in a preferred embodiment , cathodal stimulation is approximately six volts , administered for approximately 0 . 4 milliseconds . in the manner disclosed by these embodiments , as well as those alterations and modifications that can become obvious upon the reading of this specification , a maximum membrane potential without activation is achieved in the first phase of stimulation . fig6 depicts an alternative preferred embodiment of biphasic stimulation wherein a first stimulation phase , comprising anodal stimulation 602 , is administered over period 604 with rising intensity level 606 . the ramp of rising intensity level 606 can be linear or non - linear , and the slope can vary . this anodal stimulation is immediately followed by a second stimulation phase comprising cathodal stimulation 608 of conventional intensity and duration . in alternative embodiments , anodal stimulation 602 : ( 1 ) rises to a maximum subthreshold amplitude less than three volts ; ( 2 ) is of a duration of approximately two to eight milliseconds ; and / or ( 3 ) is administered over 200 milliseconds post heart beat . in yet other alternative embodiments , cathodal stimulation 608 is : ( 1 ) of a short duration ; ( 2 ) approximately 0 . 3 to 1 . 5 milliseconds ; ( 3 ) of a high amplitude ; ( 4 ) in the approximate range of three to twenty volts ; and / or ( 5 ) of a duration less than 0 . 3 milliseconds and at a voltage greater than twenty volts . in the manner disclosed by these embodiments , as well as those alterations and modifications which can become obvious upon the reading of this specification , a maximum membrane potential without activation is achieved in the first phase of stimulation . fig7 depicts biphasic electrical stimulation wherein a first stimulation phase , comprising series 702 of anodal pulses , is administered at amplitude 704 . in one embodiment , rest period 706 is of equal duration to stimulation period 708 , and is administered at baseline amplitude . in an alternative embodiment , rest period 706 is of a differing duration than stimulation period 708 , and is administered at baseline amplitude . rest period 706 occurs after each stimulation period 708 , with the exception that a second stimulation phase , comprising cathodal stimulation 710 of conventional intensity and duration , immediately follows the completion of series 702 . in alternative embodiments : ( 1 ) the total charge transferred through series 702 of anodal stimulation is at the maximum subthreshold level ; and / or ( 2 ) the first stimulation pulse of series 702 is administered over 200 milliseconds post heart beat . in yet other alternative embodiments , cathodal stimulation 710 is : ( 1 ) of a short duration ; ( 2 ) approximately 0 . 3 to 1 . 5 milliseconds ; ( 3 ) of a high amplitude ; ( 4 ) in the approximate range of three to twenty volts , and / or ( 5 ) of a duration less than 0 . 3 milliseconds and at a voltage greater than twenty volts . referring to fig8 the practice of the present invention is illustrated . sensing is used to determine the existence of atrial fibrillation 802 . sensing can be direct or indirect . for example , direct sensing can be based on data from multiple atrial sensing electrodes . the sensing electrodes sense the cardiac activity as depicted by electrical signals . for example , as is known in the art , r - waves occur upon the depolarization of ventricular tissue and p - waves occur upon the depolarization of atrial tissue . by monitoring these electrical signals the control / timing circuit of the icd can determine the rate and regularity of the patient &# 39 ; s heart beat , and thereby determine whether the heart is undergoing arrhythmia . this determination can be made by determining the rate of the sensed r - waves and / or p - waves and comparing this determined rate against various reference rates . direct sensing can be based upon varying criteria ; such as , but not limited to , primary rate , sudden onset , and stability . the sole criteria of a primary rate sensor is the heart rate . when applying the primary rate criteria , if the heart rate should exceed a predefined level , then treatment is begun . sensing electronics set to sudden onset criteria ignore those changes which occur slowly , and initiate treatment when there is a sudden change such as immediate paroxysmal arrhythmia . this type of criteria would thus discriminate against sinus tachycardia . stability of rate can also be an important criteria . for example , treatment with a ventricular device would not be warranted for a fast rate that varies , here treatment with an atrial device would be indicated . alternative embodiments , sensing can be indirect . indirect sensing can be based on any of various functional parameters such as arterial blood pressure , rate of the electrocardiogram deflections or the probability density function ( pdf ) of the electrocardiogram . while it has been known in the art to apply pdf to the global electrocardiogram and / or to the r wave , it has been unexpectedly discovered that pdf of the baseline is also indicated for the determination