Electrical Potential Distribution Surrounding the Atria during Depolarization and Repolarization in the Dog

Transcription

1 Electrical Ptential Distributin Surrunding the Atria during Deplarizatin and Replarizatin in the Dg By Madisn S. Spach, M.D., Terry D. King, M.D., Rger C Barr, Ph.D., David E. Baz, B.S., Mary N. Mrrw, M.A., and Sctt Herman-Giddens ABSTRACT The ptential distributin at the atrial surface during deplarizatin and replarizatin was studied in intact dgs. A preparatin was develped by implanting 30 t 40 miniature electrdes permanently n each atrium t recrd uniplar electrgrams in the intact animal. Heart blck was created t dissciate atrial and ventricular activity. The electrgrams were recrded n magnetic tape and atrial isptential heart maps prduced with the use f a digital cmputer. The changing ptential distributin during excitatin indicated the early presence f multiple wave frnts which were related primarily t the crista terminalis, Bachmann's bundle, and a special bundle t the base f the right appendage. The interatrial septum prvided a cnducting bridge which had an imprtant influence f glbal atrial excitatin, depending n the site f impulse frmatin. Clliding excitatin wave frnts were quite prminent. During terminal atrial excitatin, replarizatin maxima were present simultaneusly with deplarizatin maxima. Replarizatin was characterized by a changing ptential distributin which fllwed the same general pattern as excitatin spread; and, furthermre, the earliest areas f excitatin were assciated with a replarizatin maximum and terminal areas f excitatin were assciated with replarizatin minima. ADDITINAL KEY WRDS activatin wave frnt atrial electrgrams atrial excitatin atrial replarizatin cardiac electrphysilgy interatrial cnductin While cnsiderable infrmatin has been accumulated fr characterizing the excitatin sequence f the ventricles and the resultant bdy surface ptential distributin, similar studies cncerning atrial activity have been sparse. Recent advances in recrding and data prcessing techniques (1-3 ) indicate that it is pssible t vercme many f the technical prblems f recrding the lw vltage signal f atrial activity n the bdy surface t explre mre fully the ultimate clinical limitatins f detecting atrial abnrmalities frm Frm the Department f Pediatrics and the Divisins f Bimedical Engineering and Bimathematics, Duke University, Durham, Nrth Carlina This investigatin was supprted in part by U. S. Public Health Service Grants HE 11307, HE 11309, HE 5372, and HE 5716 frm the Natinal Heart Institute. Received February 28, Accepted fr publicatin April 18, surface recrdings. Since we cannt hpe t ratinally understand the ultimate value and limitatins f atrial bdy surface activity until we better understand the nature f electrical activity at the atrial surface, it seemed wise t examine further tw separate but clsely related electrphysilgic questins. (1) What are the characteristics f the field f ptentials surrunding the atrium during deplarizatin? (2) What is the nature f the atrial ptentials generated during replarizatin? The classic wrk f Lewis and c-wrkers (4) in the dg initially presented evidence fr radial spread f activatin f the atria. Eyster and Meek (5) challenged Lewis's wrk since they fund evidence f asymmetrical spread frm the SA nde area. Puech and c-wrkers (6), van der Ki et al. (7), and San and Yamagishi (8) demnstrated early elngatin Circulatin Research, Vl. XXIV, June

2 858 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS f initial wave frnts which spread inferirly frm the SA nde; hwever, these investigatrs cncluded that atrial excitatin was principally characterized by radial spread. The questin f radial versus asymmetrical spread f atrial activatin remains unsettled. This is due primarily t the lack f detailed physilgic data t deny r cnfirm the thery f cncentric cnductin as related t the ttal excitatin sequence f bth atria. Additinally, the nature f replarizatin f a mass f mycardial tissue remains at a primitive stage f understanding. T gain insight int this latter prblem, we cnsidered the atrium t prvide the particular advantage f a twrather than three-dimensinal prblem due t the thin chamber walls, althugh the glbal gemetry f the atrium is smewhat cmplex (9) because f the appendages and venus cnnectins. This study was designed t define the nature f the electrical ptential distributin at the atrial surface during deplarizatin and replarizatin in the intact dg. Althugh atrial excitatin can be characterized by ischrnus time lines t depict the mvement f wave frnts ver the atria, the time curse f replarizatin precludes the use f this apprach. We have develped an experimental mdel with numerus electrdes permanently implanted n the atrial surface t recrd electrgrams at each site and frm these have cnstructed isptential atrial heart maps. We thught that this apprach might imprve insight int the abve questins by apprximating nrmal cnditins mre clsely than can be achieved in the pen-chest dg. e.g., n the atrial septum. Particular attentin was given t precise placement f electrdes surrunding the sinus nde area, alng the taenia terminalis, ver the right atrial free wall tward the A-V grve, bth surfaces f the right atria] appendage, the prximal superir vena cava, and n and arund Bachmann's bundle. In eight animals cmplete heart blck was prduced by intrducing a small iridectmy scalpel thrugh the lateral area f the right atrium with a single incisin acrss the A-V grve. The insulated wires were then apprximated, channeled thrugh the apprximated pericardium, lped inside the chest, and brught ut superficially fr permanent implantatin beneath the skin f the neck. Three weeks after recvery frm the initial surgery, at secnd peratin, thrugh a left thractmy, 25 t 30 similar electrdes were implanted ver the left atrial surface. Particular attentin was given t placement in the area f Bachmann's bundle superirly, n bth surfaces f the left atrial appendage, the main bdy f the left atrium laterally and inferirly, and inferirly at the junctin f the tw atria (representing the exterir psitin f the atrial septum). The wires were lped within the chest and the distal ends Methds A. PREPARATIN F EXPERIMENTAL MDEL Twelve dgs weighing 18 t 25 kg initially underwent right thractmy with implantatin f 30 t 40 insulated silver-cated cpper electrdes,1 1 mm in diameter, n the external surface f the right atrium. Each electrde was anchred by fine silk superficial stitch in the atrial wall. N electrdes were implanted internally, *N silver-cated cpper, Tefln insulated, Belden Crpratin, P.. Bx 5070-A, Chicag, Illinis FIGURE 1 Lateral rentgengram f chrnic preparatin shws the electrde psitins ver the right atrium (right superir) and left atrium (left inferir). Multiple wires n each atrium were cnfined t bundles and led ut thrugh the pericardum. A large intrathracic lp was created t minimize tensin at the electrdemuscle junctin. Circulatin Research, Vl. XXIV, June 1969

