The reason is that, soon after birth, the pressure in the aorta becomes very high. It also increases the lung blood pressure, and causes the blood vessels to become hard and thick - Pulmonary Hypertension. When the PDA is large, and particularly in small children, the enormous blood flow may cause Heart Failure. Early repair may be needed in these cases. The changes I have mentioned earlier become worse.
Heart failure sets in.
The child does not grow and develop normally. The condition is called Eisenmenger syndrome. This is called a duct dependent circulation. The PDA then should be kept open, until repair of the associated condition is performed. A medicine called Prostaglandin is used to keep the ductus open.