Variability of resting respiratory drive and timing in healthy subjects

Studies of breathing pattern have focused primarily on changes in the mean values of the breathing pattern components, whereas there has been minimal investigation of breath-to-breath variability, which should provide information on the constancy with which respiration is controlled. In this study we examined the variability of breathing pattern both on a breath-to-breath and day-to-day basis by calculating the coefficient of variation (i.e., the standard deviation expressed as a percentage of the mean). By examining breath-to-breath data, we found that the coefficients of variation to tidal volume (VT) and fractional inspiratory time (TI/TT, an index of timing) obtained with an inductive plethysmograph and spirometer were within 1% of each other. Examination of breath-to-breath variability in breathing pattern over a 15-min period in 65 subjects revealed large coefficients of variation, indicating the need to base calculations on a relatively large number of breaths. Less breath-to-breath variability was observed in respiratory frequency [f, 20.8 ± 11.5% (SD)] and TI/TT (17.9 ± 6.5%) than in VT (33 ± 14.9%) and mean inspiratory flow (VT/TI, an index of drive; 31.6 ± 12.6%; P < 0.0001). Older subjects (60 - 81 yr) displayed greater breath-to-breath variability than young subjects (21 - 50 yr). Use of a mouthpiece did not affect the degree of variability. By examining day-to-day data, the average breathing pattern (mean of a 15-min recording) was quite reproducible: coefficients of variation for VT/TI and TI/TT of 8.5 ± 3.2 and 3.1 ± 2.4%, respectively; this variability was considerably less than that of the ventilatory response to hypercapnia (39.9 ± 9.8%; P < 0.005). In conclusion, the smaller variability in f and TI/TT compared with VT/TI suggests that the rhythm-generating function of the respiratory control system is more constant than the drive component of the system.