The pNNx Heart Rate Variability Statistics: An Application to Neuroautonomic Dysfunction of Clozapine-Treated Subjects

OBJECTIVE: The percentage of successive normal cardiac interbeat intervals greater than 50 msec (pNN50) is a widely used heart rate variability measure, which is useful in identifying the neuroautonomic dysfunction of psychiatric disorders. However, pNN50 is only one member of a larger family of pNNx statistics, where x is greater than 0 msec. The potential application of the general pNNx statistics has not yet been explored in the psychiatric field. The authors examined the pNNx statistics in clozapine-treated subjects and normal controls to evaluate the usefulness of the general pNNx statistics. METHODS: Sixty-one schizophrenic patients treated with clozapine and fifty-nine normal controls were evaluated. Probability values for the differences between the groups at each pNN value (range: pNN1-pNN100) were calculated using data obtained from a 30-minute electrocardiogram. RESULTS: The conventional pNN50 and pNNx values with x<50 msec were all significantly lower in the patient group (p<0.05). The distinction between the two groups was more prominent at pNN values less than 50 msec than that observed at pNN50. The maximum separation between groups occurred at pNN5 (68.2+/-19.1 vs. 22.5+/-20.5, p<10(-22)). CONCLUSION: The pNNx with x<50 msec provided more robust discrimination between the groups than the conventional pNN50, suggesting the importance of analyzing very small variations of interbeat interval in discriminating normal and pathological heart rate patterns. The results also suggest that the general pNNx statistics may be applied and useful in evaluating the neuroautonomic dysfunction in patients treated with clozapine, complementing the traditionally computed pNN50 value.