Abstract
BackgroundA 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.MethodsFetal 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.ResultsHeart 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.ConclusionsHeart 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.
Highlights
Our current understanding of the cardiorespiratory responses to birth asphyxia in humans is primarily based on experiments first conducted in the 1960’s by Dawes and colleagues [1]
The duration of asphyxia was similar between groups at,11 min (Table 1) and the target blood pressure (BP) for terminating the asphyxia was the same, the BP at end asphyxia was higher in the ex utero lambs compared to in utero lambs (22.364.7 and 17.863.6 mmHg respectively)
BP was higher in ex utero lambs from 6.5 to 10 min after asphyxia onset (Figure 2)
Summary
Our current understanding of the cardiorespiratory responses to birth asphyxia in humans is primarily based on experiments first conducted in the 1960’s by Dawes and colleagues [1]. The first sign of compromise is cessation of respiratory efforts, commonly referred to as primary apnea, which is accompanied by a profound bradycardia. This bradycardia is mediated by vagal inputs and is most strongly initiated by both hypoxia and acidosis [2]. 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
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