Electrophysiology Basics
First Time Seeing an ECG, ever?
I highly recommend a slightly more "didactic" approach to looking at ECGs - which thanks to the many free material out there, they aren't hard to come by. For the basics of electrophysiology and the ECG, the following are good introductions to the subject matter:
Electrocardiograms (ECGs) measures the waves of depolarization and repolarization in the heart. When depolarization happens, the heart muscle contracts. When repolarization happens, the ions in the cardiac membrane "reset" to allow for the next action potential. "Waves" is key here: ECGs can't measure activity at a specific location of the heart - only the general direction that the electrical activity is moving. These electrical waves propagate out of the heart and towards the skin where they can be recorded by ECG leads.
The Effect of Depolarization and Repolarization
For a particular view of the heart, one of the ECG leads will be designated as the positive lead, and will essentially record all the electrical activity coming towards it. If there's a wave of depolarization coming towards it, the ECG will deflect up.
If there's a wave of depolarization going away from it, the ECG will deflect downwards.
Repolarizations will deflect in the opposite direction. If there's a wave of repolarization going towards the positive lead, the ECG will deflect down.
If there's a wave of repolarization going away from it, the ECG will reflect upwards.
When there's no electrical activity in the heart because either the cells have been fully depolarized or fully repolarized, the ECG will show no activity. Keep this in mind: a flat ECG gives no indication if the heart is depolarized or repolarized.
The Effects of Direction and Size
The largest deflection will occur when an electrical wave is going "straight-on" to the positive lead. If the electrical wave is going at an angle to the lead, then the size of the deflection will be lower.
If it's going perpendicular to the lead, then there will be no deflection on the ECG.
The size of the electrical wave also has a huge factor in size. The atria, being relatively small, will have relatively small deflections.
The ventricles, being relatively large, will have relatively larger deflections.