COPYRIGHTED MATERIAL. Anatomical and Electrophysiological Bases CHAPTER The h eart w alls The s pecific c onduction s ystem (Fig. 1.

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1 CHPTER ntomicl nd Electrophysiologicl ses.. The h ert w lls The hert hs four cvities, two tri nd two ventricles, comprised minly of contrctile cells clled crdiomyocytes. The electricl stimulus originting in the sinus node (SN) is distributed through the entire hert by mens of specific conduction system (SCS). The left ventricle (LV) hs four wlls: nterior, septl, inferior, nd lterl. Figure. shows the three segments of the nterior nd inferior wlls, the five segments of the septl nd lterl wlls, nd the pex segment (segment ). Mgnetic resonnce imging hs now shown tht the previously-nmed posterior wll corresponds to the inferobsl segment of the inferior wll (segment in Fig.. ) ( yés de Lun et l., ; yés de Lun nd Fiol-Sl, 8 )... Coronry c ircultion (Fig.. ) sed on coronry perfusion, the hert is divided into two zones: the nteroseptl zone, perfused by the left nterior descending rtery (LD) (Fig.. ) nd the inferolterl zone, perfused by the right coronry rtery (RC) nd circumflex rtery (CX) (Figs.C nd. D). The hert hs res of shred perfusion (shown in grey in Fig.. ) in which one of the two rteries domintes. For exmple, segment (pex) is perfused by the LD, if long; otherwise by the RC nd even prtilly by the CX... The s pecific c onduction s ystem (Fig.. ) Electricl stimuli pss through the internodl pthwys (chmnn, Wenckebch nd Thorel bundles), from the sinus node to the V node nd the His bundle. From there stimuli rech the ventricles through the ventriculr conduction system: the right brnch (R) nd the trunk of the left brnch (L), nd its divisions (superonterior nd inferoposterior fscicles nd the middle fibers tht exist between them) (Figs. nd. ). Figure. C shows in grey the structures tht the V junction encompss. Figure. D shows the three ctivtion entry points in the left ventricle (LV) ( Durrer et l., 9 )... The u ltrstructure of c rdic c ells There re two types of cell in the hert:. Contrctile cells or crdiomyocytes, which re responsible for crdic pump function (contrctile). Under norml conditions these cells do not hve utomtic cpcity nd cnnot generte stimuli. COPYRIGHTED MTERIL C D ECGs for eginners, First Edition. ntoni yés de Lun. John Wiley & Sons, Inc. Published by John Wiley & Sons, Inc.

2 CHPTER D X sl RV nterior wll X 8 b Medil 8 9 RV Y b 8 9 picl Y 9 b pex C Inferior wll b Septl wll Lterl wll Figure. () Segments into which the left ventricle is divided ccording to the trnsverse (short-xis) sections performed t the bsl (), medil (M), nd picl () levels. The bsl nd medil sections delinete into six segments ech, while the picl section shows four segments. Together with the pex, they constitute the segments into which the left ventricle cn be divided, ccording to the clssifiction performed by the mericn Imging Societies ( Cerqueir et l., ). lso shown is the view of the segments with the hert open in horizontl long-xis plne () nd verticl long-xis (sgittl-like) plne (C). In D, the segments nd the four wlls of the hert re shown in bull s-eye view. RV = right ventricle.. Specific cells (cells of specific system of conduction), which re in chrge of impulse formtion (utomticity) nd impulse trnsmission to the contrctile myocrdium. Contrctile cells (Fig.. ) re comprised of the following:. The contrctile system mde up of myofibers in which the contrctile unit is the srcomere (Figs.,., nd.), structure tht cn contrct nd relx. The energy for this ctivity is provided by mitochondri.. The cellulr ctivtion relxtion system consisting of the cellulr membrne formed by lipid bilyer (srcolemm) (Figs. nd. ). Ions (N, K, nd especilly C ), responsible for ctivtion, depolriztion nd repolriztion, systole, nd cellulr rest (distole) phses, flow through chnnels tht exist in this membrne.. The trnsverse tubulr (T) system (Fig.. ), which llows electricl excittion to enter the cell, nd the srcoplsmtic reticulum (Fig.. ) which contins the clcium necessry for cellulr contrction.. The specific cells, non-contrctile, re of three types: () P cells, the ones with more utomticity, locted especilly in sinus node ; (b) Purkinje cells, with less utomticity, locted especilly in the His bundle, bundle brnches nd Purkinje network; nd (c) trnsitionl cells... The e lectrophysiology of c rdic c ells... Trnsmembrne d istolic p otentil ( TDP ) nd t rnsmembrne ction p otentil ( ) in utomtic nd c ontrctile c ells ll contrctile crdic cells t rest show equilib - rium between the externl electricl chrges nd the internl negtive chrges (Fig.. ). When micro-electrode is locted inside contrctile cell t rest while second micro-electrode remins in

