SURGICAL ANATOMY OF CONDUCTION SYSTEM IN NORMAL AND CONGENITAL HEART

DISEASE AND APPLIED IMPORTANCE WITH RELATION TO VARIOUS CARDIAC OPERATIONS

Presented by- dr.Kiran.GModerater – dr.RV Kumar

Anatomy of conduction system

Sinoatrial node

1. The sinoatrial (SA) node is a spindle-shaped structure composed of a fibrous tissue matrix with closely packed cells. It is 10-20 mm long, 2-3 mm wide, and thick, tending to narrow caudally toward the inferior vena cava (IVC). The SA node is located less than 1 mm from the epicardial surface, laterally in the right atrial sulcus terminalis at the junction of the anteromedial aspect of the superior vena cava (SVC) and the right atrium (RA).

2. The artery supplying the sinus node branches from the right coronary artery in 55-60% of hearts or the left circumflex artery in 40-45% of hearts. The artery approaches the node from a clockwise or counterclockwise direction around the SVC–RA junction.[3]

3. The SA node is densely innervated with postganglionic adrenergic and cholinergic nerve terminals. Neurotransmitters modulate the SA node discharge rate by stimulation of beta-adrenergic and muscarinic receptors. Both beta1 and beta2 adrenoceptors subtypes are present in the SA node. The human SA node contains a more than 3-fold greater density of beta-adrenergic and muscarinic cholinergic receptors than the adjacent atrial tissue.[4]

internodal pathway Anatomic evidence suggests the presence of 3

intra-atrial pathways: (1) anterior internodal pathway, (2) middle internodal tract, and (3) posterior internodal tract.

The anterior internodal pathway begins at the anterior margin of the SA node and curves anteriorly around the SVC to enter the anterior interatrial band, called the Bachmann. This band continues to the left atrium (LA), with the anterior internodal pathway entering the superior margin of the AV node. The Bachmann bundle is a large muscle bundle that appears to conduct the cardiac impulse preferentially from the RA to the LA.

The middle internodal tract begins at the superior and posterior margins of the sinus node, travels behind the SVC to the crest of the interatrial septum, and descends in the interatrial septum to the superior margin of the AV node.

The posterior internodal tract starts at the posterior margin of the sinus node and travels posteriorly around the SVC and along the crista terminalis to the eustachian ridge and then into the interatrial septum above the coronary sinus, where it joins the posterior portion of the AV node. These groups of internodal tissue are best referred to as internodal atrial myocardium, not tracts, as they do not appear to be histologically discrete specialized tracts

Atrioventricular node

The compact portion of the atrioventricular (AV) node is a superficial structure located just beneath the RA endocardium, anterior to the ostium of the coronary sinus, and directly above the insertion of the septal leaflet of the tricuspid valve. It is at the apex of a triangle formed by the tricuspid annulus and the tendon of Todaro, which originates in the central fibrous body and passes posteriorly through the atrial septum to continue with the eustachian valve

Triangle of koch

In 85-90% of human hearts, the arterial supply to the AV node is a branch from the right coronary artery that originates at the posterior intersection of the AV and interventricular grooves (crux). In the remaining 10-15% of the hearts, a branch of the left circumflex coronary artery provides the AV nodal artery.

Fibers in the lower part of the AV node may exhibit automatic impulse formation.

The main function of the AV node is modulation of the atrial impulse transmission to the ventricles to coordinate atrial and ventricular contractions.

Bundle of His

The bundle of His is a structure that connects with the distal part of the compact AV node, perforates the central fibrous body, and continues through the annulus fibrosus, where it is called the nonbranching portion as it penetrates the membranous septum.

Connective tissue of the central fibrous body and membranous septum encloses the penetrating portion of the AV bundle, which may send out extensions into the central fibrous body. Proximal cells of the penetrating portion are heterogeneous and resemble those of the compact AV node; distal cells are similar to cells in the proximal bundle branches.

Branches from the anterior and posterior descending coronary arteries supply the upper muscular interventricular septum with blood, which makes the conduction system at this site more impervious to the ischemic damage, unless the ischemia is extensive

Bundle branches The bundle branches originate at the superior margin of

the muscular interventricular septum, immediately below the membranous septum, with the cells of the left bundle branch cascading downward as a continuous sheet onto the septum beneath the noncoronary aortic cusp. The right bundle branch continues intramyocardially as an unbranched extension of the AV bundle down the right side of the interventricular septum to the apex of the right ventricle and base of the anterior papillary muscle. The anatomy of the left bundle branch system may be variable and may not conform to a constant bifascicular division. However, for clinical purposes and electrocardiography (ECG), the concept of a trifascicular system remains useful

Terminal Purkinje fibers

The terminal Purkinje fibers connect with the ends of the bundle branches to form interweaving networks on the endocardial surface of both ventricles, which transmit the cardiac impulse almost simultaneously to the entire right and left ventricular endocardium. Purkinje fibers tend to be less concentrated at the base of the ventricle and the papillary muscle tips. They penetrate only the inner third of the endocardium. Purkinje fibers appear to be more resistant to ischemia than ordinary myocardial fibers.

