Page 4, Cardiac Performance, Dr. D. Penney



Heart Rate Control (continued..)

Chronotropicity is altered by the release of several neurotransmitter hormones. Acetylcholine from parasympathetic endings on the SA and AV nodes decrease heart rate and slow AV conduction , whereas norepinephrine from sympathetic endings also on the SA and AV nodes and epinephrine and norepinephrine from the adrenal medulla, increase heart rate and speed AV conduction. The adrenal medullary catecholamines reach the heart via the blood. SA and/or AV nodal depression caused by parasympathetic (vagal) stimulation (Figure 2.01), acetylcholine administration, or direct electrical stimulation can be sufficiently powerful to cause cardiac standstill; however, ventricular or bundle branch pacemakers firing at a slower rate permit vagal escape within a few seconds, thus preventing death.



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