3 ELECTRICAL PTENTIAL DISTRIBUTIN 859 implanted beneath the skin f the neck. Due t difficulties in separating the atrium frm psterir mediastinal structures, n electrdes were implanted psterirly between the superir pulmnary venus atrial cnnectins (1-cm 2 area). B. DATA ACQUISITIN The animals appeared healthy and were quite active fllwing recvery frm the secnd peratin. n the day f study, 5 t 8 weeks pstperatively, x-ray films were taken t cnfirm the psitin f electrde sites (Fig. 1). A plastic cube clck calibrated with 1-cm wire markings was psitined adjacent t the chest at the level f the atrium t estimate interelectrde distances frm the x-ray film. Three animals had rentgengraphic evidence f atelectasis f the right upper lbe, but nne had evidence f pleural fluid accumulatin. Each animal was studied under pentbarbital anesthesia (30 mg/kg). The electrdes in the neck were exterirized and cnnected t a Atrial Leads Atrial Ptential Map 0 Amplifiers C Analg Recrder Cmputer ^ - > JEAN SPNTAN PA Plts fr Editing FIGURE 2 Summary f data acquisitin and prcessing methds. The atrial leads were exterirized and cnnected t a selectr switch which allwed rapid sampling f all pints. Five uniplar atrial electrgrams were recrded simultaneusly with the right atrial reference tracing. The left leg was used as a cmmn reference pint. After representative beats were selected fr prcessing, analg-t-digital cnversin was perfrmed at a sampling rate f 926 samples/sec fr each lead. The cmputer prgram was designed t prvide tw types f utput: (1) the individual electrgrams were pltted in graphic frm fr final editing fr artifact and baseline shifts; and (2) the ptentials at each site were printed in a matrix crrespnding t the relative atrial psitin f each recrding site; thus, the atrial ptential maps cnsisted f the ptential values f each pint at apprximately each msec. The atrial ptential maps were cmpleted by hand drawings f isptential lines. (See text fr detailed discussin.) Circulatin Research, Vl. XXIV, June 1969

4 860 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS switching bx (Fig. 2). The a-c amplifiers 2 had a high frequency respnse f 5 kc/sec and a lw frequency respnse f 0.1 cycles/sec. The electrgrams were recrded in blcks frm five electrdes simultaneusly, alng with a right atrial reference tracing. All atrial ptentials were recrded in reference t the left leg. The switching bx allwed sequential sampling f the ttal 60 t 70 pints within 15 t 20 minutes. The data were recrded with an Ampex FR 1300 tape recrder at a tape speed f 15 inches/sec and were mnitred scillgraphically t ensure that the reference tracing remained stable. The average spntaneus rate f each dg was within the range f 105 t 145 beats/min and varied less than 15 beats/min. Fllwing each recrding sequence, repeat tracings were btained frm selected sites ver bth atria t ensure reprducibility f results. Thereafter, the recrding sequence was repeated while the atria were paced frm varying sites at a cnstant rate (the cnstant pacing rate was in the range f 170 t 190 beats/min). C. DATA PRCESSING The scillgraphic recrds were edited t select beats fr analysis which maintained baseline stability. In animals with cmplete heart blck, nly thse atrial beats ccurring utside the time f the ventricular QRS-T were chsen fr analysis. All data were transcribed by an Airbrne Instruments Labratries analg-t-digital cnverter at a sampling rate f 926 samples/sec fr each lead. The data were then prcessed by an IBM cmputer. The cmputer prgram was designed t prvide tw types f utput (Fig. 2). (1) Each lead underging analysis was displayed in graphic frm with a Calcmp pltter t allw an additinal check fr artifact and baseline shifts. (2) A matrix was created whereby the psitin f each electrde was identified as t its relative psitin n the atrial surface. The numerical utput cnsisted f ptential values at each psitin t demnstrate the instantaneus field f ptentials fr each millisecnd thrughut atrial excitatin and replarizatin. The atrial heart maps were cmpleted by drawing equiptential lines n the cmputer sheets. The nise level in the final maps was cnsidered t be ± 0.2 mv (peak-t-peak), as judged frm the Calcmp plts; therefre, nly when ptentials exceeded this value were they included in the drawings depicting the ptential distributin. D. CNSIDERATINS REGARDING THE METHDS T gain insight int the characteristics f atrial -'Ampex High Gain A-C Amplifier, N electrical activity by depicting the field f ptentials surrunding the atria, several cnsideratins arse which required study. 1. What is the general relatinship between the excitatin wave frnt depicted by instantaneus ischrnus time lines t the psitin f maxima and minima and the field f psitive and negative ptentials? Van der Ki and c-wrkers (7) nted that lcal excitatin as timed with a biplar lead cincided precisely with the mst rapid prtin f the intrinsic deflectin f atrial uniplar electrgrams. T examine this time relatinship, fur dgs underwent acute atrial excitatin studies whereby a biplar electrde with the terminals 1 mm apart was used t recrd biplar and uniplar electrgrams simultaneusly 1. The uniplar electrgrams recrded during the acute studies in the pen-chest dgs and thse recrded in the intact animals with chrnically implanted electrdes were cmpared. This shwed that fr cmparable atrial sites the shapes f the P waves were quite similar. These data were displayed scillgraphically at a paper speed f 800 mm/sec. The lcal excitatin times were measured bth frm the biplar tracings and frm the simultaneus uniplar tracings by selecting pints during the rapid prtin f the intrinsic deflectin. nly uniplar tracings which prvided a single majr intrinsic deflectin were used fr analysis. The cmparisn f 60 recrds indicated that the timing f lcal excitatin by either methd was in agreement within ±1.5 msec. n the basis f these results, lcal excitatin times were determined frm the uniplar electrgrams in fur intact animals t cnstruct the activatin wave frnt at several instants f time during atrial excitatin fr cmparisn with the distributin f ptentials at the same time. 2. Since the implantatin f electrdes directly n the atrial surface prduces lcal injury, such effects, if prminent, wuld prduce spurius values in depicting the atrial ptential distributin. Three additinal animals were studied t evaluate the rapidity f injury current changes. Immediately after suturing the electrdes in psitin, injury prduced psitive shifts f the curve during bth P (atrial excitatin) and Ta (atrial replarizatin). These effects disappeared within 15 minutes t 6 hurs. After this time, pacing different sites f the atrium prduced alteratins in the P and Ta wave cnfiguratin with return t the riginal pattern upn resumptin f nrmal sinus rhythm. These findings, as well as the tracings in the dgs underging study, indicated that there were n detectable lcal injury effects in the final electrgrams f the intact preparatins. At the cmpletin f each study, the animal was killed. Autpsy revealed mderate cllapse f Circulatin Research, Vl. XXIV, June 1969

