VOLUNTARY CONTROL OF BREATHING DOES NOT ALTER VAGAL MODULATION OF HEART-RATE

Variations in respiratory pattern influence the heart rate spectrum. I t has been suggested, hence, that metronomic respiration should be use d to correctly assess vagal modulation of heart rate by using spectral analysis. On the other hand, breathing to a metronome has been report ed to increase heart rate spectral power in the high- or respiratory f requency region; this finding has led to the suggestion that metronomi c respiration enhances vagal tone or alters vagal modulation of heart rate. To investigate whether metronomic breathing complicates the inte rpretation of heart rate spectra by altering vagal modulation, we reco rded the electrocardiogram and respiration from eight volunteers durin g three breathing trials of 10 min each: 1) spontaneous breathing (mea n rate of 14.4 breaths/min); 2) breathing to a metronome at the rate o f 15, 18, and 21 breaths/min for 2, 6, and 2 min, respectively; and 3) breathing to a metronome at the rate of 18 breaths/min for 10 min. Da ta were also collected from eight volunteers who breathed spontaneousl y for 20 min and breathed metronomically at each subject's mean sponta neous breathing frequency for 20 min. Results from the three 10-min br eathing trials showed that heart rate power in the respiratory frequen cy region was smaller during metronomic breathing than during spontane ous breathing. This decrease could be explained fully by the higher br eathing frequencies used during trials 2 and 3 of metronomic breathing . When the subjects breathed metronomically at each subject's mean bre athing frequency, the heart rate powers during metronomic breathing we re similar to those during spontaneous breathing. Our results suggest that vagal modulation of heart rate is not altered and vagal tone is n ot enhanced during metronomic breathing.