Diagnosing level of AV Block, PA interval, AH interval and HV interval

AV node is subendocardial structure measuring 1 X 3 X 5 mm, placed in right atrium, at apex of Koch’s triangle. Boundaries of Koch’s triangle are posterior boundary is by ostium of coronary sinus, anterior boundary is by septal tricuspid leaflet, and superior boundary is by tendon of Tadaro
AV bundle or His bundle which continues from compact AV node pierces central fibrous body in close proximity of aortic, Mitral and Tricuspid valve annulus. This close proximity of AV bundle is basis of injury of AV bundle from valve problems.
On ventricle side, AV bundle emerges close to membranous septum, Right bundle (RBB) is band which emerges from AV bundle traverses on right ventricle ( moderator band). Left bundle (LBB) is short and broad, divides into anterior and posterior fascicles.
AV junction
AV blocks
AV blocks at nodal level are benign, but Infra-hisian AV blocks need pacemaker implantation.
Escape rhythm from nodal block are narrow complex and stable but that of Infra-hisian are wide complex and unstable. If there was no pre-existing bundle branch block, then this finding i.e. narrow versus wide complex can be used to diagnose level of block

Vagal manures and Atropine
Owing differential nerve supply to nodal and infranodal tissue, response to vagal stimulation from carotid sinus massage and atropine varies. Nodal tissue is supplied by autonomic nerves, but infranodal tissue is sparsely supplied by autonomic nerves.
On vagal stimulation in AV block patients, ventricular rate decreases in nodal diseases, but that of infranodal diseases it may in fact increase.
Atropine administration will have opposite effect, increased ventricular rate with nodal diseases and decrease with infranodal diseases

This provides precise information about level of block. Catheter is placed in right atrium along the superior margin of tricuspid annulus and electrical activity is recorded. This records atrial, His bundle and also ventricular activity. Apart from intra-atrial catheter simultaneous surface ECG is also recorded.
Definition of intervals
·         PA interval is time taken from onset of P wave in surface ECG to onset deflection in atrial catheter place at His bundle. this is index of intra-arterial conduction normal less than 55milli secs
·         AH interval represents conduction through AV node, this is measured from rapid deflection on His bundle recording to His electrocardiogram. normal is less than 130milli secs
·         HV interval is this represents conduction through His-Purkinje system. This is measured from onset His electrocardiogram to onset QRS on Surface ECG. Normal is less than 50milli secs.
Intra-cardiac recording showing the long AH interval (210 milliseconds) and normal HV interval (50 milliseconds). I, III and V1: surface electrocardiogram leads. HBE: His bundle recording
·        Atrial pacing is done in incremental fashion to expose AV block. First degree heart block and Mobitz type one heart block are common but if these occur at cycle length more than 500 millisecs i.e. heart rate less than 120 beats per min, than it is pathological in absence of high vagal tone.
·        First degree AV block and Mobitz type one heart block is typically nodal disease, so there is prolongation of AH interval but HV interval is normal.
·        Block below nodal level manifest by prolongation of HV interval.
·        Prolongation of HV interval even in asymptomatic is associated with increased risk of high grade AV block. More so if HV interval is more than 100milli secs. Annul incidence of complete heart block (CHB) is 10%, so this is indication of pacing.
·        If there is CHB already or there is intermittent CHB than Electrophysiological (EP) study is little useful, wise to pace them

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