Changes at Birth

00:00 The Changes That Happen in Birth. It's very important for us to understand the process involved with how birth happens and how you account for these four big differences between the fetal circulation and the adult circulation and those are once again the placenta having the foramen ovale open, the ductus arteriosus and the ductus venosus. Overall, it's important to think about what types of resistances occur in the various circulations. So in terms of the fetus, pulmonary vascular resistance is greater than systemic vascular resistance. So the resistance across the lungs is greater than around the whole body of the fetus, much different than in the adult. The adult pulmonary vascular resistance is a fraction of what systemic vascular resistance is. It's much easier for the adult to pump blood across the lungs than it is around to the whole body. The other big difference have to do with the pressures. The right ventricular pressure is lower than the left ventricular pressure in the adult and this makes a lot of sense because in the right side of the heart we're under low pressures and low resistances and the left side higher pressures, higher resistances. So let's now start the birth process with those two big topics in mind. What are the pulmonary vascular resistances and how are the changes between right ventricular pressure and left ventricular pressure? So the first thing that happens during birth is there is damage to the umbilical cord. This decreases the umbilical cord blood flow. Why is this important for us to first think about is this is now changing placental flow. Remember, 40% of placental flow or cardiac output is going to the placenta. Therefore, now 40% of flow is available to go elsewhere. This dramatically increases SVR because the placenta was a low-resistant circuit and you remove it then resistance in the whole system is going to climb and this occurs very quickly. This quick change in systemic vascular resistance closes the ductus venosus. The increase in systemic vascular resistance makes left ventricular pressure go up and as left ventricular pressure goes up, left atrial pressure goes up as well and this occurs with some changes that happen in the lungs.

02:51 When you decrease placental blood flow, you are no longer exchanging gases with the mother.

02:58 This causes asphyxia, which stimulates the very first breath of the now infant. That lung inflation does two things. It first decreases pulmonary vascular resistance and the second thing it starts to increase the amount of PaO2. These are important processes. This first inflation of the lungs of the new infant neonate is very very important. This drops pressure in the right ventricle and in the right atrium then this closes the ductus arteriosus and the foramen ovale. So all of these closing of the ductus venosus, the ductus arteriosus and the foramen ovale happen almost instantaneously once the placenta, the blood flow is decreased and the first breath happens. That is a lot of changes in the cardiovascular system in just a few seconds.