THE EFFECT OF HYPERCAPNIA AND HYPERCAPNIA ASSOCIATED WITH CENTRAL COOLING ON BREATHING IN UNANESTHETIZED FETAL LAMBS

In utero breathing activity is present periodically, but it must becom e continuous at birth. We investigated the effect of hypercapnia and o f hypercapnia combined with central cooling on fetal breathing in seve n chronically instrumented fetal lambs of 131-134-d gestation, using a n extracorporeal membrane oxy enation (ECMO) system to control fetal b lood gases and fetal temperature. During fetal hypercapnia (from a par tial pressure of arterial CO2 (Paco(2)) 6.18 +/- 0.11 kPa to Paco(2) 7 .39 +/- 0.15 kPa) frequency, amplitude, and incidence of fetal breathi ng movements during low voltage electrocortical activity (LV ECoG) inc reased significantly compared with isocapnic control on ECMO, but brea thing remained absent during high voltage electrocortical activity (HV ECoG). During hypercapnia accompanied by central cooling (Paco(2) 7.9 0 +/- 0.13 kPa, temperature decreased by 2.1 degrees C) there were sim ilar changes in fetal breathing movements during LV EC6G, bur in four out of seven fetuses fetal breathing movements continued throughout HV ECoG. Hypercapnia accompanied by central cooling can thus override th e inhibitory effects of HV ECoG on fetal breathing movements. This may be due to changes in sensitivity to CO2 produced by an increase in af ferent input to the CNS.