His-bundle

Automa-ticity. Special cardiac cells, such as SA and AV nodal cells, His-bundle cells, and Purkinje fibers, spontaneously generate an impulse. This is the property of automaticity. Ectopic sites can act as pacemakers if the rate of phase 4 depolarization or resting membrane potential is increased, or the threshold for excitation is reduced. 

The quinolinone derivative, OPC-88117 (73), is yet another compound described as possessing both Class I and Class III electrophysiological activities. Studies in guinea-pig papillary muscle showed that OPC-88117 at 30 increased APDgo by about 15% and decreased F ,ax by only 4% however, at 100 /iM APDgo was prolonged by 23 % and was decreased by 23 % [206]. Further experiments in isolated rabbit hearts demonstrated that OPC-88117 increased atrio-His bundle (A-H) and His bundle-ventricle (H-V) conduction times and refractory periods with a profile that was similar to, but more potent than that of lidocaine [207]. 

The AV node lies directly above the insertion of the septal leaflet of the tricuspid valve and anterior to the ostium of the coronary sinus. It is part of the AV junction area, which is divided into three regions. The transitional cells, or nodal approaches, connect the atrial myocardium to the compact portion of the AV node. The slowest conduction time occurs within the AV node [8]. At its distal end, the compact portion of the AV node enters the central fibrous body, becoming the penetrating portion, or His-bundle [9]. 

The blood supply to the AV node is via the AV nodal artery, a braneh of the right eoronary artery in 90% of hearts, with the remaining 10% arising from the eireumflex artery [10], The main His bundle is typieally supplied by the AV nodal artery with a minor eontribution from septal perforators, but the bundle branehes have a blood supply that is more dependent on septal perforators. 

The antiarrhythmic drugs in class I suppress both normal Purkinje fiber and His bundle automaticity in addition to abnormal automaticity resulting from myocardial damage. Suppression of abnormal automaticity permits the sinoatrial node again to assume the role of the dominant pacemaker. 

The most important electrocardiographic change produced by verapamil is prolongation of the PR interval, a response consistent with the known effects of the drug on A-V nodal transmission. Verapamil has no effect on intraatrial and intraventricular conduction. The predominant electrophysiological effect is on A-V conduction proximal to the His bundle. 

PR prolongation QT prolongation QRS prolongation Increased anticholinergic and no-radernergic tone at the His bundle, leading to blockade of electrical impulses Bradycardia Palpitations Fainting Sudden death Monitor EKG parameters frequently Wilens et al., 1996... 

A quad-polar electrode catheter is inserted through the left femoral artery and positioned at the noncoronary cup of the aortic valve to record a His bundle electrogram. A bidirectional steerable monophasic action potential (MAP) recording/pacing combination catheter is inserted through the left femoral vein and positioned at the endocardium to obtain MAP signals. The signals are amplified with a DC amplifier. 

Scherlag BJ, Lau SH, Helfant RH, Berkowitz WD, Stein E, Damato AN. Catheter technique for recording His bundle activity in man. Circulation 1969 39(l) 13-8. 

A 57-year-old man with bipolar disorder taking olanzapine, lithium, and other drugs had underlying mitral valve prolapse, left ventricular hypertrophy, and His bundle anomalies he died suddenly, probably because of a cardiac dysrhythmia. 

It has long been believed that cardiac arrhythmias result from changes in the conducting properties and/or automaticity of the myocardium. Normally, each cardiac impulse arises in the sinoatrial node in the right atrium and then is rapidly transmitted throughout the atria and via the atrioventricular node, His bundle, bundle branches and specialized conduction fibers (Purkinje fibers) to all regions of the ventricles to assure coordinated activation and contraction. 

Conduction delay or block may occur in any area of the AV conduction system the AV node, the His bundle, or the bundle branches. AV block usually is categorized into three different types based on surface ECG... 

