Why can't an EKG be used to diagnose all diseases or defects of the heart?

All an EKG does is to measure the electrical signal in the heart.

Think of your car as a heart. The EKG could measure most problems your battery is hooked into, but won't measure things outside of the electrical bits- things like the airflow, fuel flow, exhaust, tires, body, etc. would not be detected.

In the human heart, the electrical flow shows a lot of the glitches, but not everything. You also have the problems of intermittant problems that only show up under certain conditions, etc.

cause not all diseases are easy to find not from EKG's , not from blood tests, and not from MRI's. I wish it were that easy really. I was diagnosed with MS in 2004 and between all the EKG's, MRI;s and blood tests it all came up negative. The only way a doctor can find out if someone has MS is to draw fluid from the spinal cord. So you see all diseases has their own way of being found in one way of another.

Because an EKG only shows the electric conduction of the heart and it only shows how long or short it takes for the blood to go through the heart and what rhythm your heart is in. Stress Test measure how much stress your heart can or can not take. (Physical stress). Echos show in depth how it is working on the inside.
Echo on carotids show if their is blockage in the arteries or not. so EKG's can not diagnose ever thing of the heart its more kind of just the top lay picture. then you get in to the actually pictures of what the heart is doing.
EKG's can not show if their are blood clots in the arteries a TEE test can show that.
But EKG's can show if their is a blockage of the flow of blood threw the heart such as Bundle Branch Blocks (BBB), First degree heart block (AVB1), Second degree heart block (2 Degree AVB), Third degree heart block (3 degree or Complete heart block).