Atrioventricular conduction system

Sites of endothelin-receptor expression. ETA receptors are expressed in the smooth muscle cells of the vascular medial layer and the airways, in cardiac myocytes, lung parenchyma, bronchiolar epithelial cells and prostate epithelial cells. ETB receptors are expressed in endothelial cells, in bronchiolar smooth muscle cells, vascular smooth muscle cells of certain vessels (e.g. saphenous vein, internal mammary artety), in the renal proximal and distal tubule, the renal collecting duct and in the cells of the atrioventricular conducting system. 

Elnatan J, Molenaar P, Rosenfeldt FL, Summers RJ. Autoradiographic localization and quantitation of pr and p2-adrenoceptors in the human atrioventricular conducting system a comparison of patients with idiopathic dilated cardiomyopathy and ischemic heart disease. J Mol Cell Cardiol 1994 26 313-323. 

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...

Adenosine is produced by many tissues, mainly as a byproduct of ATP breakdown. It is released from neurons, glia and other cells, possibly through the operation of the membrane transport system. Its rate of production varies with the functional state of the tissue and it may play a role as an autocrine or paracrine mediator (e.g. controlling blood flow). The uptake of adenosine is blocked by dipyridamole, which has vasodilatory effects. The effects of adenosine are mediated by a group of G protein-coupled receptors (the Gi/o-coupled Ai- and A3 receptors, and the Gs-coupled A2a-/A2B receptors). Ai receptors can mediate vasoconstriction, block of cardiac atrioventricular conduction and reduction of force of contraction, bronchoconstriction, and inhibition of neurotransmitter release. A2 receptors mediate vasodilatation and are involved in the stimulation of nociceptive afferent neurons. A3 receptors mediate the release of mediators from mast cells. Methylxanthines (e.g. caffeine) function as antagonists of Ai and A2 receptors. Adenosine itself is used to terminate supraventricular tachycardia by intravenous bolus injection. 

FIGURE 6-1. The cardiac conduction system. AV, atrioventricular. (Reprinted with permission from Cummins RO, (ed.) ACLS Provider Manual. Dallas American Heart Association 2003 253.)... 

AF is characterized as an extremely rapid (400 to 600 atrial beats/min) and disorganized atrial activation. There is a loss of atrial contraction (atrial kick), and supraventricular impulses penetrate the atrioventricular (AV) conduction system in variable degrees, resulting in irregular ventricular activation and irregularly irregular pulse (120 to 180 beats/min). 

Their efficacy in many illnesses is explained by the competitive binding of )3-adrenore-ceptors in the autonomic nervous system by basically any of the employed drags of the l-aryloxy-3-aminopropanol-2 class, which result in reduction of heart rate and strength of cardiac beats, slowing of atrioventricular conductivity, reduction of the level of renin in the plasma, and reduction of blood pressure. The main effects of 8-adrenoblockers are expressed at the level of the vasomotor center in the hypothalamus, which result in a slowing of the release of sympathetic, tonic impulses. Included in the main group of... 

Drugs of this group are calcium channel blockers that inhibit slow transmembrane calcium ion flow in the cell of the conductive system of the heart during depolarization, which causes a slowing of atrioventricular conductivity and increased effective refractive period of atrioventricular ganglia, which eventually leads to the relaxation of smooth muscle of heart musculature and restores normal sinus rhythm during supraventricular tachycardias. 

Historically and romantically, the heartbeat is recognized as the quintessential hallmark of life. Normally, the heart beats at 60-100 beats per minute (bpm), with each beat yielding a ventricular contraction that ejects blood out to the body. Each heartbeat is an electrical event that originates from a collection of electrically excitable cells within the heart called the sinoatrial node (SA), anatomically located at the upper pole of the heart. The sinoatrial node is the primary pacemaker of the heart. The electrical impulse generated in the sinoatrial node spreads rapidly downward from the atria chambers of the heart and reaches the atrioventricular node (AV), a collection of electrically excitable cells that constitutes the electrical interface between the atria and ventricles of the heart. Erom the AV node, the impulse propagates throughout the ventricles via an electrical conduction system referred to as the His-Purkinje system. The electrical transmission... 