These drugs are cardiac calcium channel blockers. By decreasing calcium conduction in the calcium-dependent portions of the AV node, His bundle and in the myocardium itself, ectopic foci can be suppressed. 

Figure 24.7 shows an example of a His bundle recording. The top two traces are leads II and Vq of the EGG and the bottom trace is the voltage difference from two electrodes on the indwelling electrode catheter. This internal view of cardiac activation combined with the His bundle electrogram has been referred to as His bundle electrocardiography [20]. Atrial activation on the catheter recording is called the A deflection and ventricular activation called the V deflection. The His bundle potential is the central H deflection. Since the catheter is located very close to the His bundle and AV node, it is assumed that the A deflection arises from atrial muscle tissue close to the AV node. When combined with the surface lead information a number of new intervals can be obtained. These are the PA, AH, and HV intervals. The PA interval is a measure of atrial muscle activation time, the AH interval is a measure of AV nodal activation time, and the HV interval is a measure of the ventricular conduction system activation time. 

FIGURE 24.7 The top two traces are EGG leads II and V-2 and the bottom trace is abipolar catheter recording, properly positioned inside the heart, showing the His Bundle deflection (HB), and intracardiac atrial (A) and ventricular... 

Scherlag B.J., Samet R, and Helfant R.H. His bundle electrogram a critical appraisal of its uses and hmitations. Circulation 46 601-613,1972. 

Fig. 4. 96 Right anterior oblique (RAO) (panel A) and left anterior oblique (LAO) (panel B) sections of the male heart obtained from the EPFL s visible human surface server, EPFL 1998. Panel A shows the inferior caval vein (ICV), the inferior isthmus (CTI), the supraventricular crest (SVC), the aorta (Ao), and right ventricular outflow tract (RVOT). The white dot signals the site corresponding to the membranous septum or the maximal His-Bundle potential is usually recorded. In the LAO projection, the right atrial appendage (RAA) and the right and left atria at the level of the atrial ventricular junction s are depicted. The white dot also signals the area were the his bundle is recorded. The left atrial appendage (LAA) is superior, (from Farre J, Anderson RH, Cabrera JA, et al Fluorscopic cardiac anatomy for catheter ablation of tachycardia. PACE 25 88, 2002)...

Deshmukh PM, Romanyshyn M. Direct His-bundle pacing, present and future. PACE 2004 27 862. 

Fig. 10.1 Diagrammatic representation of the atrioventricular conduction system. The bundle branch system consists of a three-pronged system (two on the left side and one on the right side). At the bottom, the surface ECG is depicted simultaneously with a His bundle electrogram (HBE). The HBE is recorded near the septal leaflet of...

Fig. 10.5 Sinus rhythm with second-degree Type 13 2 iniranodal AV block, and RBBB. Note that the AH interval remains constant. TTie HV interval increases from 80 (following first P-wave) to 150ms (following second P-wave). The third P-wave is followed by an H deflection but no QRS complex. AV block occurs in the His-Purkinje system below the site of recording of the His bundle potential (arrow). Note the shorter PR interval after the nonconducted P-wave, a feature typical of Type I second-degree AV block. HBE = His bundle electrogram, A = atrial deflection, H = His bundle deflection, V = ventricular deflection, P = P-wave. TL = time lines 50ms. (Barold SS. Pacemaker treatment of bradycardias and selection of optimal pacing modes. In Zipes DP (Ed.). Contemporary Treatments in Cardiovascular Disease, 1997 1 123, with pamission.)...

Type II according to the strict definition occurs in the His-Purkinje system and rarely above the site of recording of the His bundle potential in the proximal His bundle or nodo-Hisian junction. Type II block has not yet been convincingly demonstrated in the N zone of the AV node (3). Most if not all the purported exceptions involve reports where type I blocks (shorter PR interval after the blocked beat) are claimed to be type II blocks by using loopholes in the definitions of second-degree AV block. Because type II invariably occurs in the His-Purkinje system, it should be a class I indication for pacing. 

---