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ANATOMICAL CORRELATION OF CARDIAC ELECTRICAL ACTIVITY Nasser A. Mahdi, MD. Cardiac Anatomy. Heart Chambers The Right ventricle is an anterior structure comprised of an inlet, trabecular and outflow segments.
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ANATOMICAL CORRELATION OF CARDIAC ELECTRICAL ACTIVITYNasser A. Mahdi, MD
Cardiac Anatomy Heart Chambers The Right ventricle is an anterior structure comprised of an inlet, trabecular and outflow segments. It is thinner than the left ventricle and the the RV apex is much more trabeculated than the LV apex. The left ventricle is thickest towards the base and thinner towards the apex. The left ventricle is 3 times thicker than the right ventricle.
Cardiac Anatomy Heart Chambers The Right Atrium is comprised of a smooth free wall posterior region that receives the the venae cavae and coronary sinus, and an anterior muscular region that is lined by parallel pectinate muscles and from which the right atrial appendage arises. The right atrial free wall is quite thin and can be perforated easily by stiff catheters. The Left atrium receives 4 pulmonary veins (2 upper and 2 lower). Left atrial muscle extends some distance within the pulmonary veins resulting in a cuff like muscle which acts as sphincter during atrial systole and may be the source of focal atrial fibrillation amenable to RF ablation. The atrial appendage arises anterolaterally and is smaller and less pyramidal the right atrial appendage. The atrial appendage is multilobed in 80% of the time( 2 lobes). The esophagus and descending thoracic aorta lie posteriorly to the left atrium. The left atrium enlarges with age and may explain the age related increase in atrial fibrillation.
Cardiac Anatomy Blood Supply • The right coronary artery arises from the right aortic sinus. In 50% of subjects, the first anterior branch is the conus artery which nourishes the right ventricular outflow tract. The descendingseptalartery which arises from the proximal right coronary artery, or early from the conus artery or right aortic sinus supplies the infundibular septum and the distal AVbundle (known also as the HIS bundle). In 70% of human hearts, the right coronary artery gives rise not only to the posteriordescendingartery which travels in the inferior interventricular septum but also to branches that supply the inferior left ventricular free wall. These arteries nourish the inferior third of the ventricular septum including the right bundle branch and posterior portion of the left bundle branch and the inferior left ventricular free wall, including the posteromedial mitral papillary muscle. Co- domonance in 20%. • The left main coronary artery originates from the left aortic sinus. It travels between the pulmonary artery and the left atrium and is covered in part by the left atrial appendage. In the majority of patients (about 75%), it bifurcates into a left anterior descending artery and circumflex artery and in the remainder 25% of patients, it trifurcates into the left anterior descending artery, the intermediate artery also known as ramus intermediate vessel and the left circumflex artery.
Blood Supply • The left anterior descending artery travels within the anterior interventricular groove and wraps around the apex and may ascent a variable distance along the inferior interventricular. Septal perforating branches can be seen from the left anterior descending artery which supply the HIS bundle and the anterior aspect of the left bundle branch. The left anterior descending artery also gives rise to diagonals which nourish the anterior left ventricular free wall and the medial third of the anterior right ventricular free wall. It is not uncommon to see myocardial bridges in the LAD in about 12% of patients and they produce systolic luminal narrowing in only 1-2% of hearts and most likely have a benign prognosis. • The left circumflex coronary artery travels within the left atrio ventricular groove and often terminates just beyond the obtuse marginal branch. The circumflex artery supplies the lateral left ventricular free wall; however, in 10% of patients the circumflex artery gives rise to a posterior descending branch and therefore it is called a left dominant system. The Circumflex and the anterior descending arteries nourish the anterolateral mitral papillary muscle and the circumflex, and the right coronary artery supply the postero medial mitral papillary muscle. It is of note to mention that the origin of the posterior descending artery determines the blood supply of the inferior portion of the left ventricle and thereby defining coronary dominance.
