Respiratory impedance in healthy unsedated South African infants: effects of maternal smoking

Abstract

Background and objective: Non-invasive techniques for measuring lung mechanics in infants are needed for a better understanding of lung growth and function, and to study the effects of prenatal factors on subsequent lung growth in healthy infants. The forced oscillation technique requires minimal cooperation from the individual but has rarely been used in infants. The study aims to assess the use of the forced oscillation technique to measure the influence of antenatal exposures on respiratory mechanics in unsedated infants enrolled in a birth cohort study in Cape Town, South Africa.

Methods: Healthy term infants were studied at 6-10 weeks of age using the forced oscillation technique. Respiratory impedance was measured in the frequency range 8-48 Hz via a face mask during natural sleep. Respiratory system resistance, compliance and inertance were calculated from the impedance spectra.

Results: Of 177 infants tested, successful measurements were obtained in 164 (93%). Median (25-75%) values for resistance, compliance and inertance were 50.2 (39.5-60.6) cmH2 O.s.L(-1), 0.78 (0.61-0.99) mL.cmH2 O(-1) and 0.062 (0.050-0.086) cmH2 O.s(2) .L(-1), respectively. As a group, male infants had 16% higher resistance (P = 0.006) and 18% lower compliance (P = 0.02) than females. Infants whose mothers smoked during pregnancy had a 19% lower compliance than infants not exposed to tobacco smoke during pregnancy (P = 0.005). Neither maternal HIV infection nor ethnicity had a significant effect on respiratory mechanics.

Conclusions: The forced oscillation technique is sensitive enough to demonstrate the effects of tobacco smoke exposure and sex in respiratory mechanics in healthy infants. This technique will facilitate assessing perinatal influences of lung function in infancy.

Keywords: forced oscillation technique; respiratory compliance paediatric; respiratory function test; respiratory resistance.

Figure 1

Schematic representation of the forced oscillation equipment. A loudspeaker delivers the computer-generated multicomponent forcing function. Pressure is measured at each end of a wave-tube (P1 and P2) for the estimation of respiratory impedance, and a pneumotachograph is used to monitor tidal airflow (V'). A bias flow reduces the influence of equipment dead-space on the infant's breathing pattern.

Figure 2

Resistance (R) and compliance (C) by sex, ethnicity, maternal smoke exposure and maternal HIV. Data are shown as median, 25–75% and 95% confidence intervals.