5 ELECTRICAL PTENTIAL DISTRIBUTIN 861 the upper lbes f bth lungs in fur animals. There was n pericardial r pleural fluid present. A thin layer f cnnective tissue envelped the atrial surface and wires. In each heart, the wires were dissected free fr final identificatin f electrde sites. Histlgic studies (Massn's stain) indicated that, except fr the verlying cnnective tissue, the atrial muscle was nrmal. 3. In dgs with cmplete heart blck, the amplitude f the P waves varied with their phase in the ventricular cycle. P waves during the first 120 msec after QRS nset shwed n amplitude change; hwever, if the P wave ccurred during the last 200 msec f the ventricular T wave, the amplitude either remained cnstant r decreased as much as 21% (it never increased). Anther type f amplitude alteratin fund in three dgs was mst prminent n the atrial appendages. If the P wave n the appendages ccurred within 200 msec after cmpletin f the ventricular T wave, the largest amplitude thrughut the cycle was recrded. With the subsequent tw r three atrial beats (withut a superimpsed ventricular beat) there was gradual decrease, as much as 25%, in the amplitude f the P wave prir t the next ventricular beat. This phenmenn remained reprducible fr 5 hurs. If the atrial beats were selected within a cnstant time interval between ventricular beats, the peak-t-peak amplitude shwed less than 5% variability. Therefre, t minimize errr in the final ptential maps in the animals with cmplete heart blck, beats were chsen fr analysis within a 70-msec range within the R-R interval. Results A. CNFIGURATIN F ELECTRGRAMS In the earliest and latest areas f excitatin, the P and Ta waves were always f ppsite plarity regardless f the site where excitatin was initiated, as shwn in Figure 3. The atrial T waves were predminantly upright ver the right atrium and superir left atrium and negative r flat ver the inferir left atrium during nrmal sinus rhythm. During nrmal sinus rhythm, the P wave was negative with a psitive T in the area near the SA nde, whereas ver the latest area f excitatin n the left atrium, the P wave was predminantly upright with a flat r negative T deflectin. When the inferir lateral left atrium was paced, the curves were reversed with negative left atrial P and upright T waves ver the early excited areas; the upper right atrium, which excited late, demnstrated predmi- Circulatin Research, Vl. XXIV, June 1969 nantly psitive P waves with negative T deflectins. The shapes f the P waves at varius sites during nrmal sinus rhythm (Fig. 4) were similar in frm with thse reprted by Puech and c-wrkers (6), van der Ki et al. (7), and Matsuka (10). Additinally, there were irregular wavefrms with high frequency cmpnents at sites alng the brders f the atrial appendages. The site n the SA nde area where earliest negativity was recrded yielded entirely negative deflectins; these frequently were assciated with a shrt and small pre-ptential (apprximately 20 msec and 70 fiv) and high frequency ntching during the dwnstrke (7). Electrgrams recrded 1 t 3 cm frm the SA nde n bth l.. I (ten) N s 6ZYG0S/SVC FIGURE 3 Relatinship f P and Ta waves at earliest and latest areas f excitatin. The tracings are illustrative f the grup and were recrded in an animal with cmplete heart blck. During nrmal sinus rhythm (NSR) the earliest sites f excitatin adjacent t the SA nde inscribed predminantly negative P waves with a psitive Ta; the latest areas f excitatin, n the lateral left atrium, inscribed either biphasic r predminantly psitive P waves with a negative Ta. When the lateral inferir left atrium was paced (pulse square), the P waves became negative with a psitive Ta and there was a predminantly psitive P wave with a negative Ta deflectin ver the right upper atrium, which excited late. APPEN = appendage; AZYGS = azygs vein; BACHMANN = Bachmann's bundle; IVC = inferir vena cava; LA = left atrium; NSR = nrmal sinus rhythm; PACE = site f initiatin f beat via pacemaker; PV = pulmnary vein; RA = right atrium; SVC = superir vena cava. The taenia terminalis is the external landmark fr crista terminalis; the venus area is the venus prtin f right atrium lateral t the taenia terminalis.

6 862 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS the taenia terminalis and Bachmann's bundle yielded rs waves while adjacent atrial areas demnstrated RS cnfiguratins. ver the lwer right atrium and lateral left atrium the wavefrms were primarily RS. The latest sites f excitatin yielded curves that were predminantly Rs. B. ATRIAL PTENTIALS DURING DEPLARIZATIN AND REPLARIZATIN The results t be presented f the atrial ptential distributin during atrial excitatin and replarizatin were the same fr all dgs fr any cmparable pacemaker site. Minr variatins will be nted where apprpriate. Since the results were similar in the different dgs, the findings in a single dg with cmplete heart blck will be used t illustrate excitatin and replarizatin ptential distributin changes when atrial activatin is initiated at three different sites: (1) sinus FIGURE 4 Typical atrial electrgrams in a dg with nrmal sinus rhythm. The vertical line accmpanying each tracing is the time reference. Nte that the earliest activity is assciated with a negative deflectin in the area f the SA nde. There was high frequency ntching in the wavefrms recrded n the appendages (1, 3, 12). Wavefrms recrded in area 11 usually cnsisted f a W cnfiguratin. Ta wave cnfiguratin during the early prtin f replarizatin can be seen as predminantly upright curves ver the right atrium and negative r flat deflectins ver the left atrium. nde area (nrmal spntaneus atrial rhythm), (2) area adjacent t the crnary sinus (attempt t simulate crnary sinus and/r ndal rhythm), and (3) area n inferir-lateral wall f left atrium (attempt t simulate left atrial rhythm). In the presentatin f the results, the term wave frnt will be used t signify the demarcatin line between psitive and negative ptentials during deplarizatin. This line frequently separated clsely appsed maxima and minima. Additinal infrmatin cncerning the relatinship f the wave frnt as cnstructed frm ischrnus time lines and the ptential distributin is presented in sectin C. Nrmal Sinus Nde Atrial Rhythm (Fig. 5). Atrial activity was initiated by develpment f an islated minimum ver the SA nde area. The minimum remained lcalized t a small area ver the SA nde fr 3 t 5 msec and in tw dgs it spread ver the adjacent area 1 cm laterally n the superir vena cava; then it extended dwn alng the taenia terminalis with the develpment f maxima and surrunding psitive ptentials (Fig. 5, tp, 9 msec). Thereafter, as the minimum prgressed further dwn alng the taenia terminalis, anther minimum prgressed rapidly tward the left atrium in Bachmann's bundle area; additinally there was a small irregular r branch area ("pseudpd") alng the anterir base f the right atrial appendage. This resulted in early atrial excitatin characterized by a nnunifrm, nnsymmetrical spread f activity which appeared mre cmplex than previusly described (Fig. 5, tp, 24 msec). Nte that the distributin f psitive and negative ptentials indicates wave frnts simultaneusly prgressing ver the upper left atrium, superir right atrium and appendage, inferir right atrium, and als superirly ver the prximal superir vena cava. Additinally, there were cnsiderable ptential changes ver the right atrium fr the first 15 t 20 msec, while n changes culd be detected ver mst f the left atrium. Excitatin f the base f the psterir right Circulatin Research, Vl. XXIV, June 1969

