The effect of 2% CO2, 100% O2, theophylline and 15% O2 on "inspiratory drive" and "effective" timing in preterm infants.

We studied 20 preterm infants (B.W. 1440 +/- 80 g (S.E.); G.A. 33 +/- 1 wk) to determine the effect of respiratory stimulants and depressants on respiratory output as measured by VE = VT . f, and VE = VT/Ti . Ti/Ttot. These 20 infants were divided in four groups of five infants. Each group received a respiratory stimulant (2% CO2, 100% O2 or theophylline) or'a respiratory depressant (15% O2). VT/Ti is mean inspiratory flow and represents a mechanic translation of neuronal output. Ti/Ttot is a dimensionless number and has been defined as effective timing. Each study consisted of 3-5 min while the infant breathed 21% O2, followed by 5 min breathing 2% CO2, 100% O2 or 15% O2. The effect of theophylline was assessed by 48-72 h after the initial dose. The respiratory stimulants caused an increase in VT with little or no change in f; 15% O2 produced a decrease in f primarily. According to the newer approach, 2% CO2, 100% O2 and theophylline produced an increase in "inspiratory drive" with little or no change in "effective" timing; 15% O2 decreased "effective" timing primarily via an increase in Te. These findings suggest that the paradoxical decrease in ventilation during hypoxia in preterm infants may not be solely dependent on the central depressant effects of O2. At least in part, the mechanism may be due to a direct action of low O2 on elements controlling expiratory time.