Breathing variability during wakefulness and NREM sleep in humans

Introduction: Breath-to-breath variability in humans during wakefulness and NREM sleep has been studied using flow measurements obtained while breathing through a mask connected to a pneumotachograph or using volume measurements obtained by breathing unencumbered with a respiratory inductance plethysmograph (respitrace) in place. Varying results among previous studies may be explained by instrumentation-induced changes in breathing pattern, respitrace measurement inaccuracies due to posture changes during sleep, or short recording periods. Purpose: We examined ventilation changes and breathing pattern variability in normal humans during wakefulness, stage II, and stage III-IV sleep without instrumentation using a specially constructed recumbent body plethysmograph. Our recording period during both wakefulness and NREM sleep (30 minutes each) was also extended beyond that reported in previous studies. Methods: Four subjects (mean age 29 yrs, range 18-35) underwent an overnight sleep study after a previous night of sleep deprivation. Breath-by-breath measurements of tidal volume (V_T), total respiratory cycle duration (T_TOT), inspiratory (T_I) and expiratory (T_E) duration, mean inspiratory flow (V_T/T_I), and minute ventilation (V_E) were obtained from our LabVIEW-based computerized acquisition-analysis system. We calculated mean values of V_T, T_TOT, T_I, T_E, and V_T/TI for wakefulness and sleep stages II and III-IV. We examined breath-to-breath variability by calculating the coefficient of variation. Results: During NREM sleep, time spent in stage II was 40% and in stage III-IV was 60%. The mean VT was 448±75cc during wakefulness, 408±65cc during stage II, and 400±85cc during stage HI-IV; mean coefficient of variation for V_T was 29%, 34%, and 16%, respectively. The mean T_TOT was 3.99±0.28sec during wakefulness compared with 4.39±0.17sec during stage II and 4.26±0.37sec during stage III-IV. The mean coefficient of variation for T_TOT was 21%, 13%, and 7% respectively. During wakefulness, mean V_E was 6840±1148cc, which decreased to 5593±635cc during stage II and to 5878±699cc during stage III-IV. No other systematic changes in other variables (T_I, T_E, T_I/T_TOT, and V_T/T_I) were noted. Conclusions: We conclude that V_T and V_E fall and T_TOT increases (i.e., frequency decreases) during NREM sleep compared with wakefulness. There is a greater variability in V_T than in T_TOT during both wakefulness and sleep. Variability in oreathing during NREM sleep is greater during stage II than stage III-IV. Clinical Implications: We have described a new non-invasive device to more accurately measure breathing during sleep.