The ontogeny of hemodynamic responses to prolonged umbilical cord occlusion in fetal sheep.
There is evidence that preterm fetuses have blunted chemoreflex-mediated responses to hypoxia. However, the preterm fetus has much lower aerobic requirements than at term, and so moderate hypoxia may not be sufficient to elicit maximal chemoreflex responses; there are only limited quantitative data on the ontogeny of chemoreflex and hemodynamic responses to severe asphyxia. Chronically instrumented fetal sheep at 0.6 (n = 12), 0.7 (n = 12), and 0.85 (n = 8) of gestational age (GA; term = 147 days) were exposed to 30, 25, or 15 min of complete umbilical cord occlusion, respectively. At all ages, occlusion was associated with early onset of bradycardia, profoundly reduced femoral blood flow and conductance, and hypertension. The 0.6-GA fetuses showed a significantly slower and lesser fall in femoral blood flow and conductance compared with the 0.85-GA group, with a correspondingly reduced relative rise in mean arterial blood pressure. As occlusion continued, the initial adaptation was followed by loss of peripheral vasoconstriction and progressive development of hypotension in all groups. The 0.85-GA fetuses showed significantly more sustained reduction in femoral conductance but also more rapid onset of hypotension than either of the younger groups. Electroencephalographic (EEG) activity was suppressed during occlusion in all groups, but the degree of suppression was less at 0.6 GA than at term. In conclusion, the near-midgestation fetus shows attenuated initial (chemoreflex) peripheral vasomotor responses to severe asphyxia compared with more mature fetuses but more sustained hemodynamic adaptation and reduced suppression of EEG activity during continued occlusion of the umbilical cord.
Department of Physiology, Faculty of Medical and Health Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand.
Guido
G
Wassink
Laura
L
Bennet
Lindsea C
LC
Booth
Ellen C
EC
Jensen
Bert
B
Wibbens
Justin M
JM
Dean
Alistair Jan
AJ
Gunn
D000222:Adaptation, Physiological; D000818:Animals; D021721:Balloon Occlusion; D001794:Blood Pressure / Q000502:physiology*; D002628:Chemoreceptor Cells / Q000503:physiopathology; D004195:Disease Models, Animal; D005263:Femoral Artery / Q000503:physiopathology; D005311:Fetal Hypoxia / Q000209:etiology / Q000503:physiopathology*; D005333:Fetus; D005865:Gestational Age; D006340:Heart Rate, Fetal / Q000502:physiology*; D007022:Hypotension / Q000209:etiology / Q000503:physiopathology; D012039:Regional Blood Flow / Q000502:physiology; D012756:Sheep / Q000502:physiology*; D013997:Time Factors; D014470:Umbilical Cord / Q000098:blood supply* / Q000503:physiopathology / Q000601:surgery