Chapter 12: Electrical Activity of the Heart & Cardiac Rhythm

Electrical Activity of the Heart & Cardiac Rhythm details the ionic mechanisms underlying cardiac action potentials, contrasting the fast-response potentials of ventricular muscle—distinguished by a prolonged plateau phase mediated by L-type calcium channels—with the slow-response, automatic potentials of the sinoatrial and atrioventricular nodes. The text explores the phenomenon of automaticity, driven by the funny current and potassium conductance changes, and traces the orderly conduction of electrical impulses through the specialized conduction system to ensure efficient pumping. A significant portion is dedicated to the clinical application of the electrocardiogram (ECG), explaining how cardiac dipoles generate characteristic waveforms (P wave, QRS complex, T wave) and intervals. It further describes the standard twelve-lead system, vector analysis for determining the mean electrical axis, and the diagnostic criteria for atrial and ventricular hypertrophy. The discussion extends to the pathophysiology of arrhythmias, covering mechanisms like reentry circuits, ectopic foci, and triggered activity, as well as the impact of electrolyte disturbances such as hyperkalemia and hypokalemia. Finally, the summary categorizes antiarrhythmic drugs into classes based on their electrophysiological effects on sodium, potassium, and calcium channels, providing a framework for understanding the medical management of rhythm disorders.