I want to know more about Valves in the heart.

There are four valves in the Heart.  Two are on the right side of the heart, 2 on the left side.The First valve on each side allows blood to come into the pump.  The Second valve on each side allows the Blood to leave the Pump.
The Blue blood without oxygen returning from the body to the heart enters the right sided Pump via the Tricuspid Valve. The Right Pump (Right Ventricle) pushes the blood into the lungs via the pulmonary valve.
The structure of each valve has to it 2-3 leaflets i.e. these open and allow the blood to move forward.The Valves on the Right Side of the Heart.
The Blood from the lungs enters the network within the lungs where the oxygen enters the blood and becomes red in color called oxygenated blood. This blood enters the left pump (the Left ventricle) via the Mitral valve.
The left Pump pushes the blood into the body via the all important Aortic Valve.

My Child’s Valve is narrowed. What is this condition ?
Valve between the heart & lungs: the pulmonary valve
The Pulmonary Valve is narrowed often. Narrowing of the valve is usually due to the leaflets not opening completely.  The blood cannot move forward normally in a laminar flow. The pressure increases below the valve in the pump as this tries to overcome the restriction to its flow.
The muscle constituting the pump also increases (hypertrophy) and there by keep up the amount of blood flowing through the narrowed valve (more force is required to get the same amount of blood to flow across the valve.

What causes the valve not to open up fully?
This valve most often has 3 leaflets (equivalent of petals) which have become thickened or joined at the junctions (commisures). This is important to know prior to intervention for a good result.
Sometimes the valve may not open well due to tethering of the leaflets to the upper side of the valve or the narrowing is superior to the valve and not the valve. This will not respond to ballooning.
If the valve ring diameter is narrower than normal then also it will not allow blood to flow through normally.

How much is mild, moderate or severe narrowing?
Pulmonary Stenosis is referred to as being mild, moderate or severe depending on the pressure difference across the valve.  Normally there is no pressure difference across the valve or is minimal up to 20-25mmHg.
If the pressure difference across the valve is 45 mmHg it is considered mild stenosis. If the pressure difference is in the range of 60 mmHg then it is considered moderate. If it is more than 80 mmHg it is considered severe. Usually ballooning is considered for valves which are severely stenotic.  Or earlier if the child is symptomatic.

How is the procedure done?
Refer to the document above