Cardiac Rate Control

Under normal physiological conditions, the dominant pacemaker of the heart is the sinoatrial node, which in adults fires at a rate of 60-100 beats per minute, faster than any other region. In a person at rest, modulation by the parasympathetic nervous system is dominant and slows the sinoatrial nodal rate to about 75 action potentials per minute (or beats per minute when contractions are elicited). In addition to the cells of the sinoatrial node, other conduction system cells, specifically those found in the specialized fibers in the atrioventricular junction and His-Purkinje system, are capable of developing spontaneous diastolic depolarization. Rhythms generated by impulse formation within these cells (ectopic pacemakers) range from 25-55 beats per minute in the human heart (Fig. 5). These rhythms are commonly referred to as ventricular escape rhythms. These rhythms are important for patient survival because they maintain some degree of cardiac output when the sinoatrial or atrioventricular nodes are nonfunctional or are functioning inappropriately. These various populations of pacemaker myocytes (sinoatrial and atrioventricular nodal) elicit so-called slow-type action potentials (slow-response action potential; see Section 5).

In addition to the normal sources of cardiac rhythms, myocardial tissue can exhibit abnormal self-excitability. Such a site is also called an ectopic pacemaker or ectopic focus. This pacemaker may operate only occasionally, producing extra beats, or it may induce a new cardiac rhythm for some period of time. Potentiators of ectopic activity include caffeine, nicotine, electrolyte imbalances, hypoxia, or toxic reactions to drugs such as digitalis. For more detail on rate control of the heart, refer to Chapter 10.

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