7 ELECTRICAL PTENTIAL DISTRIBUTIN 863 atrial appendage started frm Bachmann's bundle area; hwever, bth the psterir and anterir surface f the right atrial appendage excited in a base-t-apex manner. Early excitatin f the right atrial free wall appeared as an elngated wavefrnt alng the brder f the taenia terminalis (Fig. 5A, 24 msec). The mvement f clsely appsed maxima and minima tward the A-V grve prduced an elngated maximum and psitive ptential area alng the A-V grve with trailing minima and enlarging area f negative ptentials (Fig. 5A, 32 msec). The majr prtin f left atrium and inferir atrium deplarized after right atrial excitatin had been cmpleted. Wave frnts prgressed in tw directins: superirly dwnward frm the upper left atrium and inferirly in a leftward directin acrss the area adjacent t the crnary sinus (Fig. 5A, 45 msec). These tw cases require separate cnsideratin as they prgressed tward each ther t cmplete left atrial excitatin. In ne case, an excitatin maximum cntinued superirly n the left atrial appendage at the same time as a larger area f deplarizatin cntinued ver the lateral wall (Fig. 5B, 56 msec). The sequence f spread n the left atrial appendage varied smewhat, but in all cases the inferir surface f the appendage began t excite near the base next t its right edge with base-t-apex spread. Invasin f the superir surface f the appendage started at the base as a result f the wave frnt prgressing ver the superir (psterir) wall f the left atrium. The last area f the appendage t excite was at the tip (Fig. 5B, 56 and 60 msec); in ne dg this was the latest area f ttal atrial excitatin, and in the remaining animals, the left atrial appendage excitatin maximum cntinued t within 5 t 10 msec f final atrial deplarizatin, which ccurred n the inferir-lateral wall f the left atrium. In the secnd case, with leftward mvement f the inferir right atrial maximum-minimum acrss the crnary sinus area, islated excitatin develped rapidly n the central inferir atrium, as indicated by the central Circulatin Research, Vl. XXIV, June 1SC9 minimum at 56 msec. The minimum can be accunted fr by excitatin via the interatrial septum. Thereafter, multiple maxima and minima ccurred transiently n the inferir atrium, indicating wave frnts mving bilaterally frm the central regin. Multiple maxima and minima, caused by the cnverging wave frnts, cmpleted excitatin f the area adjacent t the crnary sinus (Fig. 5B, 56 msec). The final area t deplarize (except in the ne dg in which the left appendage excited last) was lcated alng the inferir-lateral left atrial wall, as fund by Puech and c-wrkers (6). This area was invaded by cnverging wave frnts indicated by the mvement f separate minima prgressing int the area f the maximum frm a left superir directin and frm the right (Fig. 5B, 60 msec). During the final 15 t 20 msec f left atrial excitatin, replarizatin maxima develped ver the right atrial free wall and SA nde area. Tw types f maxima then existed simultaneusly, deplarizatin maxima ver the left atrium and replarizatin maxima ver the right atrium (Fig. 5B, 60 msec). Replarizatin was characterized by develpment f psitive ptentials ver the right atrium and negative ptentials ver the left atrium (70 msec). Thereafter, as the intensity f the maxima increased ver the right atrium, psitive ptentials migrated as pseudpd extensins t the left ver the superir and inferir atrial surfaces in a sequence similar t that f excitatin spread (80 msec). The prgressin f the pseudpds f psitive ptentials cntinued (Fig. 5C), fllwing the same general pattern f excitatin, while the minimum and surrunding negative ptential area persisted thrughut ver the latest excitatin areas f the left atrium. Thus, nrmal atrial replarizatin was characterized by a minimum (sink) ver the latest excitatin area f the left atrium while a maximum (surce) was lcated ver the right atrium. The distance separating replarizatin maxima and minima was as great as 3 t 5 cm, while that separating the excitatin maxima and minima was t small t be measured

8 864 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS 100msec 56 msec 60 msec 70 msec 80 msec FIGURE 5 Atrial ptential distributin during deplarizatin and replarizatin with nrmal sinus rhythm. These data represent the ptential distributin in a single animal with cmplete heart blck and are representative f the grup. Each instant f time is shwn with the ptential distributin superimpsed n an atrial utline f the type shwn in Figure 3. The instant f time frm nset is indicated abve each heart map; this instant als is indicated by a vertical line superimpsed n a right and left atrial electrgram. The right atrial tracing was recrded at the junctin f the superir vena cava and the base f the right atrial appendage; the left atrial electrgram was recrded frm the inferir-lateral left atrial free wall. The maxima (highest ptentials) are indicated by +, and the area f psitive ptentials surrunding the maxima is indicated by stippling. The minima (lwest ptentials) are indicated by and the surrunding negative area is indicated by diagnal lines. The demarcatin line between psitive and negative ptentials represents zer vltage in reference t the left leg. The ptential values (in millivlts) f the maxima (max) and minima (min) fr each map are nted belw. (See text fr detailed discussin.) A: At 9 msec, minimum ver SA nde = 1.8 and maxima = 0.5 t 0.7. At 24 msec, minima = 2.2 t 2.7 and maxima 0.4 t 0.7. At 32 msec, minima = 2.3 t 2.7 and maxima 1.7 t 2.5. At 45 msec, minima at left atrial appendage = 3.4 and 4.6; inferir Circulatin Resecrcb, Vl. XXIV, June 1969

