Effect of maternal smoking during pregnancy on passive respiratory mechanics in early infancy

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Abstract
We studied the effect of prenatal maternal cigarette smoking on passive expiratory mechanics in 53 healthy infants tested early in infancy (mean ± SD, 5.1 ± 1.5 weeks). Maternal smoking was measured by: 1) questionnaire reports of the number of cigarettes smoked per day; and 2) urine cotinine concentrations (corrected for creatinine) at each visit. Respiratory system mechanics were assessed by the single-breath occlusion-passive-flow-volume maneuver. In ten infants born to smoking mothers the time constant of the respiratory system was 23% reduced [0.34 vs. 0.44 s; 95% confidence interval (CI), −45% + 1%; P = 0.06]. This was related to an estimated 13% decrease in respiratory system compliance (4.86 vs. 5.62 Ml/cmH2O; 95%Cl, −33% + 6%; P = 0.18) and a 10% reduction in respiratory system resistance (0.073 vs. 0.081 cmH H2O/mL/s; 95%Cl, −42% + 22%; P = 0.56). Functional residual capacity (FRC), measured by helium-dilution, was also decreased by 13% (78 vs. 90 mL; 95%Cl, −27% + 0.3%; P = 0.06) in smoke-exposed infants. Forced expiratory flow rates at FRC obtained by thoraco-abdominal compression were reduced by 28% in infants of smoking mothers (VFRc, 99 vs. 138 rnlis; 95%Cl, −54% + 2%; P = 0.04), as reported previously in a larger sample from this population. This study was limited by small numbers of infants exposed to smoking during pregnancy and by ethnic imbalance among the smoking-exposed and unexposed groups. Nevertheless, it suggests that the diminished forced expiratory flows observed in infants exposed in utero to maternal tobacco smoking (UTS) are not attributable to increases in lung compliance, resistance, or a delay in passive lung emptying. Rather, the data support the hypothesis that UTS exposure may cause a reduction in airway size as well as alterations in the growth and/or maturation of passive mechanical properties of the respiratory system in healthy newborns.