Introduction. II—Basic terminologyBMJ 2002; 324 doi: https://doi.org/10.1136/bmj.324.7335.470 (Published 23 February 2002) Cite this as: BMJ 2002;324:470
- Steve Meek,
- Francis Morris
This article explains the genesis of and normal values for the individual components of the wave forms that are seen in an electrocardiogram. To recognise electrocardiographic abnormalities the range of normal wave patterns must be understood.
The sinoatrial node lies high in the wall of the right atrium and initiates atrial depolarisation, producing the P wave on the electrocardiogram. Although the atria are anatomically two distinct chambers, electrically they act almost as one. They have relatively little muscle and generate a single, small P wave. P wave amplitude rarely exceeds two and a half small squares (0.25 mV). The duration of the P wave should not exceed three small squares (0.12 s).
The wave of depolarisation is directed inferiorly and towards the left, and thus the P wave tends to be upright in leads I and II and inverted in lead aVR. Sinus P waves are usually most prominently seen in leads II and V1. A negative P wave in lead I may be due to incorrect recording of the electrocardiogram (that is, with transposition of the left and right arm electrodes), dextrocardia, or abnormal atrial rhythms.
The P wave in V1 is often biphasic. Early right atrial forces are directed anteriorly, giving rise to an initial positive deflection; these are followed by left atrial forces travelling posteriorly, producing a later negative deflection. A large negative deflection (area of more than one small square) suggests left atrial enlargement.
Characteristics of the P wave
Positive in leads I and II
Best seen in leads II and V1
Commonly biphasic in lead V1
<3 small squares in duration
<2.5 small squares in amplitude