Aim: To determine any variation in the respiratory responses to hypoxia/hypercapnia of infants born small for gestational age (SGA) to smoking and to non-smoking mothers.
Methods: A total of 70 average for gestational age (AGA) infants (>36 weeks gestation, >2500 g, >25th centile for gestational age, and no maternal smoking), and 47 SGA infants (<10th centile for gestational age) were studied at 1 and 3 months of age, in quiet and active sleep. Respiratory test gases were delivered through a Perspex hood to simulate face down rebreathing by slowly allowing the inspired air to be altered to a CO2 maximum of 5% and O2 minimum of 13.5%. The change in ventilation with inspired CO2 was measured over 5–6 minutes of the test. The slope of a linear curve fit relating inspired CO2 to the logarithm of ventilation was taken as a quantitative measure of ventilatory asphyxial sensitivity (VAS).
Results: There was no significant difference in VAS between the AGA and SGA infants (0.25 v 0.24). However within the SGA group, VAS was significantly higher (p = 0.048) in the infants whose mothers smoked during pregnancy (0.26 (0.01); n = 24) than in those that did not (0.23 (0.01); n = 23). The change in minute ventilation was significantly higher in the smokers than the non-smokers group (141% v 119%; p = 0.03) as the result of a significantly larger change in respiratory rate (8 v 4 breaths/min; p = 0.047) but not tidal volume.
Conclusions: Maternal smoking appears to be the key factor in enhancing infants’ respiratory responses to hypoxia/hypercapnia, irrespective of gestational age.
- AGA, average for gestatinal age
- AS, active sleep
- HR, heart rate
- IUGR, intrauterine growth retardation
- QS, quiet sleep
- RR, respiratory rate
- SGA, small for gestational age
- SIDS, sudden infant death syndrome
- VAS, ventilatory asphyxial sensitivity
- Ve, minute ventilation
- tidal volume
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