The activity of peripheral chemoreceptors was studied in 19 preterm very low birthweight infants at the postconceptional age of 36 and 40 weeks using the hyperoxic test. The infants were in a healthy condition and did not receive any extra oxygen or medication when tested. The inhalation of pure oxygen caused a decrease in mean (SE) ventilation by 16.1 (2.6)% and 15.1 (2.1)% at the 36th and 40th gestational week respectively. At the 36th gestational week the ventilatory response was significantly slower than at 40 weeks (10.9 (6) and 7.3 (3) sec). Six infants who had been on supplemental oxygen for more than 21 days (from 21 to 56 days) responded with significantly lower response to hyperoxia at the 36th gestational week (-7.9 (3.6)%) than those receiving oxygen treatment for a shorter period of time, 0 to 16 days (-19.9 (3.2)%). The 'low responding' group included three infants who had suffered from chronic lung disease. Those infants showed the lowest hyperoxic response (-4.3 (3.9)%). There was no difference in the response among healthy preterm infants (eight infants) and infants with respiratory distress syndrome. At the 40th gestational week the differences, even though showing the same characteristics, were not statistically significant. No statistically significant relationship was found between the strength of the ventilatory response to oxygen versus gestational, postnatal age, nor the time interval between the termination of supplemental oxygen treatment and the test. No relationship was found between the number of apnoeic/bradycardic spells and the strength of the ventilatory depression caused by hyperoxia. In conclusion we found that the very preterm infants, with the exception of those who received long periods of oxygen treatment, have stronger peripheral chemoreceptor responses than those reported for 2-4 day old full term infants. However, infants who had suffered from chronic lung disease show a depressed hyperoxic response.
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