Transient ventilatory response to CO2 as a function of sleep state in full-term infants

The influence of sleep state on the transient (i.e., initial 60 s) and steady-state ventilatory responses to 2% CO2 inhalation was studied in 19 healthy full-term infants. A nasal mask pneumotachometer was used to measure ventilation and end-tidal CO2 partial pressure (PCO2) and enabled abrupt changes in the inspired gas concentration to be made. The magnitude of the change in minute ventilation for both the transient and steady-state responses to CO2 was not statistically different between active (AS) and quiet (QS) sleep. Nonetheless the greater variability in minute ventilation during AS compared with QS continued throughout the period of CO2 inhalation and was associated with a more variable change in ventilation in the individual infants during AS. There was a greater increase in end-tidal PCO2 over the first 60 s during AS (3.3 +/- 0.3 vs. 2.6 +/- 0.2 Torr, in AS and QS, respectively, P less than 0.03). This may indicate a smaller initial increase in alveolar ventilation, relative to CO2 delivery to the lungs, in response to CO2 inhalation during AS. Asynchronous chest wall movements were more common during AS than QS (P less than 0.005) and did not change with CO2. The inconsistent transient ventilatory response to CO2 during AS compared with QS may be important in the behavior of infants to spontaneous episodes of hypercapnia occurring during AS.