9 ELECTRICAL PTENTIAL DISTRIBUTIN 865 accurately by the electrde spacings used (3 t 15 mm). Atrial Rhythm Initiated in Crnary Sinus Area (Fig. 6). When excitatin was initiated in an area adjacent t the crnary sinus, tw wave frnts began early, as shwn at 14 msec, with clsely appsed maxima and minima mving superirly ver the lwer right atrium and left acrss the inferir atrium. This prgressin cntinued ver bth atria. n the right atrium, the maxima and minima frmed a widened frnt which mved ver the free wall superirly; the ther wave frnt migrated arund the inferir prtin f the left atrium (30 and 43 msec). While these wave frnts were prgressing ver the free walls, an excitatin minimum suddenly appeared n the middle superir surface f the atria (43 msec). This area was prbably excited via the interatrial septum. Subsequently, wave frnts mved bilaterally away frm the superir central atrium tward the wave frnts migrating upward ver the free wall f each atrium (51 msec). The remainder f atrial excitatin (Fig. 6B, 62 t 87 msec) cntinued simultaneusly in three separate areas the right and left atrial appendages and the prximal superir vena cava. n the appendages, the wave frnts cllapsed while migating distally t the tip. The latest excitatin maximum ccurred n the left atrial appendage. The duratin f excitatin was lnger than nrmal when the crnary sinus area was paced (92 versus 72 msec). During the final 20 t 25 msec f atrial activatin, excitatin maxima and minima existed ver the atrial appendages and prximal superir vena cava simultaneusly with an enlarging replarizatin maximum which began ver the lwer right atrium near the site f pacing. Nte the enlarging area f psitive ptentials f replarizatin which began inferirly n the right atrial free wall and prgressed superirly ver the right atrium and laterally ver the inferir prtin f the atria (Fig. 6B, 62 t 118 msec). The salient feature f the ptential distributin after the cmpletin f excitatin was the presence f replarizatin minima ver the three terminal but separate areas f excitatin (the tw atrial appendages and prximal superir vena cava) while replarizatin maxima were present ver the right atrial free wall and inferir atrium (118 msec). Again, replarizatin maxima and minima were separated mre than excitatin maxima and minima. As replarizatin cntinued, the maxima migrated tward the appendages (Fig. 6C). The minima remained statinary ver the three areas representing late sites f excitatin. Thus replarizatin was characterized again by (1) a cnsiderable time interval (25 t 30 msec) when bth replarizatin maxima and deplarizatin maxima were simultaneusly present, (2) develpment f a maximum in the area f earliest excitatin while minima develped ver the latest areas t excite, (3) greater distances separating maxima and minima during replarizatin than during atrial minimum = 2.4; maxima at left atrial appendage = 3.0 and 3.2, inferir atrial maximum 1.2. B: Atrial ptentials during late excitatin and early replarizatin during nrmal sinus rhythm. At 56 msec, the left atrial appendage maximum = 4.3 and minimum = 3.7; the tw maxima inferirly = 1.7 and 1.8, while the multiple minima ranged frm 0.6 (crnary sinus) t 3.1 (central atrium) and 2.0 (lateral inferir left atrium). At 60 msec, replarizatin maxima n the right atrium = 0.3 and 0.4 and the intervening minimum = 0.3. The excitatin maxima n the left atrium 1.4 and 2.0 and the assciated minima = 2.0 t 2.6. At 70 msec, the increased intensity f the stippling surrunding replarizatin maxima n the right atrium indicates the areas where values were greater than 0.5. The tw right atrial maxima = 0.7 and the left atrial minima = 0.7. At 80 msec, right atrial maxima = 0.7 and left atrial minimum = 0.3. C: Cntinued atrial replarizatin in nrmal sinus rhythm. At 100 msec, right atrial maximum = 0.7 and left atrial minimum = 0.3. At 130 msec, right atrial maxima = 0.5 t 0.9; left atrial minimum = 0.3. Circulatin Research, Vl. XXIV, Jane 1969

10 866 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS 30 msec 43 msec 100 msec B 62 msec 118msec 100msec FIGURE 6 Atrial ptential distributin with pacemaker site in crnary sinus area. The frm f presentatin is the same as fr Figure 5. The right atrial electrgram was recrded frm the lwer free wall f the right atrium and the left atrial electrgram was recrded frm the left atrial appendage. The initial spike in each tracing indicates the pacing artifact. ithe circled square pulse indicates the site f the pacing n the external atrium in the area adjacent t the crnary sinus. See text fr detailed discussin. A: At 14 msec, right atrial maximum = 1.4 and minimum = 0.6; maximum and minimum n the mid-inferir atrium = 0.4 and 0.6. At 30 msec, right atrial maximum and minimum = 2.6 and 1.9; the inferir atrial maximum and minimum = 1.3 and 1.2. At 43 msec, nte the islated minimum ( 1.7) with accmpanying maximum (2.1) superirly between the appendages. Maxima ver the right atrium = 0.7 and 0.8, assciated minima = 3.8 and 1.3; the inferir left atrial maximum and minimum =z 1.2 and 1.8. At 51 msec, the psitive ptentials ccupying the upper left atrium and right atrium encmpass the appendages; the clear area at the tip f each appendage signifies vltage levels less than ±0.2 mv. (nise level). Nte the superirly psitined central minimum ( 3.3) with adjacent maximum (2.1) at the base f the left atrial appendage. Right atrial maxima= 1.2 and 1.3 and the assciated minima 1.3 (near A-V grve) and 3.3 (ver SA nde). 180 msec Circulatin Research, Vl. XXIV, June 1969

