Conventional Methodologies

Currently, approx 10 million Americans are afflicted with cardiac arrhythmias (both ventricular and atrial) every year; nonetheless, only a small percentage of patients are expected to have electrophysiological mapping procedures. Yet, cardiac electrical mapping is considered as critical for understanding the patho-physiological mechanisms that underlie arrhythmias as well as the mechanisms of their initiation and sustenance. Further, cardiac mapping is commonly used for evaluating the effect of pharmacological therapies and for directing surgical or catheter ablation procedures; this occurs in the electrophysiology laboratory, as well as for experimental studies on arrhythmias.

More specifically, mapping of the endocardial activation and repolarization processes is critical for the selection of optimal therapeutic procedures. In particular, the mapping of endocar-dial potential distributions and their evolutions in time are required for precisely determining activation patterns, locating specific arrhythmogenic sites, and identifying areas of abnormal activity or slow conduction pathways.

Fig. 1. Image illustrating fluoroscopy's poor soft tissue contrast.

In short, the purpose of cardiac mapping is to characterize and localize the arrhythmogenic structure, and this can be accomplished by a variety of different methods. Cardiac mapping is a broad term that encompasses many applications, such as body surface mapping or epicardial mapping, as well as approaches that include activation maps or isopotential maps. There are fundamental similarities in all of these techniques.

Currently, the gold standard is the clinical electrophysi-ological study, which is primarily used to determine the source of cardiac arrhythmias and to support the management of treatment through pharmacological means or nonpharmacological interventions such as implantable pacemakers, implantable defibrillators, or radiofrequency ablation therapies. More specifically, this method is used to assess the timing and propagation of cardiac electrical activity involving the 12-lead ECG or recordings of electrical activation sequences termed extracellular electrograms, which are obtained using multiple intravascular electrode catheters positioned at various locations within the heart. The technique of catheter-based mapping not only permits better understanding of the underlying mechanisms of various arrhythmias, but also has served as the basis of most of the emerging concepts for treatment. Most important, these methodologies have allowed for widespread applications of ablative techniques in almost all known cardiac arrhythmias. Subsequently, the need for invasive arrhythmia surgery has significantly decreased as a result of these particular catheter-based endocardial mapping and ablation methodologies (14).

Nevertheless, the electrophysiological study is not without limitations. The electrophysiologist can only record electrical activity from the tip of the catheter, which must be in contact with the chamber wall. Such tip areas are relatively small in comparison to the heart's total surface area. Thus, to obtain adequate electrical activity for activation patterns, it often dictates the placement of multiple catheters at numerous locations within the chamber of interest, which in turn requires a considerable amount of time; this also leads to extensive use of fluo-roscopy, hence exposing the medical staff and patients to undesirable levels of ionizing radiation (15). Second, and perhaps more important, fluoroscopy does not sufficiently allow for the visualization of the complex 3D cardiac anatomy and/or soft tissue characteristics of the heart's chambers (Fig. 1).

As a direct result, the expedient and reproducible localization of sites of interest is often poor. More specifically, this inability to relate electrophysiological information precisely to a specific spatial location in the heart limits conventional techniques for employing radiofrequency ablation catheters for treatment of complex cardiac arrhythmias. Last, such techniques for mapping electrical potential activity from multiple sites do so sequentially over several cardiac cycles without accounting for likely beat-to-beat variability in activation patterns. Despite these known limitations, electrophysiologists still use these conventional techniques as the gold standard for validation purposes.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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