An unborn baby’s heart develops in stages. Frame 1 illustrates that the embryo’s main vein and artery pass close to each other in the chest cavity. Three weeks after conception these two tubes link together, fusing to form a single chamber (Frame 2) before the upper atria and lower ventricles begin to take shape (shown in Frame 3). Beginning in the 5th week (Frame 4), the atrioventricular canal is gradually split by growing ridges of tissue to provide two separate pathways for blood, one through each side of the heart. When the septum forms down the center (Frame 5), the heart contains four separate chambers.
In embryological development, the heart and its major blood vessels are formed and functioning long before any other organ. As seen in these slides, the heart emerges from a twisting, turning, thickening and folding of the embryo’s two main vessels.
By the eight week of development, the embryo possesses a tiny heart with four chambers, a miniature version of an adult organ. Before birth, the blood circulation must be modified because the respiratory and digestive systems are not yet fully formed. The placenta serves as the source of oxygen and nutrients. Because the fetus does not breathe, and the fluid-filled lungs do not absorb oxygen from the air, there is a hole in the septum between the right and left atria.
By birth, this hole, the foramen ovale, will close. An adult circulation pattern is established after the placenta is cut and the infant takes its first breaths. Minutes after birth, the ductus arteriosus contracts, closing completely in roughly one week. It shrinks into a fibrous band joining the pulmonary artery to the aorta.
Effectively two pumps side by side, a normal heart ensures that venous and arterial circulations are kept entirely separate. In about one baby in 200, something goes awry in the development process. Tubes link up incorrectly perhaps with serious consequences for blood flow. These problems are known as congenital heart defects.
When abnormal connections allow the two types of blood to mix, the heart has to work harder to get enough oxygenated blood to the tissues. Many such heart defects are congenital, although they may not be obvious at birth.
The period in which the atria and ventricles are being divided in the fetal heart is critical. During fetal development, there are openings (shunts) which close at birth, and a normal pattern of circulation is established. Interference with these processes by rubella or rubeola (measles) virus may lead to the following defects:
1. VSD, ventricular septal defect- an abnormal opening between the ventricles- blood passes from left to right ventricle
2. ASD, atrial septal defect- an opening in the atrial septum permits mixing of oxygenated and unoxygenated blood
3. Defects in the arteries
a. coarctation of the aorta– the aortic lumen is narrowed
b. patent ductus arteriosus– the shunt between the pulmonary artery and aorta is open (blood bypasses the lungs)
In Tetralogy of Fallot – 4 defects occur simultaneously:
a. Narrow pulmonary artery or valve
b. ventricular septal defect (opening)
c. aorta receives blood from both ventricles
d. right ventricular hypertrophy