The P wave is upright, rounded, and uniform. It usually occurs before the QRS complex, though in certain circumstances, it may occur during or after the QRS complex. The QRS complex is the most easily visualized marking on the ECG strip due to its size.

When an impulse originates somewhere other than the SA node, the P wave morphology changes, and it may sometimes have a downward deflection. Occasionally, two electrical impulses occur at the same time. For example, if both the atria and the ventricles depolarize at the same time, the P wave (atrial depolarization) and the T wave (ventricular depolarization) overlap each other, but since the T wave has a more dominant deflection, the P wave may seem to disappear (see Figure 2.11). This phenomenon is known as a buried wave or losing a wave.

Figure 2.11 First-Degree Atrioventricular Block

First-Degree AV Block ECG

Possible causes of a buried P wave: 

• Increased vagal tone
• Athletic training
• Inferior myocardial infarction
• Mitral valve surgery
• Myocarditis from Lyme disease 
• AV nodal-blocking drugs

The P wave is the most reliable wave to identify on the ECG strip. All leads should be examined for P waves. The absence of clearly identifiable P waves may indicate atrial fibrillation or another arrhythmia. 

The relationship between P waves and the QRS complex should also be established. The number of P waves should equal the number of QRS complexes. 

• If there are more QRS complexes than P waves, then the rhythm is an accelerated ventricular or junctional rhythm. 
• If there are more P waves than QRS complexes, then some type of AV block is present. 
• P waves that occur after each QRS complex may indicate a junctional rhythm, a ventricular rhythm with retrograde AV conduction, an AV nodal re-entrant rhythm, or an AV-reciprocating tachycardia.