Below are examples using each method ... and the exact heart rate is 54 bpm. There is one P wave for each QRS complex, thus the atrial rate is the same. These QRS complexes are exactly three ...

The links below take you to examples of each of these. If there are no P waves present, or the P wave morphology is not normal, then the exact rhythm must be determined. Various arrhythmias ...

The diagrams show what happens when P waves and S waves pass through the Earth. S waves do not pass through the Earth's core S waves cannot pass through the liquid outer core, but P waves can.