Effect of periodic or regular respiratory pattern on the ventilatory response to low inhaled CO2 in preterm infants during sleep.

We wanted to know whether the ventilatory response to low concentrations of CO2 is measurable in the absence of change in alveolar PCO2, is sleep state dependent, and is dictated by the resting respiratory pattern. Therefore, we gave 11 preterm infants (birth weight, 1,565 +/- 122 g; gestational age, 32 +/- 1 wk; postnatal age, 28 +/- 5 days) 0.5 to 1.5% CO2 after a control period of breathing 21% O2. They were studied on 2 or 3 occasions, the aim being to have 5 infants in each of 2 categories, periodic to regular breathing, and regular to regular breathing, after administration of CO2 in both sleep states (n = 20). In both sleep states, when low CO2 increased ventilation, alveolar PCO2 also increased. The increase in ventilation was primarily due to an increase in breathing frequency if breathing was periodic, and to an increase in tidal volume if breathing was regular. This response was not affected by sleep state. We conclude that changes in respiratory pattern with low inhaled CO2 are fundamentally dependent on whether the baseline respiration is periodic or regular.