This is also known as ‘hole in the heart’. Before birth,most oxygenated blood from the placenta enters the left atrium from the right atrium through the foramen ovale in the septum. There is a valve-like structure across the opening,consisting of two partly overlapping membranes. The valve is open when the pressure in the right atrium is higher than in the left. This diverts blood flow from the right to the left side of the heart,bypassing the pulmonary circulation,which in the unborn child is not functional because the fetus derives his oxygen supply through the placenta. After birth,when the pulmonary circulation is established and the pressure in the left atrium is higher,the two membranes come in contact,closing the valve. Letet the closure becomes permanent due to fibrosis.
When the membranes do not overlap,an opening between the atria remains patent after birth. In many cases it is too small to cause symptoms in early life but they may appear later. In severe cases blood flows back to the right atrium from the left. This increases the right ventricular and pulmonary pressure,causing hypertrophy of the myocardium and eventually cardiac failure. As pressure in the right atrium rises,blood flow through the defect may be reversed,but this is not an improvement because deoxygenated blood gains access to the general circulation.
