Comparison of spontaneous vs. metronome-guided breathing on assessment of vagal modulation using RR variability

R-R interval variability (RR variability) is increasingly being used as an index of autonomic activity. High-frequency (HF) power reflects vagal modul ation of the sinus node. Since vagal modulation occurs at the respiratory f requency, some investigators have suggested that HF power cannot be interpr eted unless the breathing rate is controlled. We hypothesized that HF power during spontaneous breathing would not differ significantly from HF power during metronome-guided breathing. We measured HF power during spontaneous breathing in 20 healthy subjects and 19 patients with heart disease. Each s ubject's spontaneous breathing rate was determined, and the calculation of HF power was repeated with a metronome set to his or her average spontaneou s breathing rate. There was no significant difference between the logarithm of HF power measured during spontaneous and metronome-guided breathing [4. 88 +/- 0.29 vs. 5.29 +/- 0.30 1n( ms(2)), P = 0.32] in the group as a whole and when patients and healthy subjects were examined separately. We did ob serve a small (9.9%) decrease in HF power with increasing metronome-guided breathing rates (from 9 to 20 breaths/min). These data indicate that HF pow er during spontaneous and metronome-guided breathing differs at most by ver y small amounts. This variability is several logarithmic units less than th e wide discrepancies observed between healthy subjects and cardiac patients with a heterogeneous group of cardiovascular disorders. In addition, HF po wer is relatively constant across the range of typical breathing rates. The se data indicate that there is no need to control breathing rate to interpr et HF power when RR variability (and specifically HF power) is used to iden tify high-risk cardiac patients.