Respiration Physiology (1985) 62, 387-397 387 Elsevier THE EFFECT OF REBREATHING CO 2 ON VENTILATION AND DIAPHRAGMATIC ELECTROMYOGRAPHY IN NEWBORN INFANTS GUY MORIETTE, PATRICK VAN REEMPTS, MICHAEL MOORE, DON CATES and HENRIQUE RIGATTO Department of Pediatrics, University of Manitoba, Winnipeg, Canada, and Service de Medecine N~onatale, H6pital Port-Royal, Paris, France Abstract. We tested the hypothesis of whether the reduced ventilatory response to CO 2 in preterm as compared to term infants is related to primary central unresponsiveness, or to mechanical impairment of the respiratory pump. Eleven preterm (n = 19; gestational age 32 + 0.4 wk) and 14 term (n = 24; GA 40 + 0.3 wk) infants were studied. Minute integrated diaphragmatic activity (EMGDi x f), and mean inspiratory diaphragmatic activity (EMGDi/TI), were used as indices of central output. After 3 min breathing 21 ~o 02 (control), infants rebreathed from a bag containing 5~ CO2 in 40~o 02 for 2 to 3 minutes. We measured VE, VT, f, VT/TI. Sleep states were monitored. Preterm infants had a decreased ventilatory response to CO2 both in quiet sleep (QS) (0.0379 + 0.067 vs 0.505 + 0.032 L' (min. kg. kPa PAco2)- I; P < 0.04) and in active sleep (AS) (0.210 + 0.032 vs 0.331 + 0.048 L. (min. kg. kPa PAco2)- 1; p < 0.04). The decrease in response primarily was a function of a lack of increase in tidal volume with CO2 in QS and a lack of increase in fin AS. Parallel to these changes there were significant correlations between the increases in EMGDi x fand VE with inhaled CO 2 (r = 0.75; P < 0.001); VT and EMGDi (r = 0.63; P < 0.0 !); and between the increases in EMGr~/TI and VT/TI with inhaled CO 2 (r = 0.64; P < 0.001). The results suggest that ventilatory response to CO 2 is (1) correlated highly with diaphragmatic indices of central output; (2) less in active than in quiet sleep; (3) less in preterm than in term infants. We conclude that despite their increased chest wall compliance, preterm infants respond less to CO 2 because of central unresponsiveness. Carbon dioxide Human Preterm infant Control of breathing Newborn Response to hypercapnia Diaphragm Premature Ventilation Decreased ventilatory response to CO 2 in preterm infants has been attributed to decreased responsiveness of the respiratory centre, to mechanical impairment of the Accepted for publication 24 August 1985 Reprint requests to: Henrique Rigatto, M.D., Department of Pediatrics, WS 108 - Women's Hospital, 700 William Avenue, Winnipeg, Manitoba, Canada R3E 0Z3. 0034-5687/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)