Perinatal aortic growth in lambs: relation to blood flow changes at birth.

We have examined aortic growth and aortic hemodynamics in lambs in the perinatal period. Morphometry of histological cross sections indicated that abdominal aortic circumference decreased by 31% between 131 days of gestation and 2-3 wk postpartum. In contrast, the internal circumference of the thoracic aorta increased by 34% over the same time interval; thus size reduction of the abdominal aorta was not part of a generalized arterial response to ex utero life. We also determined medial cross-sectional area as an index of medial tissue mass. In the perinatal period (120 days gestation to 21 days postpartum), this index increased by 144% for the thoracic aorta but only by 69% in the abdominal aorta. Differences in rate of medial tissue accumulation were much greater postpartum than in utero. The relationship between abdominal aortic growth and hemodynamic changes was examined by instrumenting fetal lambs with blood pressure catheters, abdominal aortic blood velocity transducers, and sonomicrometer diameter crystals mounted on the abdominal aorta. Parturition, and the consequent loss of the placental circulation, caused a 73% reduction in abdominal aortic blood velocity. Abdominal aortic external diameter in the period between 4 and 14 days postpartum was reduced significantly compared with in utero values. These data are consistent with the hypothesis that blood flow changes at birth significantly influence arterial growth postpartum.