 The vectorcardiograph (VCG) examines the ECG potentials generated along the three-dimensional axes of the body; that is, the x, y, and z planes.  The x vector is taken as the potential between two points under the arms, the y vector is between the head and right leg, and the z vector is from the front to the back of the body.

 The data obtained from inside of the heart by a process called catheterization and is used to make preoperative judgments on the need for surgery.  The catheter is inserted into the heart via the peripheral vascular system.  It can record intracardiac pressures and allow withdrawal of blood from the heart chambers to measure oxygen and carbon dioxide levels.

 In right catheterization the catheter is introduced through a peripheral vein to the vena cava and into the right side of the heart.

 A retrograde catheterization requires introduction of the catheter via an artery, usually the brachial or femoral artery.  The catheter is threaded through the arterial system and enters the left ventricle via the aorta.

 In transseptal catheterization, a special large diameter catheter is introduced through the femoral vein into the right atrium, where a special needle in the end of the catheter is used to puncture the septum wall dividing the right and left side of the heart.  A smaller catheter is then threaded through the large catheter and the needle to enter the left ventricle via the left atrium.  This technique is used primarily when aortic stenosis prevents use of the retrograde technique.