Cardiac Electrophysiology and Arrhythmias

• Treatment of patients with symptoms referable to an arrhythmia is possible for many physicians.

• Accurate diagnosis of the arrhythmic event may prove difficult, and additional diagnostic tests not readily available to the family physician may be needed.

• More conscientious use of antiarrhythmic drugs is necessary as their proarrhythmic effects become known.

• Implantable devices continue to evolve and promise to alter the natural history of many malignant cardiac disease states.

The accurate understanding and interpretation of electrocardiograms (ECGs), rhythm strips, and unusual cardiac beats is a rewarding practice. Understanding is based not only on recognition of patterns but also on knowledge of electrical activation and repolarization of individual cells alone and in the aggregate. Anticipation of what should be happening will aid in determining what is happening during any particular beat. Appreciation of the history of electro-physiology and arrhythmology further aids in rhythm management.

Normal cardiac cellular actions may include automaticity, rhythmicity, conductivity, and contractility. Specialized cardiac cells may perform one of these functions better than a prototypical myocyte, thus facilitating organ function. Clinical arrhythmias result from disorders of impulse formation, abnormal impulse conduction, or a combination of these events (Akhtar et al., 1988; Zipes and Jalife, 1990).

The correct action of all cardiac cells depends on a normally functioning cell membrane or sarcolemma. As with central and peripheral neurons, cardiac cells have bilayer cell membranes composed of phospholipid molecules with specialized channels or pores that function as a semipermeable membrane to a variety of molecules. Sodium (Na+), potassium (K+), calcium (Ca++), chloride (Cl-), and other ions move across the cell membrane in an organized fashion, resulting in depolarization of the cell from a resting electronegative state. Specialized structures along both the long axis and the short axis of cardiac cells facilitate in the coupling of mechanical and electrical action (Hoyt et al., 1989).

Two specialized electrical cell types with different permeability characteristics for Na+ and Ca++ give rise to specialized action. Slowly depolarizing and conducting calcium-dependent cells are more abundant in the sinus node and the atrioventricular junctional area. Rapidly depolarizing, fast-conducting sodium-dependent cells are more widespread and include atrial and ventricular myocytes and specialized His-Purkinje fibers, as well as abnormal cardiac structures such as bypass tracts, as discussed later. Medications, ischemia, injury, fibrosis, and external stimulation affect these cells differently, allowing for treatment and diagnosis of a variety of arrhythmias.

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