RESUMEN
The purpose of this article is to review the physiologic 01 mechanisms causing heart rate variability. In addition, the n rationale for using low (LF) and high frequency (HF) components of heart rate variability to study cardiac sympa- tho-vagal balance is discussed based on these physiologic mechanisms. Heart rate variability is almost exclusively IJ mediated by the autonomic nervous system, as indicated r by the finding that heart rate is virtually constant in patients I following heart transplantation. Sympathetic actions on : the sinus nade are mediated by the intracellular second I messenger cAMPo Thus, sympathetic-mediated heart rate fluctuations require likewise fluctuations in intracellular cAMP concentration, which is dependent on the activities of adenylyl cyclase and phosphodiesterase. In contrast, parasympathetic modulation of sinus node function does not depend on intracellular second messengers and can...