Circulatory responses to asphyxia differ if the asphyxia occurs in utero or ex utero in near-term lambs.
BACKGROUND
A cornerstone of neonatal resuscitation teaching suggests that a rapid vagal-mediated bradycardia is one of the first signs of perinatal compromise. As this understanding is based primarily on fetal studies, we investigated whether the heart rate and blood pressure response to total asphyxia is influenced by whether the animal is in utero or ex utero.


METHODS
Fetal sheep were instrumented at ∼ 139 days of gestation and then asphyxiated by umbilical cord occlusion until mean arterial blood pressure decreased to ∼ 20 mmHg. Lambs were either completely submerged in amniotic fluid (in utero; n = 8) throughout the asphyxia or were delivered and then remained ex utero (ex utero; n = 8) throughout the asphyxia. Heart rate and arterial blood pressure were continuously recorded.


RESULTS
Heart rate was higher in ex utero lambs than in utero lambs. Heart rates in in utero lambs rapidly decreased, while heart rates in ex utero lambs initially increased following cord occlusion (for ∼ 1.5 min) before they started to decrease. Mean arterial pressure initially increased then decreased in both groups.


CONCLUSIONS
Heart rate response to asphyxia was markedly different depending upon whether the lamb was in utero or ex utero. This indicates that the cardiovascular responses to perinatal asphyxia are significantly influenced by the newborn's local environment. As such, based solely on heart rate, the stage and severity of a perinatal asphyxic event may not be as accurate as previously assumed.
The Ritchie Centre, Monash University, Melbourne, Australia.
Kristina S
KS
Sobotka
Neonatal Research, Royal Women's Hospital, Melbourne, Australia.
Colin
C
Morley
The Ritchie Centre, Monash University, Melbourne, Australia.
Tracey
T
Ong
The Ritchie Centre, Monash University, Melbourne, Australia; Department of Obstetrics and Gynaecology, Monash University, Melbourne, Victoria, Australia.
Graeme R
GR
Polglase
The Ritchie Centre, Monash University, Melbourne, Australia.
James D S
JD
Aridas
The Ritchie Centre, Monash University, Melbourne, Australia; Department of Obstetrics and Gynaecology, Monash University, Melbourne, Victoria, Australia.
Suzanne L
SL
Miller
The Ritchie Centre, Monash University, Melbourne, Australia; Neonatal Research, Royal Women's Hospital, Melbourne, Australia.
Georg M
GM
Schmölzer
Department of Pediatrics, University Hospital of North Norway, Tromsø, Norway; Pediatric Research Group, Faculty of Health Sciences, University of Tromsø, Tromsø, Norway.
Claus
C
Klingenberg
The Ritchie Centre, Monash University, Melbourne, Australia; Department of Obstetrics and Gynaecology, Monash University, Melbourne, Victoria, Australia.
Timothy J M
TJ
Moss
The Ritchie Centre, Monash University, Melbourne, Australia; Department of Obstetrics and Gynaecology, Monash University, Melbourne, Victoria, Australia.
Graham
G
Jenkin
The Ritchie Centre, Monash University, Melbourne, Australia; Department of Obstetrics and Gynaecology, Monash University, Melbourne, Victoria, Australia.
Stuart B
SB
Hooper
D000818:Animals; D001237:Asphyxia / Q000503:physiopathology*; D001238:Asphyxia Neonatorum / Q000503:physiopathology*; D001794:Blood Pressure / Q000502:physiology*; D005333:Fetus / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology*; D034561:Sheep, Domestic / Q000502:physiology; D047929:Term Birth