ABSTRACT
The only normal circumstance in which the P wave rate is variable is sinus arrhythmia, which is usually only seen in patients below the age of 40 years. After determining the heart rate, rhythm and axis, examine each wave of the electrocardiogram (ECG), beginning with the P wave. There are two reasons why P waves may be absent from the ECG. The first is that there is no coordinated atrial depolarization so that P waves are not being formed. The second is that P waves are present, but are just not obvious. Search the ECG carefully for evidence of P waves before concluding that they are absent, as P waves will often be hidden by any rapid tachycardia. P waves can just be discerned as a small deflection immediately after the end of the QRS complexes in some leads.