11 ELECTRICAL PTENTIAL DISTRIBUTIN 867 deplarizatin, and (4) the migratin f the maxima tward the minima during terminal replarizatin. Left Atrial Rhythm riginating Inferirly and Laterally (Fig. 7). Fllwing start f excitatin n the inferir lateral left atrial wall, tw wave frnts were indicated by the ptential distributin shwn at 19 msec, ne prgressing superirly tward the left atrial appendage ver the left atrial free wall, and the ther t the right acrss the inferir atrium. As the clsely appsed maximumminimum migrated right acrss the inferir atrium, there als was spread arund the base f the left atrial appendage superirly and inferirly (33 msec). Subsequently, the ptential distributin indicated multiple wave frnts, as shwn at 44 msec, with ne mving right n the inferir atrium, anther mving right n the superir surface between the appendages, and with cntinuing excitatin f the left atrial appendage. Excitatin f the superir surface f the atrium was characterized by mvement f the maximum-minimum rightward in the area f Bachmann's bundle with less rapid invasin f the adjining atrial superir surface at the base f the right atrial appendage and the intervening area between the tw appendages (63 msec). Simultaneusly, the wave frnt n the inferir atrium mved acrss the crnary sinus area t the lwer prtin f the right atrial free wall. The sequence f excitatin cntinued with spread alng the taenia terminalis with evidence f multiple wave frnts ver the right atrium such that the ptential distributins shwn at 76 msec and 80 msec (Fig. 7B) indicated the fllwing: (1) activity alng the taenia terminalis area with wave frnts prgressing tward ne anther frm a superir and inferir directin, (2) wave frnts prgressing ver the psterir and anterir medial surface f the right atrial appendage, (3) invasin f the right atrial free wall tward the A-V grve frm the taenia terminalis, and (4) an additinal area f islated activity ver the prximal superir vena cava. Excitatin f the right atrium was cmpleted with wave frnts mving ver the free wall and appendage in a fashin similar t nrmal, as shwn at 80 and 89 msec; i.e., there were excitatin maxima and minima psitined s that the area f the taenia terminalis and Bachmann's bundle were envelped by negative ptentials while the appendage and free wall were ccupied by maxima and psitive ptentials. The demarcatin line mved tward the A-V grve frm right t left. The latest areas f excitatin were n the left edge f the right atrial appendage and alng the upper A-V grve at the base f the appendage (89 msec). The initial evidence f replarizatin (63 msec) was develpment f psitive ptentials in the areas n the left atrium which excited early; this ccurred cnsiderably befre atrial B: Cmpletin f atrial excitatin and early replarizatin. At 62 msec, the prximal (SVC) maximum = 1.0, and the right atrial appendage maxima-=1.7 and 2.2, and the intervening minimum = 2.0. The left atrial maxima = 5.4 and 6.2, and the superirly psitined minima = 5.7 and 5.8. The inferir left atrial wall minimum = 0.5. Nte the nset f replarizatin signified by the maximum (0.3) and surrunding psitive ptentials n the inferir right atrium. At 72 msec, prximal SVC maximum = 0.7; right atrial appendage maximum and minimum = 3.4 and 0.8; left atrial appendage maximum = 3.0; inferir right atrial replarizatin maximum = 0.5. At 87 msec, prximal SVC minimum = 0.3; left atrial appendage maximum and minimum = 5.8 and 2.4; right atrial free wall replarizatin maximum = 0.9. At 118 msec, prximal SVC minimum = 0.3; right atrial appendage minimum = 0.3; left atrial appendage minimum = 0.8; right atrial maximum = 1.0 and inferir atrial maximum = C: Terminal replarizatin. At 157 msec, nte the three minima ver the latest separate sites f excitatin: SVC = 0.3; right atrial appendage = 0.4; and left ventricular appendage = 0.9; right atrial maximum = 0.7 and left atrial maximum = 0.3. At 180 msec, the three minima = 0.3 t 0.5; right atrial appendage maximum = 0.7. Circulatin Research, Vl. XXIV, June 1969

12 868 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS 33 msec 63 msec 100 msec B 76 msec 93 msec 108 msec 100 msec FIGURE 7 Atrial ptentials with pacemaker site n inferir lateral left atrium. The time identificatin electrgrams beneath each map were recrded frm the right atrial appendage (RA) and inferir left atrium (LA). The initiating spike represents pacemaker artifact. The circled square indicates the site f pacemaker stimulatin. See text fr detailed discussin. A: At 19 msec, inferir atrial maximum = 0.8; left atrial maxima = 0.8 and 0.9; bth left atrial minima = 1.0. At 33 msec, inferir atrial maximum and minimum = 1.0 and 3.1; LA appendage inferir maximum and minimum = 0.8 and 1.5; superir LA appendage maximum and minimum = 2.6 and 2.8. At 44 msec, inferir atrial maximum and minimum = 1.2 and 2.1; upper right atrial maximum = 1.4, and LA appendage maximum = 2.7; inferir LA appendage minimum = 5.2 and upper LA appendage minimum = 3.0. At 63 msec, inferir RA maximum and minimum = 3.0 and 2.0; superir RA maxima = 1.5 (medial) and 0.9 (SA nde); LA appendage maximum = 2.0 and superirly psitined minimum = 1.3. Nte the area f psitive ptentials (early replarizatin) ver the lateral left atrium. The highest value in this area was 0.3. B: Cmpletin f atrial excitatin and early replarizatin. At 76 msec, nte the prgressin f the upper and lwer areas f negative ptentials prgressing tward ne anther ver Circulatin Research, Vl. XXIV, June 1969

13 ELECTRICAL PTENTIAL DISTRIBUTIN 869 activatin had been cmpleted. As excitatin cntinued n the right atrium, a replarizatin maximum develped superirly at the base f the left atrial appendage while pseudpds f psitive ptentials prgressed superirly and inferirly in the same general sequence as fllwed by excitatin. At the cmpletin f excitatin, the ptential distributin shwn at 93 msec was typical in that there were psitive ptentials ver the inferir atrium, left atrium, and in the area f Bachmann's bundle, while negative ptentials envelped the right atrium. As replarizatin cntinued, pseudpds f psitive activity migrated ver the right atrium frm bth a superir and inferir directin (Fig. 7C, 108 and 155 msec). Again, during this prtin f the replarizatin the distances separating maxima and minima were much greater than the clsely spaced maximaminima f excitatin. During final replarizatin, psitive ptentials invaded areas adjacent t the minima (197 msec). C. RELATIN BETWEEN ACTIVATIN WAVE FRNT (ISCHRNUS TIME LINES) AND FIELD F PTENTIAL DISTRIBUTIN We cnstructed the ischrnus time lines by the cnventinal methd (11) t cmpare them t the ptential distributin present at the same time. Fr each recrding site, lcal activatin was determined frm the uniplar electrgram, with the cmplex curves n the appendages excluded. The derived excitatin wave frnts were drawn as ischrnus time lines and, independently, the atrial isptential heart maps were drawn; then the tw were cmpared. Figure 8 shws tw instants during nrmal sinus atrial rhythm. At 30 msec nte that (1) there is gd agreement between the psitin f the wave frnt (ischrnus time line) and the psitin f psitive ptentials in the areas tward which the wave frnt is mving and negative ptentials ccupy the areas which have already been excited; (2) distant areas ver the left atrium yet t be excited remain unperturbed; (3) the maxima (highest ptentials ) and minima (lwest ptentials) ccur in clse prximity in the vicinity f the excitatin wave frnt. At 60 msec there are multiple cllapsing excitatin wave frnts n the left atrium. At this time nte that (1) the maxima ccur in the areas being invaded (surce side f the wave frnt) and there are clsely psitined minima in the areas which have just been excited (sink side f the wave frnt); it is nt clear why the minima were lcated at particular sites alng the wave frnt and nt at thers; (2) during late atrial excitatin there are nt nly multiple excitatin wave frnts with assciated multiple maxima and minima, but simultaneusly n the right atrium replarizatin maxima are present. Discussin A. LIMITATINS F THE EXPERIMENTAL MDEL The fllwing limitatins shuld be nted fr this experimental mdel in the dg. the area surrunding Bachmann's bundle and the taenia terminalis. Lwer RA free wall maximum and minimum = S.I and 2.9; superir vena cava (SVC) maximum = 0.8; RA appendage maximum = 2.1. Upper atrial minima varied frm 1.0 t 1.7. Replarizatin maximum n superir lateral left atrium = 0.7. At 80 msec, nte the excitatin ptential distributin ver right atrium as regards its similarity t that encuntered during nrmal sinus rhythm (cf. Figure 5A, 24 msec). (SVC) maximum = 0.7; upper right atrial maximum and minimum = 2.8 and 2.4; lwer right atrial maximum and minimum = 3.2 and 2.5; left atrial maximum and minimum = 0.7 and 0.8. At 89 msec, RA appendage tip maximum z= 2.0; maximum and minimum at base f RA appendage near A-V grve = 5.5 and 2.3; superir minimum = 1.4; inferir LA appendage minimum = 0.6; LA maxima = 1.0 (upper) and 0.6 (lwer). At 93 msec, minima = 0.4 t 0.8; LA maxima = 1.1 (upper) and 0.6 (lwer). C: Cmpletin f replarizatin assciated with left atria pacing fcus. At 108 msec, minima vary frm 0.3 t 0.7; LA maximum = 1.2. At 155 msec, RA minimum = 0.8; inferir maximum = 0.9; upper maximum = 1.0. At 177 msec, RA minimum = 0.7; inferir maximum = 0.5; upper maximum = 0.4. At 197 msec, minima = 0.3 and 0.4; maxima = 0.3 and 0.5. Nte the disappearance f ptential changes ver the left atrial free wall. Circulatin Research, Vl. XXIV, June 1969