of atrial abnormalities . here , the electrodes are specific to the atrium and data related to the r wave is canceled out . thus , whether or not to administer treatment can also be affected by pdf monitoring of the time the signal spends around the baseline . lastly , to determine whether an arrhythmia comes from the atria or the ventricles , a test impulse ( s ) can be given to one chamber to see if capture occurs and perturbs the rhythm . for example , in a ventricular rhythm , an atrial test impulse can capture the atrium , but the ventricular rhythm will continue unchanged afterwards . in an atrial rhythm , ( or sinus rhythm ), if the atrial test pulse captures , the timing of all subsequent beats is changed . to determine if a pulse captures , the baseline immediately after the beat can be examined to determine if it is different from zero ( or from a baseline template ). if so , the beat can be inferred to have captured . in addition , the pdf pattern of the rhythm can be shown to have changed , inferring capture . thus , in a preferred embodiment , sensing electronics are based upon multiple criteria . in addition , the present invention envisions devices working in more than one chamber such that appropriate treatment can be administered to either the atrium or the ventricle in response to sensing electronics based upon a variety of criteria , including those described above as well as other criteria known to those skilled in the art . if atrial fibrillation occurs , a baseline of cardiac activity or a template can be recorded 804 . the template can be based on parameters such as electrocardiogram data , mechanical motion and / or probability density function data . in an alternative embodiment , the template is established after capture has occurred . pacing is initiated 806 . in a preferred embodiment , stimulation is administered at threshold until capture has occurred , at which time stimulation is administered at a subthreshold level . in alternative embodiments , stimulation is : ( 1 ) initiated at threshold and remains at threshold ; ( 2 ) initiated subthreshold and remains subthreshold ; ( 3 ) conventional prior to capture and then biphasic ; ( 4 ) biphasic prior to capture and then conventional or ( 5 ) biphasic throughout . the atrium is monitored throughout this initial pacing period to determine the status of capture 808 . capture can be determined by multiple means . first , capture or the loss thereof , can be determined by monitoring cardiac rhythm . loss of capture can result in a change in timing of the heart beat . second , capture or the loss thereof , can be determined through monitoring the previously described template . where the template is established pre - stimulation , a change in the baseline signifies capture . where the template is established after capture has occurred , a change in the template characteristics signifies loss of capture . the templates can be established and / or updated at any time . once capture occurs the stimulation protocol of the entrained sites is adjusted 810 . in a first embodiment , the stimulation rates of the entrained sites are slowed simultaneously , and then stopped . in a second embodiment , the spread of conduction is slowed . in a third embodiment , the stimulation speed is increased and stimulation is then stopped . in addition to adjusting stimulation rates upon the occurrence of capture , the stimulation protocol can also be adjusted such that ( 1 ) if stimulation of a conventional nature was administered prior to capture , biphasic stimulation is administered post - capture ; ( 2 ) if biphasic stimulation was administered prior to capture , conventional stimulation is administered post - capture or ( 3 ) if biphasic stimulation was administered prior to capture , biphasic stimulation continues to be administered post - capture . having thus described the basic concept of the invention , it will be readily apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only , and is not limiting . various alterations , improvements and modifications will occur and are intended to those skilled in the art , but are not expressly stated herein . these modifications , alterations and improvements are intended to be suggested hereby , and within the scope of the invention . further , the pacing pulses described in this specification are well within the capabilities of existing pacemaker electronics with appropriate programming . accordingly , the invention is limited only by the following claims and equivalents thereto .
atrial sensing and stimulation as intervention for atrial fibrillation . the present invention relates to a method of atrial defibrillation using an implanted electronic stimulator . in a variety of protocols varying combinations of conventional and biphasic stimulation are applied at threshold and sub - threshold levels . in a preferred embodiment , the implantable electronic stimulator of the present invention includes multiple electrodes having stimulating and sensing capabilities . the small size of these electrodes allows for intravenous insertion into the patient .