3 ntomicl nd Electrophysiologicl ses LD nteroseptl 8 9 Inferolterl S 8 9 D C RC 8 D 8 LCX DP 9 PL 9 P Figure. ccording to the ntomicl vrints of coronry circultion, the res of shred vrible perfusion re shown in grey (). The perfusion of these segments by the corresponding coronry rteries ( D) cn be seen in the bull s-eye imges. For exmple, the pex (segment ) is usully perfused by the LD but sometimes by the RC, or even the LCX. Segments nd 9 re shred by LD nd RC, nd lso the smll prt of the mid-low lterl wll is shred by LD nd LCX. Segments, nd correspond to the RC or the LCX, depending on which of them is dominnt (the RC in >8% of the cses). Segment often receives blood from LD. E the exterior (Fig.. ), difference in trnsmembrne potentil, clled the trnsmembrne distolic potentil (TDP), is produced. Under norml conditions the voltge is 9 mv (Fig.. ). s contrctile cells re not utomtic; TDP is rectiliner (Fig..). This mens tht during the distolic phse, n equilibrium between the potssium outwrd ionic current, nd the sodium nd clcium inwrd ionic current tkes plce. When the contrctile cell receives stimulus trnsmitted from neighboring cell, sodium current enters the cell rpidly. This cretes stimulus upon reching the threshold potentil, which forms the trnsmembrne ction potentil () (Fig.. ). Thus, in the contrctile cells the formtion of (Fig.. ), which is the bsis of cellulr ctivtion (depolriztion nd repolriztion), tkes plce becuse stimulus () trnsmitted from neighboring cell, originting from rpid entry of sodium, reches the threshold potentil () nd results in (b nd c re stimuli under threshold potentil) (Fig..8 ). The hs four phses: phse tht is the depolriztion, the loss of externl electricl chrges, nd phses to involve repolriztion, the recovery of these chrges. Cells of the specific conduction system (SCS) hve n scending TDP, becuse they present some distolic depolriztion due to rpid F

4 CHPTER Superonterior fscicle () VCS Inferoposterior fscicle () FO VCI SC VCI Middle fibers () C Todro s tendon D RV LV CS NC β α NH PH CF RH L R S. Fibrous ring of the tricuspid vlve Figure. () Right lterl view of the specific conduction system., nd : internodl trcts; : V node; : bundle of His; : left brnch; : right brnch with its rmifictions; o: ort; VN: V node; CS: coronry sinus; FO: foss ovlis; IVC: inferior cv vein; SN: sinus node; SVC: superior cv vein. () Left lterl view of LV: see the superonterior (S) division (), the inferoposterior (IP) division () nd the middle fibers () (qudrifsciculr theory) or rther qudruple input of ventriculr ctivtion theory. (C) Structure of the V junction extending further thn the V node (the compct node). The zone shded in gry is included in the V junction, nd my be involved in the reentry circuits exclusive to the V junction: CF: centrl fibrous body; N: compct V node; PH: bundle of His penetrting portion; RH: bundle of His rmifying portion; L: left brnch; R: right brnch. Slow conduction ( α) nd rpid conduction (β) pthwys; : entry of fibers of internodl pthwys into the V node: NH: nodl-his trnsition zone; CS: coronry sinus. (D) The open left ventricle shows the three points of LV ctivtion ccording to Durrer (see text). inctivtion of the potssium outwrd currents. The sinus node (SN) is the SCS structure with the gretest scending TDP, nd thus it presents the gretest utomticity, nd is the physiologicl pcemker of the hert. The in utomtic cells (Fig.. ) tkes plce when the TDP reches the threshold potentil. This occurs t the exct point when the ionic curves of sodium (rising) nd potssium (decresing) cross (Fig..8 ), resulting in the entry of sodium into the cell. This occurs more rpidly when the TDP curve is shrper, s in SN utomtic cells. SCS cells fter cellulr depolriztion () tht present slow scent (phse ) (contrctile cells) experience shorter repolriztion phses ( nd ).... Electroionic c orreltion in f ormtion (Figs.8 nd.9 ) In both contrctile (Fig.. ) nd utomtic cells (Fig.. ) curves originte becuse first rpid entry of sodium occurs, followed by sodium nd clcium, into the cell during phse, or cellulr depolriztion. Lter this is followed by slow exit of potssium, resulting in the repolriztion process G

