Wandering pacemaker

Abstract: The sinoatrial node is the primary pacemaker of the heart. Nodal dysfunction with aging, heart failure, atrial fibrillation, and even endurance athletic training can lead to a wide variety of pathological clinical syndromes. Recent work utilizing molecular markers to map the extent of the node, along with the delineation of a novel paranodal area intermediate in characteristics between the node and the surrounding atrial muscle, has shown that pacemaker tissue is more widely spread in the right atrium than previously appreciated. This can explain the phenomenon of a "wandering pacemaker" and concomitant changes in the P-wave morphology. Extensive knowledge now exists regarding the molecular architecture of the node (in particular, the expression of ion channels) and how this relates to pacemaking. This review is an up-to-date summary of the current state of our appreciation of the above topics.

Abstract: Cardiac arrhythmias are probably more common in horses than in any other domestic animal species. The most frequent clinical complaint associated with cardiac arrhythmias is exercise intolerance. Physical examination is characterized by auscultation abnormalities such as fast or slow heart rate, irregular rhythm, extra sounds, long pauses, or abnormal heart sounds. The electrocardiogram is used to make a definitive diagnosis of the dysrhythmia. Other laboratory and cardiac function tests are employed to determine the etiology and to assess the significance of the arrhythmia. Antiarrhythmic therapy is given when clinical signs specifically related to the arrhythmia are present, when hemodynamic parameters are compromised by the arrhythmia, or when the ECG reveals abnormalities that put the patient at risk for development of more severe arrhythmias. The cardiovascular drugs most frequently used are digoxin and quinidine. Digoxin is most commonly used for supraventricular arrhythmias, especially arrhythmias characterized by fast heart rates. Quinidine is very effective for short-term treatment of ventricular and supraventricular arrhythmias but must be used with caution because of the potential for toxic side effects. The cardiac arrhythmias due to vagal tone (sinus arrhythmia, sinus block, sinus arrest, sinus bradycardia, wandering pacemaker, first-degree AV block, and second-degree AV block) that are found in resting horses are generally considered to be normal and generally do not require therapy.

Abstract: We have previously shown in rabbit that the pacemaker of the heart (the sinus node) is widespread and matches the wide distribution of the leading pacemaker site within the right atrium. There is, however, uncertainty about the precise location of the pacemaker in human heart, and its spatial relationships with the surrounding right atrial muscle. Therefore, the aim of the current study was to investigate the distribution of the sinus node tissue in a series of healthy human hearts and, for one of the hearts to construct a computer three-dimensional anatomical model of the sinus node, including the likely orientation of myocytes. A combination of experimental techniques--diffusion tensor magnetic resonance imaging (DT-MRI), histology, immunohistochemistry, image processing and computer modelling--was used. Our data show that the sinus node was larger than in previous studies and, most importantly, we identified a previously unknown area running alongside the sinus node (the "paranodal area"), which is even more extensive than the sinus node. This area possesses properties of both nodal and atrial tissues and may have a role in pacemaking. For example, it could explain the wide spread distribution of the leading pacemaker site in human right atrium, a phenomenon known as the wandering pacemaker observed in clinics. In summary, a novel 3D anatomical reconstruction presents a new picture of the distribution of nodal cells within the human right atrium.