Cardiac Conduction System Sinus Node The sinus node is the primary pacemaker of the heart. It is an epicardial structure that measures approximately 15x5x2 mm and is located in the sulcus terminalus near the superior cavoatrial junction. Histologically, the sinus node consists of specialized cardiac muscle cells embedded within a permanent collagenous stroma. Its myocardial cells are smaller than ventricular muscle cells and contain only scant contractile elements. Ultra structurally, the sinus node comprises transitional cells and variable numbers of P cells centrally and atrial myocardial cells peripherally. The P cells are thought to be the source of normal cardiac impulse formation. Because the sinus node occupies an epicardial position, its function maybe affected by pericarditis or metastaticneoplasms. The sinus node is rarely infarcted but its function can be altered by adjacent atrial infarction. AV Node The AV node is a subendocardial right atrial structure that measures approximately 6x4x1.5 mm. It is located within the triangle of Koch bordered by the tendon of todaro, septal tricuspid annulus and coronary sinus ostium. The AV nodal artery courses near the bundle but not necessarily through it. Sympathetic and parasympathetic nerves enter the AV node and greatly influence its function. With advancing age, the AV node acquires progressively more fibrous tissue although not as extensively as the sinus node.
Atrioventricular Bundle The AV bundle or known as the HIS bundle arises from the distal portion of the AV node and courses through the central fibrous body to the summit of the muscular ventricular septum adjacent to the membranous septum. It affords the only physiologic avenue for electrical conduction between the ventricles. By its proximity within the central fibrous body (the right fibrous trigon) the AV bundle is closely related to the annuli of the aortic, mitral and tricuspid valves. The AV bundle has a dual blood supply from the AV nodal artery of the right coronary artery and the first septal perforator branch of the left anterior descending artery. The entire AV bundle is insulated by a collagenous sheet. With increasing age the fibrous septa become thicker and the functional elements maybe partially replaced by adipose tissue. Ultra structurally the AV bundle is made of purkinje cells and ventricular myocardial cells in parallel arrangement. In certain subjects, other conduction pathways exist between the atria and ventricles either within the existing AV conduction system or along the fibrous skeleton and therefore may produce certain arrhythmia. Atrio nodal bypass tracts of James connect the atria to the distal AV node and atrio fasicular tracts of Breckenmachar connect the atrial to the AV bundle. Bypass fibers of Mahaim connect the AV node and the AV bundle respectively to the underlying ventricular septal summit. These bypass fibers are quite commonly observed histologically and are apparently not functional in most persons although they may produce a ventricular pre excitation in some instances.
Internodal Tracts Although by light microscopy there are no morphologically distinct pathways between the sinus node and the AV node, EP studies support the concept of three functional pathways. These pathways represent 3 prominent muscle bundles. Lipomatous hypertrophy of the atrial septum may interfere with internodal conduction and induce various atrial arrythmias. Balloon septostomy does not produce internodal conduction disturbances since these 3 functional pathways do no travel in the fossa ovalis but only in the limbus. Bundle Branches The right bundle branch forms a cord like structure approximately 50 mm in length and 1 mm in diameter. It courses along the septal and moderator bands to the level of the anterior tricuspid papillary muscle. The left bundle branch forms a broad fenestrated sheet of conduction fibers which spreads along the septal subendocardium of the left ventricle and separates incompletely and variably into two or three indistinct fossicles.
The bundle branches are supplied by the septal perforators arising from the anterior and posterior descending coronary arteries. Right bundle branch block maybe idiopathic or associated with ischaemic heart disease, chronic systemic hypertension, or pulmonary hypertension. Right ventriculotomy which happens in certain cardiac surgeries produces features of right bundle branch block even though the bundle may not have been transected. Left bundle branch block maybe associated with fibro calcificdegeneration as a result of chronic ischaemia, left ventricular hypertension, calcification of the aortic or mitral valves or any form of cardiomyopathies such as dilated cardiomyopathy. Cardiac Innervation Cardiac innervation arises from the cervicalganglia that originate from three pairs of cervical sympathetic cardiac nerves which intermingle as they join the cardiac flexes between the great arteries and the tracheal bifurcation. Several thoracic sympathetic cardiac nerves arise from the upper thoracic ganglia and also join the cardiac flexes. The vagus nerves give branches to the superior and inferior cervical vagal cardiac nerves and the thoracic vagal cardiac nerves which likewise interweave within the cardiac Plexus. The sympathetic and parasympathetic nerves then descent from these plexus on to the heart. Afferent nerves concerned with pain and various reflexes ascend from the heart towards the cardiac plexus.