Blood supply of conduction system

Myocardial cells have several different electrophysiologic properties: 

1. Automaticity - refers to the ability to spontaneously generate an electrical impulse.

2. Excitability - means that the cardiac cells have the ability to respond to an electrical impulse.

3. Conductivity - allows for transmission of the electrical impulse to another cardiac cell.

4. Contractility - refers to the ability to contract after an electrical impulse is received.

5. Rhythmicity - is the cell's ability to send electrical impulses in a regularly and evenly paced manner., and

Refractoriness - refers to the cell's inability to respond to another electrical impulse

Physiology

Myocardial cells contain actin and myosin filaments and contracts by means of the sliding filament mechanism. Myocardial cells are joined by gap junctions, due to which electrical impulses can spread to all cells in the mass.

Av node crossing fibrous tissue

SA node Represented on the ECG as P wave

AV node conduction is represented on the ECG as the PR Interval

The Bundle Branch and purkinje fibre depolarisation constitutes ventricular depolarisation Represented on the ECG as the QRS

Atrial repolarisation occurs within the QRS & therefore is maskedVentricular repolarisation is represented on the ECG as a T wave

Ecg recordings of counducting system

ASD

Conduction system in ASD1. RBBB( rt vent vol overload)2. Prolonged PR interval (atrial enlargement

leading to increassed internodal conduction distance)

3. Crochetage (M shaped) in inferior ECG leads

4. ostium primum a - left axis deviation. ostium secundum - right axis deviation.

SURGICAL IMPLICATIONS IN ASD

The relationship of the defect to the ostium of the coronary sinus, membranous portion of the AV septum, and commissural area between the septal and anterior tricuspid leaflets is studied because these features serve as guides to the location of the AV node and penetrating portion of the bundle of His

To avoid damaging the AV node,the sutures must not be placed too far from the edge anteriorly.

Coronary sinus ASD

Because coronary sinus ASDs are close to the AV node, stitches must be placed near the edge of the defect superiorly in tissue that may not be strong. For these reasons, patch closure is generally advisable.

Warden operation-sv ASD1. When the right superior pulmonary veins

enter more cephalad in the SVC (more than about 2 cm above the cavoatrial junction)

2. when the SVC is small (as in the presence

of bilateral SVCs)

To avoid sinus node dysfunction with incisions across the cavoatrial junction

sinus venosus malformation with right upper and middle lobe pulmonary veins entering superior vena cava (SVC). A, Right upper and middle pulmonary veins entering SVC. Right atrial appendage is amputated. B, High SVC or innominate vein is cannulated. Dashed line indicates the transecting incision in SVC

C, Cephalad end of transected SVC is anastomosed to amputated right atrial appendage. For mobilization, azygos vein is divided. D, Small incision is made in right atrial wall. Lateral edge of SVC orifice is sutured to lower rim of subcaval atrial septal defect (ASD), or the pathway is completed with a pericardial or polytetrafluoroethylene patch. Cardiac end of transected SVC is closed. E, Right pulmonary vein blood now flows (arrows) across the roofed ASD into left atrium

1. Closure of ASDs in children has been shown to improve AV conduction, decrease AV nodal refractory periods, and improve sinus node function in most patients early postoperatively

2. Presumably this improvement results from reduction in RV and right atrial volume after ablation of the left-to-right shunt at the atrial level.

In patients over about age 40, almost half of those not in atrial fibrillation preoperatively develop it late postoperatively. This tendency to atrial fibrillation or flutter late postoperatively may be less when venous cannulation is directly into the venae cavae rather than through the right atrial appendage

prevalence of changed heart rhythms after repair is similar in patients with fossa ovalis ASDs and those with sinus venosus malformation

sick sinus syndrome or junctional rhythm late postoperatively are seen in patients where incision is made across the cavoatrial junction for SV type ASD

Types of VSDs

1. Truex described the location of the specialized conduction tissue in hearts with VSD.

2. In a more detailed study, Lev expanded on this topic

3. Kirklin and DuShane developed a surgical technique that avoided producing heart block during VSD repair

Relation with conduction system

Perimembranous Ventricular Septal Defect

the AV node and penetrating portion of the bundle of His are in their normal position in hearts with perimembranous VSDs.

As the bundle penetrates the fibrous right trigone of the central fibrous body at the base of the noncoronary cusp of the aortic valve, it lies along the posteroinferior border of perimembranous and inlet-type VSDs. As the bundle continues along the inferior border of the VSD (at times slightly to the left or right of the free edge),the left bundle branch fascicles emerge from the branching portion. Only the right bundle branch remains when the bundle reaches the level of the muscle of Lancisi.

VSD The perimembranous VSD is intimately

associated with the bundle of His which in a d-loop heart passes through the tricuspid annulus at the posterior and inferior corner of the VSD.

