Heart 88:378-380 doi:10.1136/heart.88.4.378
  • Cardiovascular medicine

The pNNx files: re-examining a widely used heart rate variability measure

  1. J E Mietus1,
  2. C-K Peng1,
  3. I Henry1,
  4. R L Goldsmith2,
  5. A L Goldberger1
  1. 1Margret and H A Rey Institute for Nonlinear Dynamics in Physiology and Medicine, Cardiovascular Division, Harvard Medical School/Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA
  2. 2Division of Circulatory Physiology, Columbia-Presbyterian Medical Center, New York, New York, USA
  1. Correspondence to:
    Dr A L Goldberger, Cardiovascular Division, GZ-435, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215, USA;
  • Accepted 21 May 2002


Objective: To re-examine the standard pNN50 heart rate variability (HRV) statistic by determining how other thresholds compare with the commonly adopted 50 ms threshold in distinguishing physiological and pathological groups.

Design: Retrospective analysis of Holter monitor databases.

Subjects: Comparison of HRV data between 72 healthy subjects and 43 with congestive heart failure (CHF); between sleeping and waking states in the 72 healthy subjects; and between 20 young and 20 healthy elderly subjects.

Main outcome measures: Probability values for discriminating between groups using a family of pNN values ranging from pNN4 to pNN100.

Results: For all three comparisons, pNN values substantially less than 50 ms consistently provided better separation between groups. For the normal versus CHF groups, p < 10−13 for pNN12 versus p < 10−4 for pNN50; for the sleeping versus awake groups, p < 10−21 for pNN12 versus p < 10−10 for pNN50; and for the young versus elderly groups, p < 10−6 for pNN28 versus p < 10−4 for pNN50. In addition, for the subgroups of elderly healthy subjects versus younger patients with CHF, p < 0.007 for pNN20 versus p < 0.17 for pNN50; and for the subgroup of New York Heart Association functional class I–II CHF versus class III–IV, p < 0.04 for pNN10 versus p < 0.13 for pNN50.

Conclusions: pNN50 is only one member of a general pNNx family of HRV statistics. Enhanced discrimination between a variety of normal and pathological conditions is obtained by using pNN thresholds as low as 20 ms or less rather than the standard 50 ms threshold.