Lateral pontine lesions affect central chemosensitivity in unanesthetized fetal lambs

The mechanisms underlying the apnea associated with both hypoxia and high-voltage electrocortical activity (HV ECOG) in the fetal lamb are not clearly understood. We have previously shown that lesions in the rostral lateral pons change the depressive response to hypoxia to one of stimulation. We now present evidence that lesions in this area also affect the fetal response to H+/CO2. Lactic acidemia and 5-6% hypercapnia both caused almost continuous breathing throughout both HV and low-voltage (LV) ECOG in seven of eight fetuses with lesions that reversed the depressive response to hypoxia. In five intact control fetuses and seven fetuses with lesions that did not affect the response to hypoxia, neither CO2 nor lactic acidemia induced continuous breathing. We conclude that the lateral pontine area, which is involved in the hypoxic depression of breathing in the fetus, is also involved in the inhibitory mechanisms operating during HV ECOG. Furthermore the inhibition of breathing during HV ECOG appears to be related to a raised threshold for fetal breathing, which is sensitive to H+/CO2.