14 870 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS 30 msec 60 msec FIGURE 8 Relatinship f activatin wavefrnts as derived by ischrnus time lines and the field f ptential distributin. Lcal excitatin times were determined frm the uniplar electrgrams. This shws excitatin wave frnts derived frm ischrnus time lines (abve) fr tw instants f time as cmpared t the assciated ptential distributin (belw). Nte that the area f psitive ptentials is lcated in the area tward which the wave frnt is mving (30 msec); the minima and area f negative ptentials ccur in areas previusly invaded. Nte the varied psitins f the maxima and minima in relatin t the wave frnt. A cmparisn at 60 msec indicates (1) cllapsing wave frnts n the left atrial appendage and n the inferir lateral left atrial wall with psitive ptentials ccurring in the directin f the mvement f the wave frnt, and (2) the discrepancy between the field f ptentials and psitin f the excitatin wavefrnts due t the simultaneusly present replarizatin maxima n the right atrium. Ptential values f maxima and minima in millivlts: 30 msec, minima 2.3 t 2.7, maxima varied frm 1.7 t 2.5; 60 msec, deplarizatin LA maxima = 2.0 and 2.3, LA minima = 1.9 t 2.6, RA replarizatin maxima = 0.3 and Increasing rate shrtens atrial replarizatin (12) and thereby alters the T waves recrded frm the atrial surface. We perfrmed pacing experiments t vary the rate and fund that the atrial T wave remained cnstant in amplitude and frm ver a range f 180 t 245 beats/min; with further increase in rate, T wave amplitude increased. During the data-recrding prcess, rate remained cnstant thrughut all studies which invlved pacing; hwever, there was slight variability in rate (maximum f 15 beats/min) thrughut the recrding sequence fr the runs during nrmal sinus atrial rhythm. 2. Anther prblem cncerns injury currents induced by directly implanted electrdes. We were unable t demnstrate direct injury effects in the chrnic preparatins; in fact, ne f the impressive aspects f the study was the remarkable reprducibility f the P and Ta curves ver lng perids f time. Thus, it was ur impressin that atrial excitatin and replarizatin may be subject t change when the electrdes are manipulated during acute experiments; hwever, with the vlume cnductr intact and during prlnged anesthesia, atrial P and Ta waves remained remarkably cnstant n a beat-t-beat basis. 3. Althugh the number f sampling pints was large, and althugh many f the electrdes were within 3 t 4 mm f ne anther, this separatin still des nt allw precise definitin f the distances between maxima and minima assciated with the prpagating atrial excitatin wave frnt. Hwever, since the distances were larger between maxima and minima during replarizatin, rather cnfident estimates are pssible during replarizatin. B. CNSIDERATINS REGARDING HEART ISPTENTIAL MAPS Taccardi and Marchetti, in studies f the islated turtle heart, demnstrated that multiple maxima and minima adjacent t the ventricular surface were related t separate wave frnts (13). ur studies f the atrium shw a similar relatinship f changing maxima and minima as related t atrial wave frnts. The significance f the zer isptential line (with respect t left leg reference) during excitatin is that its psitin clsely apprximated the distributin f the majr wave frnts. Hwever, it shuld be emphasized that at times when multiple areas f right atrium were being excited, the venus area f the Circulatin Research. Vl. XXIV, June 1969