Oosthoek P, Viragh S, Lamers WH, Moorman AFM Immunohistochemical delineation of the conduction system. I. The atrioventricular node and Purkinje fibers. Circ Res 1993a 73 482-491. 

FIGURE 23-2 Schematic representation of the conduction system of the heart. Conduction normally follows the pathways indicated by the dashed lines. Impulses originate in the sinoatrial node and are transmitted to the atrioventricular node. Impulses are then conducted from the atrioventricular node to the ventricles by the bundle of His and bundle branches. 

Although cardiac arrhythmias involve the electrical conduction system of the heart including the sinus node, atrioventricular... 

Septal ablation related mortality at experienced centers is currently 1% to 2%, similar to that of surgical myectomy (Table 4). Conduction system abnormalities are relatively common complications of septal ablation, Permanent right bundle branch block occurs in about 50% of patients and transitory complete heart block in 60% and permanent pacemakers required for high grade atrioventricular block in about 5% to 20%, Concerns of late occurrence of complete heart block following septal ablation mandates in-patient monitoring for 4 to 5 days,... 

In a healthy heart, the primary rhythm is generated by the sinuatrial node. If the latter is damaged or disconnected from the subsequent parts of conducting system, lower centers such as the atrioventricular node or the bundle of His can take over and supply a somewhat slower rhtythm. 

Figure 5.8. The conduction system of the heart, a Anatomy, b Electrical rhythm in the sinoatrial node (top), atrioventricular node (center), and the heart muscle (bottom). The dotted line inb (center) represents the own rhythm of the AV node that normally gets overridden by the faster sinoatrial rhythm (solid line).

Like all caldum channel blodcers, verapamil modulates ionic calcium influx across cell membranes of conductile and contractile myocardial cells, as well as arterial smooth muscle. The modulation of calcium influx slows atrioventricular conduction, reduces myocardial contractility and systemic vascular resistance, and results in coronary and peripheral vaso dilation. Verapamil is currently indicated for controlling angina, hypertension, paroxysmal supraventricular tachycardia, and rapid ventricular atrial flutter or fibrillation (1-5),... 

Atrioventricular node and conduction system Pi Increased conduction velocity and automaticity... 

Calcium channel blockers differ in their effects on the myocardial conduction system. Both verapamil and diltiazem have significant inhibitory effects on both sinoatrial and atrioventricular nodal function, whereas nifedipine has little or no effect. Nevertheless, nifedipine can on occasion cause troublesome bradydysrhythmias (44,45). 

Carbamazepine can depress the cardiac conducting system (SEDA-16, 71). There have been a few reports of reversible atrioventricular block (SED-12,130) (2-4), and asystole has been described in a patient with GuiUam-Barre syndrome (SEDA-18, 62). 

The systemic hemodynamic actions of nicorandil are occasionally associated with a transient increase in heart rate of up to 18% (9,10). Larger doses are associated with cardiac depression, a dose-dependent fall in sinus rate and atrioventricular conduction velocity (11), and shortening of the cardiac action potential duration (12). However, no prodysrhythmic effects have been observed in man (13,14). Single oral doses over 40 mg have been associated with severe postural hypotension, dizziness, and syncope (10,15). 

From the SA node, electrical activity moves in a wavefront through an atrial specialized conducting system and evenmaUy gains entrance to the ventricle via an atrioventricular (AV) node and a large... 

The basic rhythm of heart rate is maintained by a part of the heart known as the pacemaker or sinoatrial (SA) node. This is a group of cardiac muscle cells in the right atrium that depolarize spontaneously. Depolarization of cardiac muscle cells brings about contraction. The pacemaker forms part of a conduction system that transmits electrical activity through the heart by means of action potentials so that contractions are coordinated and the heart can function as an efficient pump. The conduction pathway of electrical activity goes from the SA node to the atrioventricular (AV) node between atria and ventricles, then down the septum between the two ventricles to the apex of the heart and finally round the ventricles themselves. The conduction pathway in the heart is shown in Figure 4.2. 

Flecainide, one of a classic membrane-stabilizing group of antiarrhythmic agents, decreases intracardiac conduction in all parts of the heart with the greatest effects being noted in the His-Purkinje system. Effects upon atrioventricular (AV) nodal conduction time and intra-atrial conduction time, although present, are less pronounced than those on the ventricular conduction system. 

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