A Developmental Study of the Undisturbed Breathing Pattern in Human Subjects♦ 1595

Experimentally disturbed breathing pattern in human subjects has been studied extensively in the past, yet undisturbed breathing has not. Also, most data refer to infants during sleep and adults during wakefulness. We studied the undisturbed breathing pattern of preterm infants [n=6; BW 1.5±0.3 kg (Mean±SE); SW 1.5±0.3 kg; GA 31±1 wk; PNA 16±2 d]; term infants [n=6; BW 3.5±0.2 kg; SW 3.7±0.3 kg; GA 40±1 wk; PNA 15±4 d], and adult subjects [n=6; weight 60±4 kg; age 31±5 yr] during quiet sleep. A flow-through system was used to measure breathing pattern. We found: 1) instantaneous ventilation was 0.280±0.06, 0.182±0.002, and 0.226±0.003 L/min/kg in preterm, term infants, and adults; the coefficients of variation were 43%, 26% and 19% (p<0.01). The greater coefficient of variation in neonates compared to adults related to increased variability in Vt (39% and 33% in preterm and term vs 15% in adults; p<0.01) and f (42% and 32% vs 16%; p<0.01). The major determinant of frequency in preterm infants was Te (86% variability), Ti remaining relatively fixed (25% variability); 2) Vt/Ti (14±1 and 13±1 ml/s in preterm and term) and Ti/Ttot (0.37±0.01 and 0.37±0.01) were lower in neonates than in adults (28±1 ml/s and 0.51±0.01 respectively, both p<0.01); 3) the high breath-to-breath variability in preterm infants was paralleled by larger changes in alveolar PCO₂ (1.5±0.3 Torr) and a larger coefficient of variation in oxygen saturation (3%) as compared to that in term infants (1.1±0.2 and 2%, both p<0.01) and in adult subjects (0.8±0.6 and 1%, both p<0.01). We conclude: 1) high breath-to-breath variability in frequency in preterm infants closely relates to variation in Te; 2) the lower inspiratory flow and smaller tidal volumes in neonates than in adults likely reflect their high pulmonary impedance; and 3) greater breath-to-breath variability in ventilation in neonates parallels large oscillations in alveolar PCO₂ and O₂ saturation, reflecting a highly unstable chemical control of breathing. We speculate that vagal control, upper airway tone and oxygenation(pulmonary shunt) mediate changes in Te which ultimately determines developmental changes in breathing pattern.