Respiration Physiology (1983) 52, 99-111 99 Elsevier Biomedical Press CENTRAL CHEMICAL REGULATION OF BREATHING MOVEMENTS IN FETAL LAMBS A. ROGER HOHIMER, JOHN M. BISSONNETTE, BRYAN S. RICHARDSON and CINDY M. MACHIDA Department of Obstetrics and Gynecology, Oregon Health Sciences University, Portland, OR 97201, U.S.A. Abstract. In 12 chronically prepared fetal lambs between 126 and 136 days of gestation, 17 ventriculo- cisternal perfusions (123 #l/min) were performed. The concentration of bicarbonate ([HCO3-]) in the mock cerebrospinal fluid (CSF) perfusate was altered to manipulate the [HCO3-] in the cerebral ventricles. These perfusions did not systematically alter fetal arterial Pco2, Po2, pH, heart rate or mean blood pressure. Fetal breathing movements (FBM) were noted as rhythmic negative intrathoracic pressures with reference to amniotic fluid pressure. The incidence of FBM was determined and expressed as the percent of the 3 h observation period during which breathing movements were present. Perfusions with mock CSF having an approximately normal [HCO3], resulted in cisternal [HCO3] of 23.9 + 0.8 (SEM) meq/L and an incidence of fetal breathing movements (FBM) of 22.1 +_ 10.0%. Perfusions with an elevated [HCO3- ] in the mock CSF increased the cisternal [HCO3- ] to 30.7 + 0.4 meq/L and lowered the incidence of FBM to 0.6 + 0.2%. Perfusions with a lowered [HCO3- ] mock CSF decreased the cisternal [HCO3-] to 17.3 ± 0.8 meq/L and increased the incidence of FBM to 45.7 + 6.8%. These perfusions did not alter the relative incidence of low voltage (LV) versus high voltage (HV) electro- cortical (ECOG) state. In 4 animals, low [HCO3- ] perfusions induced FBM in the normally apneic HV ECOG state. We conclude that in the near-term fetal lamb, central chemoreceptors can modulate the incidence of rhythmic respiratory efforts, that tonic afferent stimuli arising from these receptors are critical for the generation of spontaneous FBMs and that central acidosis is capable of evoking FBMs in the normally apneic HV ECOG state. Apnea Control of respiration Central chemoreceptors Fetal breathing Cerebrospinal fluid Sleep state Many investigations on anesthetized or exteriorized fetal sheep have produced evidence suggesting that developing chemoreceptors, either arterial or central medul- lary, were almost totally ineffective in stimulating ventilatory efforts (Barcroft and Karvonen, 1948, Chernick et al., 1975; Herrington et al., 1971 ; Hodson et al., 1968; Jansen et al., 1975; Woodrum et al., 1977). In contrast, studies on chronically Accepted for publication 17 January 1983 0034-5687/83/0000-0000/$03.00 © 1983 Elsevier Science Publishers