Transcript ANATOMICAL CORRELATION OF CARDIAC ELECTRICAL

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.
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.
False tendon
Ratio of ventricular wall thickness
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.
P wave on EKG
Cardiac Anatomy
Blood Supply
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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 descending septal artery 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 AV bundle (known also as the HIS bundle). In
70% of human hearts, the right coronary artery gives rise not only to the posterior
descending artery 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%.
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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.
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Blood Supply
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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.
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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.
Right Coronary Artery
Left Coronary Artery
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 metastatic neoplasms. 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 calcific degeneration
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 cervical ganglia 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
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The sinus nodal artery arises from the right coronary artery in about 60% of
patients and from the circumflex artery in 40%.
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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.
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Other septal perforators of the left anterior descending artery supply the anterior
aspect of the left bundle branch, and
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Septal perforators of the posterior descending and posterolateral branches supply
the posterior portion of the left bundle branch.
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The right bundle branch receives a dual blood supply from the septal perforators of
the anterior and posterior descending arteries.
AV Node
Right bundle branch
Myocardial cells
Myocardial cells
SA node
Left bundle branch
Purkinje
fibersfibers
Purkinje
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 1015% 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
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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
Acute Inferior Myocardial Infarction
Early Repolarization Pattern
Electrocardiographic Location of Infarction Sites Based on the Presence of Abnormal Q -Waves
SITE
LEADS
FALSE PATTERNS
Inferior (diaphragmatic)
II, III, aVF
WPW (PSAP), HCM
Inferolateral "True" posterior (postero-basal)
II, III, aVF, V4-V6, V1*
RVH, "atypical" incomplete RBB, Left AP
Inferoposterior
II, III, aVF, V1*
WPW (left PSAP), HCM
Inferior-right ventricular
II, III, aVF plus V4R-V6R or V1-V3 ASMI as defined from axis
Anteroseptal
V1, V2, V3
LVH, chronic lung disease,
LBB, chest electrode misplacement
Anterolateral
I, II, V4 -V6
HCM, ventricular septal defect
Extensive anterior
I, AL1, V1-V6
High lateral
I, aL1
Anterior (apical)
V3 -V4
Posterolateral
V4 -V6 , V1
WPW (LFWAP)
Right ventricular
V4R with V4R-V6R or V1 -V3
ASMI as defined from axis
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