Yeah, don't accept it.

The heart -"cardiac muscle"- secretes  hormones, has it's own nervous system, and bridges the divide between parasympath and sympath NS. (vastly oversimplified statement, but...still true)
therefore, the findings of heartmath institute may be a good place to start your research:

http://www.heartmath.org/research/research-overview/

Heart-Rate Variability and Autonomic Function:

IHM conducts ongoing research into heart-rate variability (HRV), a measure of the naturally occurring beat-to-beat changes in heart rate. HRV analysis is a powerful, noninvasive measure of autonomic nervous-system function and an indicator of neurocardiac fitness. The HeartMath Research Center maintains an extensive HRV normals database, which provides data on the HRV of healthy individuals. HeartMath has published research demonstrating how HRV varies with age and gender and on the use of HRV analyses to assess alterations in autonomic function in conditions such as panic disorder and chronic fatigue. IHM provides HRV analysis services for studies conducted by other institutions. For related Research Publications, go to Clinical and Health Research.

good luck!