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

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.

Retrospective analysis of Holter monitor databases.

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.

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

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.

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.