VARIABILITY OF RESPIRATORY FUNCTIONS BASED ON CIRCADIAN CYCLES

: No systematic study of circadian cycles of lung functions has been reported in the literature in which environmental influences were rigidly controlled. In this study, vital capacity, inspiratory capacity, expiratory reserve volume, maximum expiratory flow rate and maximum inspiratory flow rate were measured four times daily at four-hour intervals in two subjects during a control period, during nine days of isolation in a constant environment, and during a three-day recovery period. Temperature was kept at 27C plus or minus 0.1 degree, barometric pressure 30.560 plus or minus .004 inches. All the lung functions measured showed circadian cycles which shifted during the isolation period in the same direction as sleep-wakefulness cycles, but at a somewhat different rate. Periodicities were determined with computer analysis using a cross-correlation technique with a synthesized 24-hour sinusoid. Average daily variability of lung functions based on circadian cycles ranged from 5.6 plus or minus 1.7% for vital capacity to 20.3 plus or minus 10.4% for maximum inspiratory flow rate.