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

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.