15 ELECTRICAL PTENTIAL DISTRIBUTIN 871 right atrium lateral t the taenia terminalis became excited while in the regin f negative ptentials (Fig. 5A, 9 and 24 msec). This study f atrial excitatin in the frm f isptential heart maps prvides tw types f additinal infrmatin when cmpared t that presented by depicting excitatin in the frm f ischrnus time lines. (1) Ttal atrial excitatin duratin is measured t be slightly lnger, since the uniplar tracings prvide ptential measurements at each site thrughut atrial excitatin, whereas in ischrnus time line maps nly ne instant in time is used fr each recrding site. Thus the field maps shw ptential changes in areas adjacent t very early and late excitatin sites. (2) The field maps prvide infrmatin during the latter prtin f atrial excitatin f the superimpsed replarizatin ptential changes n thse f excitatin. C. FEATURES F ATRIAL PTENTIAL DISTRIBUTIN A salient bservatin f the ptential distributin during atrial excitatin was the establishment f multiple maxima and minima simultaneusly present ver varius prtins f the atria, indicating wave frnts prgressing in multiple directins. The lngstanding questin f radial versus nnradial spread has been cnfined primarily t prpagatin frm the area f the SA nde. The changing ptential distributin thrughut atrial excitatin indicates that excitatin phenmena f the atria are mre cmplex than can be accunted fr by a simple mdel f purely radial versus nnradial spread. Als, clliding wave frnts were quite prminent, much mre s than during ventricular excitatin. They presented the majr mechanism fr terminating atrial excitatin. This cntrasts t terminal ventricular excitatin, which is characterized by unifrm wave frnts prpagating t the bundary f ventricular muscle. This fundamental difference between atrial and ventricular excitatin may be imprtant clinically in accunting fr the mre frequent develpment f atrial than ventricular arrhythmias. Early excitatin appeared t be influenced by the prminent muscle bundles cntiguus with the SA nde; i.e., the crista terminalis, Circulatin Research, Vl. XXIV, June 1969 Bachmann's bundle, and a prminent bundle t the base f the right atrial appendage. The results are in agreement with thers that cnductin velcity is increased in these bundles (8, 14, 15). The gemetry f the atrium is such that with the psitin f Bachmann's bundle superirly and the crista terminalis laterally n the right atrium, early multiple wave frnts nrmally are established s that the ptential distributin is far frm symmetrical, as illustrated in Figure 5A, 24 msec. As shwn by San and Yamagishi (8), rapid cnductin dwn the crista terminalis establishes the majr wave frnt ver the right atrial free wall in a directin parallel t this bundle. Wave frnts were similarly aligned alng Bachmann's bundle fr invasin f the psterir right atrial appendage and superir interatrial area. Fr beats initiated ver the lateral inferir left atrium, the influence f Bachmann's bundle and the taenia terminalis was indicated by the sudden prgressin f excitatin minima with surrunding negative ptentials ver these areas nce they had been invaded (Fig. 7, 63 t 76 msec). This resulted in excitatin f the majr prtin f the right atrium in a sequence similar t nrmal; i.e., frm the area f the taenia terminalis tward the A-V grve (right t left), althugh the initiating fcus was n the left atrium. These data indicate that the interatrial septum prvides a cnducting bridge which has an imprtant influence n glbal atrial excitatin, depending upn the site f rigin f the impulse. These results prvide n infrmatin as t specialized cnductin pathways within the septum (16); hwever, they shw the imprtant influence f the septum n glbal excitatin during nrmal sinus atrial rhythm and fr beats initiated in the area in the crnary sinus regin (Figs. 5 and 6). Nrmally, the inferir atrium was excited via the atrial septum at apprximately 18 t 32 msec frm nset. The start f excitatin f the inferir central prtin f the atrium prduced clliding wave frnts fr the cmpletin f excitatin f the crnary sinus area and fr the inferir lateral left atrium (Fig. 5B). Als, when the atrial pacemaker was lcated in the

16 872 SPACH, KING, BARR, BAZ, MRRW, HERMAN-GIDDENS crnary sinus area, excitatin superirly in the central area between the appendages was initiated via the atrial septum at apprximately 30 msec, prducing wave frnts spreading bilaterally ver the upper atrium. Replarizatin maxima were present during terminal atrial excitatin. This perhaps was t be expected cnsidering the time curse f replarizatin in atrial muscle (12). A prminent finding was that the ptential changes during replarizatin fllwed the same general prgressin as activatin. This cntrasts with the results f Irisawa and cwrkers (17) in acute studies n the dg right atrium with the use f suctin electrdes, and als is different frm that fund by Pipberger el al. (18) fr the dg ventricle in acute studies; bth grups fund n general pattern f replarizatin. ur studies cnsistently shwed that an initial replarizatin maximum appeared in the area f earliest excitatin. Fllwing this, the majr changes during replarizatin ccurred with extensins by pseudpds, in a sequence similar t that f excitatin, frm the area f psitive ptentials int that f the minima and surrunding negative ptentials, which were lcated ver areas that were the last t underg excitatin. The great distances between maxima and minima during replarizatin, as cmpared t excitatin, indicate that the nature f replarizatin is quite different frm that f deplarizatin (19). Particularly, this finding suggests that the length cnstant (20) fr current flw thrugh atrial mycardial fibers is much greater during replarizatin than during deplarizatin. Furthermre, terminal replarizatin was characterized by the maximum becming mre clsely assciated with the replarizatin minimum, suggesting that there is a decrease in the length cnstant during terminal replarizatin. Acknwledgments We wish t express ur appreciatin t Dr. E. A. Jhnsn and Dr. Melvyn Lieberman, Department f Physilgy, Duke University, fr their helpful suggestins during the curse f this study. References 1. DE CMNENE, J., STEPHANPLJ, BLADIER, B., CLMBAUI, F., AND BKIFACI, J.: Studies f the atrigram. Am. Heart J. 50: 666, IRISAWA, H., AND SEYAMA, I.: Cnfiguratin f the P wave during mild exercise. Am. Heart J. 71: 467, WLSEY, M. D., BBDY, D. A., AND ARZBAECHER, R. C: Measurement f spntaneus mrphlgic variatins in the electrcardigraphic P wave. Cmputers and Bimed. Res. 1: 265, LEWIS, T., MEAKINS, J., AND WHITE, P. D.: Excitatry prcess in the dg's heart: I. The auricles. Phil. Trans. Ry. Sc. Lndn, Ser. B. 205: 375, EYSTER, J. A. E., AND MEEK, W. J.: Experiments n the rigin and cnductin f the cardiac impulse: VI. Cnductin f the excitatin frm the sinauricular nde t the right auricle and auriculventricular nde. Arch. Int. Med. 18: 775, PUECH, P., ESCLAVISSAT, M., SDI-PALLARES, D., AND CISNERS, F.: Nrmal auricular activatin in the dg's heart. Am. Heart J. 47: 174, VAN DER KI, M. W., DURREH, D., VAN DAM, R. TH., AND VAN DER TWELL, L. H.: Electrical activity in sinus nde and atriventricular nde. Am. Heart J. 51: 684, SAN, T., AND YAMAGISHI, S.: Spread f excitatin frm the sinus nde. Circulatin Res. 16: 423, ABILDSKV, J. A., CRNVICH, J. A., AND BUHCH, G. E.: An analysis f activatin in human atria. Circulatin 11: 97, MATSUKA, S.: Experimental studies n the auricular waves. Japan. Circulatin J. 21: 25, SCHER, A. M., YUNG, A. C, MALMGREN, A. L., AND PATN, R. R.: Spread f electrical activity thrugh the wall f the ventricle. Circulatin Res. 1: 539, HFFMAN, B. F., AND CRANEFIELD, P. F.: Electrphysilgy f the Heart. New Yrk, McGraw-Hill Bk C., Inc., 1960, p TACCABDI, B., AND MAHCHETTI, G.: Electrphysilgy f the Heart. New Yrk, Pergamn Press, 1965, p CHILDERS, R. W., MERTDETH, J., AND ME, G. K.: Supernrmality in Bachmann's bundle. Circulatin Res. 22: 363, WAGNER, M. L., LAZZARA, R., WEISS, R. M., AND HFFMAN, B. F.: Specialized cnducting fibers in the interatrial band. Circulatin Res. 18: 502, JAMES, T. N.: Cnnecting pathways between the sinus nde and A-V nde and between the right and left atrium in the human heart. Am. Heart J. 66:498, Circulatin Research, Vl. XXIV, June 1969