The bundle soon branches into the right and left bundle branch

Repair of perimembranous ventricular septal defect (VSD) from right atrium, continuous suture technique. Right atriotomy is parallel to atrioventricular groove from right atrial appendage toward inferior vena cava. Stay sutures are placed to expose tricuspid orifice. Superior edge of VSD is not visible because of overlying anterior leaflet of tricuspid valve. Atrioventricular node lies within triangle of Koch,with bundle of His penetrating to ventricular septum at posterior angle of VSD, where it is particularly vulnerable to injury.

Bundle of His crosses beneath septal leaflet of tricuspid valve. Stitches must not penetrate tricuspid anulus or into atrial myocardium, to preserve integrity of conduction system

Via rt ventriclotomy At a point 5 to 7 mm below inferior margin of VSD, suture line is transitioned from septal leaflet onto ventricular septum. Stitching continues along inferior rim of VSD, with stitches placed 5 to 7 mm below rim until reaching muscle of Lancisi. An alternativetechnique uses interrupted pledgeted mattress sutures for very thin portions of the septal leaflet and posteroinferior edge of defect (conduction system) to facilitate secure suture placement while avoiding conduction system. Remainder of suture line employs a continuous suture technique.

Right Atrial versus Right Ventricular Approach

for Perimembranous Ventricular Septal Defect the RV approach may be used for repair of perimembranous VSDs and results in low hospital mortality even in patients with high Rp.

The RV approach has the advantage that the nadir of the noncoronary cusp of the aortic valve, which is the area of the right trigone and bundle of His, can be accurately visualized, which may be helpful in choosing the suture technique that will minimize prevalence of heart block

The RV approach has the disadvantages of (1) leaving a scar in the RV, (2) being associated with a higher prevalence of

complete RBBB than with an atrial approach,and (3) possibly resulting in more ventricular arrhythmias

late postoperatively.

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The right atrial approach may be used almost exclusively, a practice begun in about 1960 at the Mayo Clinic. An accurate repair can be obtained through a right atrial approach in nearly all cases. Associated infundibular pulmonary stenosis can be excised. An RV scar is avoided, and occurrence of RBBB is lower than with the transventricular approach.H11 With the right atrial approach, however, techniques must be accurate to avoid damaging the tricuspid valve , leaflets or chordae

CORRECTED TGA Since the right atrium must connect with the left ventricle

(i.e. atrioventricular discordance), it is not surprising that the conduction system is abnormal. Pioneering work in this area was undertaken by Anderson and colleagues.

In corrected transposition (C-TGA), the functional atrioventricular node arises anteriorly and superiorly and is usually lodged between the annulus of the mitral valve .

This functional AV node is therefore superior to the usual location of the AV node which may be present as an accessory node.

CORRECTED TGA

Often there is a posterior atrioventricular node in its usual position within the triangle of Koch, but it is usually disconnected from the remainder of the conduction tissue.

The conduction system in C-TGA is more tenuous than that of normal hearts. Fibrosis of the junction between the atrioventricular node and the atrioventricular bundle has been seen in older patients

Artrial switch operation- Post operative arrhythmia less compared to Mustard and Senning operation.

UNIVENTRICULAR HEART Categorised – Left or right – based on

morphological operative single ventricle

Single right ventricle- no conduction disturbances.

Single left ventricle- AV node is hypoplastic - Prolonged PR interval

culminating in complete heart block

Tricuspid Atresia Complete

absence of communication between the right atrium and right ventricle

About 3 % of congenital heart disease

Tricuspid atresia with / with out transposition

SA node is normal. Posterior small AV node originates in close

relation to Tendon of Todaro.

Occasionally, the branching bundle may be in close proximity to the posteroinferior rim

of the foramen and the right bundle-branch may lie subendocardially in the rim of the defect.

Surgical implication It is important, therefore, to appreciate that the atrioventricular node is in close

relation to the tendon of Todaro, which is a readily identifiable landmark during surgical exposure.

Closure of the foramen should usually be

accomplished safely provided that deep sutures are not placed in the

posteroinferior quadrant.

POST OP ARRHYTHMIAS

Usual type of arrythmias with Atrial surgery are SVT of which AF, Atrial flutter and junctional rhythms are most common.

In large ostium primum type defect because of posteriorly displaced AV node , it is frequently associated with prolonged AV conduction.

Small osteum secundum defect causes no problem during repair.

Hypothermia, ischemic arrest , direct injury to conduction system, haematoma, injury to SA nodal artery , oedema , forign body reaction to suture material all are responsible for arrhythmias

The most common conduction disturbance that occurs after ventricular surgery is RBB block .

RBBB can be due to direct injury to main RBB or right ventriculotomy ( in fallots tetrology surgery ) by disrupting the right ventricular subendocardial purkinje network

Clockwise from anterolateral commissure are the aortic cusps, conduction system, Atrioventricular septum, coronary sinus, and circumflex coronary artery, which are at risk when bites are taken too deeply

Mitral valve

Aortic valve and conduction system

Care must be taken while removing calcium from the region of the membranous septum and right trigone, Beneath the noncoronary cusp–right coronary cusp commissuret and rigorous avoidance of suture penetration of the membranous septum nearits junction with the muscular septum

Tricuspid valve and conduction system

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