Cardiovascular Responses During Transition from Fetal to Neonatal Life in Intact and Vagotomized Lambs † 1664

Abstract

Evidence suggests that pulmonary vagal innervation contributes more significantly to the alveolar ventilation in newborns than in older animals. To investigate whether vagal efferents play a role in the control of pulmonary blood flow during transition from fetal to neonatal life, we studied 12 unanesthetized, spontaneously breathing, chronically instrumented newborn lambs at 143-147 d gestation (Term=147±2d) after unassisted vaginal delivery. Surgery was performed between 130-134 d gestation to implant jugular venous and carotid arterial catheters, diaphragmatic electrode and left pulmonary artery blood flow probe (Transonic Inc). Six of 12 fetuses underwent intrathoracic vagotomy (VAG), whereas the remainder six were sham-operated(SHAM). Breathing frequency (breaths per minute) remained higher in the SHAM animals as compared with the VAG group, during the entire postnatal period(92-104 vrs 30-40, respectively; p=0.001). Inspiratory and expiratory times were significantly longer in the VAG group; p<0.01). Furthermore, similar to our previous studies, vagotomized animals were unable to establish adequate alveolar ventilation and remained hypothermic despite attempts to rewarm. Left pulmonary arterial (LPA) blood flow (ml/min/kg) increased from pre-birth values of 2.67±2.45 and 0.2±0.2 to 48±7 and 57±17 at five minutes after birth, in sham-operated and vagotomized animals, respectively (p<0.05). LPA continued to increase gradually reaching the maximum value of 101±10 and 126±19 ml/kg/min at 12 minutes of age in both SHAM and VAG groups, respectively. LPA blood flow pleatued at 85±8 and 84±6 ml/kg/min by 40 minute of age. Heart rate(beats/min) increased from 128±11(pre-birth) to 180±18 and 260±7(p<0.05),and from 134±15 (pre-birth), to 167±25 and 227±9 (p<0.05) at 10 and 30 minutes of age in SHAM and VAG groups, respectively. Heart rate was significantly higher in the SHAM group as compared with the VAG group at 30 minutes of age (p=0.02) whereas no significant changes were observed in systolic, diastolic or mean blood pressure between the two groups. We conclude that vagal innervation does not play significant role in cardiovascular adaptation at birth and inadequate pulmonary gas exchange in vagotomized animals is not due to lack of an increase in pulmonary blood flow. In addition, our study provides the first evidence of sequential changes in pulmonary blood flow during the first hour after birth in unanesthetized, spontaneously breathing newborn animals.