RT Journal Article SR Electronic T1 Heart rate variability: why do spectral analysis? JF Heart JO Heart FD BMJ Publishing Group Ltd and British Cardiovascular Society SP 99 OP 101 DO 10.1136/hrt.77.2.99 VO 77 IS 2 A1 Karemaker, J. M. YR 1997 UL http://heart.bmj.com/content/77/2/99.abstract AB