5 ntomicl nd Electrophysiologicl ses Interclted disc Srcolemm T system Srcoplsmic reticulum Mitochondri nd srcoplsmic reticulum Myofibers Myofibers Srcomere M Z nd Srcomere digrm Trnsverse section z H z Myosin M ctin... Figure. () Microphotogrphy of srcomere where ctin nd myosin filments re observed (see -). (-) Structure of the cellulr membrne (or srcolemm) showing n ionic chnnel. (-) Section of myocrdil contrctile cell including ll different elements. (-) Enlrged srcomere scheme. N C N Cl N C K N Cl C : non-difussible nions K 9 DP Figure. () The predominnt negtive chrges inside the cell re due to the presence of significnt non-diffusible nions which outweigh the ions with positive chrge, especilly K. () Two microelectrodes plced t the surfce of myocrdil fiber record horizontl reference line during the resting phse (zero line), signifying no potentil differences on the cellulr surfce. When one of the two electrodes is introduced inside the cell, the reference line shifts downwrds ( 9 mv). This line (the DP) is stble in contrctile cells nd hs more or less scending slope in the specific conduction system cells (Figs. nd. ). (phses nd ). Figure.8 shows this process in contrctile cell through the formtion of the depolriztion nd repolriztion dipoles, which will be explined in the next chpter ( yés de Lun, ). Figure.9 shows the most relevnt ionic current in utomtic (), nd contrctile cells () during systole.... Stimuli t rnsmission from the s inus n ode to the c ontrctile m yocrdium Figure. shows how stimuli re trnsmitted from the SN (the most utomtic cells) to the V node, brnches nd ventriculr Purkinje fibers, which present progressively less utomtism, nd

6 8 CHPTER 8 9 TDP Figure. Trnsmembrne distolic or resting potentil (TDP) nd trnsmembrne ction potentil () of contrctile cells. b c 8 TDP Figure. Trnsmembrne distolic or resting potentil (TDP) nd trnsmembrne ction potentil () of utomtic cells. 9 mmh cm gk gcn E E gk gn gc Isi IN IC IK IN IC Ito IKs IKr IKi Figure.8 The most relevnt ionic currents in utomtic () nd contrctile () cells during systole. Contrctile cells re chrcterized by n erly nd brupt N inwrd flow nd n initil nd trnsient K outwrd flow (I to). These re not present in utomtic cells.

7 ntomicl nd Electrophysiologicl ses 9 N C N C K N C C N K N N C K N C Out Cell T K ST J N C N N C K N C Ito K K Cel. Mem. In. Cell Src. Ret. N N C IONIC PUMP K K C Figure.9 Digrm of the electro-ionic chnges occurring during cellulr depolriztion nd repolriztion of contrctile myocrdium cells. In phse, when the N inwrd flow occurs, the depolriztion dipole ( ) is formed. In phse, when n importnt nd constnt K outwrd flow is observed, the repolriztion dipole is formed ( ). Depending on whether we exmine single cell or the whole left ventricle, negtive repolriztion wve (broken line) or positive repolriztion wve (continuous line) is recorded respectively (see Section.. in Chpter ).

8 CHPTER Sinus node finlly to the ventriculr muscles (nonutomtic contrctile cells). This process is explined in the next chpter in the section on hert ctivtion nd the domino theory. C V junction Self-ssessment. Which segment of the LV does the re previously known s the posterior wll correspond to? D Ventriculr Purkinje. Which rtery perfuses the LV pex? C. How mny ctivtion entry points re in the LV? Ventriculr muscle Figure. Sinus node P () trnsmitted to the V junction (), the ventriculr Purkinje (C) nd ventriculr muscle (D) (: threshold potentil). D. How mny types of crdic cells exist? E. Wht is TDP? F. Wht is? G. Wht role do ions ply in the formtion of?