Blood Supply of the Cardiac Conduction System • The sinus nodal artery arises from the right coronary artery in about 60% of patients and from the circumflex artery in 40%. • The AV nodal artery originates from the dominant artery and accordingly arises from the right coronary artery in 90% of cases and the circumflex artery in 10% of cases. The AV nodal artery and the first septal perforator of the left anterior descending artery offer dual blood supply to the AV bundle or known as the HIS bundle. • Other septal perforators of the left anterior descending artery supply the anterior aspect of the left bundle branch, and • Septal perforators of the posterior descending and posterolateral branches supply the posterior portion of the left bundle branch. • The right bundle branch receives a dual blood supply from the septal perforators of the anterior and posterior descending arteries.
AV Node SA node Right bundle branch Left bundle branch Purkinje fibers Myocardial cells Myocardial cells Purkinje fibers
Cardiac Valves The AV valve, the right tricuspid and left mitral AV valve have five components three of which form the valvular apparatus, the annulus, the leaflets and the commissures, and two of which form the tensor apparatus, the chordae tendineae, and the papillary muscles. Tricuspid Valve The plain of the tricuspid annulus faces towards the right ventricular apex along the free wall, the annulus inserts into the AV junction whereas along the septum it separates the AV and interventricular portions of the septum. Mitral Valve The plain of the mitral annulus faces towards the left ventricular apex. The orifice changes shape during the cardiac cycle from elleptical during ventricular systole to more circular during diastole. In living subjects, the normal mitral annular circumference is maximum during ventricular diastole and decreases 10-15% during systole. Mitral annular calcification almost invariably involves only the posterior mitral leaflet and forms a C-shaped ring of annular and subannular calcium which may impede basal ventricular contraction and thereby produce mitral regurgitation.
The Aortic Valve The plain of the aortic valve usually is directed towards the right shoulder. Its area averages about 3 cm. sq. The mitral valve is prone to stenosis or regurgitation depending on the disease etiology. In cases of infective endocarditis, subvalvular extension may involve the anterior mitral leaflet and left bundle branch causing left bundle branch block or ventricular septal myocardium causing a VSD or involvement of the ventricular septal myocardium may produce a large abscess cavity which may rupture into a ventricular chamber with formation of either an aorto right ventricular or aorto left ventricular fistula. The Pulmonary Valve The plain of the pulmonary annulus is directed towards the left mid scapula with an area of about 3.5 cm.sq. Pulmonary regurgitation occurs in conditions that produce dilatation of the pulmonary artery and annulus such as pulmonary hypertension or heart failure.
Pericardium The fibrous parietal pericardium is a resilient sac that envelops the heart and attaches onto the great vessels. Almost the entire ascending aorta and main pulmonary artery and portions of both venae cavae and all 4 pulmponary veins are intrapericardial. The serous pericardium forms the delicate inner lining of the fibrous pericvardium as well as the outer lining of the heart and great vessels. After surgery localized accumulation of fuid or blood within the pericardium (localized) can produced tamponade. With age adipose tissue can accumulate within the pericardium and can be misinterpreted as tumor.
ST- Changes • Injury implies abnormal ST changes whereas necrosis implies Q waves and ischemia implies T wave inversions or elevation. • Injury pattern occurs because the affected cells are unable to maintain their normal repolarization during diastole. • ST segment elevation occurs when the injured muscle is located between normal muscle and the corresponding unipolar lead. • ST segment depression occurs when normal muscle is located between the injured muscle and the electrode.
ST- Changes ST Depression ST Elevation electrode Epicardium Negative 70 mV Normal 90mV
