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Experimentally modified breathing pattern in human subjects, by varying the inspired gas mixture or administering different neuromodulators, has been studied extensively in the past, yet unmodified breathing has not. Moreover, most data refer to infants during sleep and adults during wakefulness. We studied the baseline breathing pattern of preterm infants [n = 10; GA 30 (27-34) wk (median, range)]; term infants [n = 10; GA 40 (39-41) wk)], and adult subjects [n = 10; age 31 (17-48) y)] during quiet sleep. A flow-through system was used to measure ventilation. We found: (i) instantaneous ventilation was 0.273 ± 0.006, 0.200 ± 0.003, and 0.135 ± 0.002 L · min[sup -1] · kg[sup -1] in preterm, term infants, and adult subjects; the coefficients of variation were 39%, 25%, and 14% (p < 0.01). The greater coefficient of variation in neonates compared to adults related to increased variability in Vt (39% and 25% in preterm and term infants vs 14% in adults; p < 0.01) and f (39% and 22% vs 9%; p < 0.01). The major determinant of frequency in preterm infants was Te (81% variability), Ti varying less (25% variability); (ii) V[sub T]/Ti decreased and Ti/Ttot increased with age; (iii) the higher breath-to-breath variability in preterm infants was associated with larger changes in alveolar PCO[sub 2] and a larger variability in O[sub 2] saturation than later in life. We conclude: (i) the high breath-to-breath variability in frequency in preterm infants closely relates to variation in Te; (ii) decreased effective inspiratory timing (Ti/Ttot) in preterm infants compared with adults likely reflects their high pulmonary impedance; and (iii) greater breath-to-breath variability in ventilation in neonates with large variations in alveolar PCO[sub 2] and O[sub 2] saturation remains when compared with values in the sleeping adult. We speculate that high variability in Te early in life represents an effort to maintain lung volume through increased post-inspiratory diaphragmatic activity and increased upper airway resistance in an attempt to avoid collapse due to poor chest wall recoil. [ABSTRACT FROM AUTHOR]