Circulatory Responses to Asphyxia Differ if the Asphyxia Occurs In Utero or Ex Utero in Near-Term Lambs

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.

The cardiovascular response to birth asphyxia is altered by the surrounding environment

“…However, the potential role that facial immersion in water may have on HR control at birth has recently been questioned. Significant differences in the HR response in lambs asphyxiated in utero compared with ex utero (in room air) have recently been demonstrated 9. Lambs asphyxiated in utero displayed the classic vagally mediated bradycardia, whereas ex utero lambs initially displayed a tachycardia before HR gradually decreased 9.…”

“…Significant differences in the HR response in lambs asphyxiated in utero compared with ex utero (in room air) have recently been demonstrated 9. Lambs asphyxiated in utero displayed the classic vagally mediated bradycardia, whereas ex utero lambs initially displayed a tachycardia before HR gradually decreased 9. Thus, the current understanding of the vagally mediated bradycardia associated with asphyxia is potentially influenced by the presence of liquid on the face.…”

Sustained Inflation and Its Role in the Delivery Room Management of Preterm Infants

“…In animal studies, SI and positive end-expiratory pressure provide an early functional residual capacity and uniformity of lung aeration, thus facilitating respiratory and cardiovascular transition at birth [7,8]. Moreover, SI improves heart rate recovery and lung compliance in near-term asphyxiated lambs [9]. …”

Perimortem Demonstration and Treatment of Recipient-to-Donor Transfusion in Monochorionic-Diamniotic Twin Gestation

“…Bradycardia is also a well-documented response to asphyxia, mediated by the vagal nerve and associated with an increase in blood pressure from downstream vascular resistance. However, it typically is an acute response occurring within the first 2 min [7] . This is in contrast to our case where the fetal bradycardia did not occur until the patient was in the recovery room.…”