Effects of lung distension and spontaneous fetal breathing on hemodynamics in sheep

In previous studies, we determined that pulmonary ventilation and oxygenation were largely responsible for increasing pulmonary blood flow and for altering central blood flow patterns but could not account for the increase in combined ventricular output (the sum of left and right ventricular outputs) that usually occurs after birth. We had ventilated fetuses with oscillating positive pressures; the possibility that high intrathoracic pressures adversely affected cardiac output could not be ruled out. To determine the effects of negative intrathoracic pressure ventilation, we occluded the umbilical cord of nine long-term instrumented fetal sheep at 135 +/- 1.2 d gestation while introducing humidified oxygen gas intratracheally. This stimulated continuous fetal respiratory activity, which resulted in negative intrathoracic pressures and increased fetal oxygenation. Pulmonary blood flow increased from 121 +/- 60 to 901 +/- 139 mL.min-1.100 g-1 (mean +/- SD) and descending aortic blood pressure increased from 7.0 +/- 1.2 to 9.6 +/- 2.0 kPa. Central blood flow patterns were altered so that right to left shunting of blood through the foramen ovale and ductus arteriosus was abolished and a left to right shunt developed across the ductus arteriosus. However, cardiac output did not increase significantly. This may be related, in part, to the increase in afterload. It is clear that pulmonary ventilation and oxygenation are the major factors responsible for changing pulmonary blood flow and central blood flow patterns at birth. Other birth-related events, such as a decrease in environmental temperature and an increase in metabolic rate, are likely to induce cardiac output to increase.