Knowing the pressure in each chamber of the heart can help doctors diagnose heart problems. Imagine you place a pressure millimeter on the left and right sides of the heart to monitor pressure in the heart. If you make a connection between the left and right sides of the heart, for example if there were a hole between the lower chambers of the heart (Ventricular Septal Defect or VSD) the blood would flow from the left side to the right side. Now the right side would be getting more blood which would result in the delivery of more blood than normal to the lungs. We call this situation Intercardiac Shunt or Left-to-Right Shunt which means the blood is shifting from the left side of the heart to the right side. This happens in any condition where there is a communication between the left and right sides of the heart because the pressure in the left side of the heart is higher than the right side of the heart.

During the process of the cardiac catheterization doctors measure the pressures in the heart so they can figure out if they are normal or high. For example, if the pressures are higher than normal in the Right Ventricle, the doctor will know that there must be a hole between the the Right Ventricle and the Left Ventricle chambers (Ventricular Septal Defect or VSD). Because the pressure in the Left Ventricle is the highest, the blood would flow from the higher pressure in the Left Ventricle through the hole and on to the lower pressure chamber in the Right Ventricle.

If there was a hole between the Right Atrium and the Left Atrium in the heart (Atrial Septal Defect or ASD) the blood would flow from the higher pressure in the Left Atrium to the lower pressure in the Right Atrium.

The pressure in any heart chamber may be high because that chamber may be pumping against a narrowed valve, or the chamber may be receiving extra blood from another chamber through a hole in its walls.