What is the P wave on electrocardiography (ECG)?

Answer

The P wave represents atrial depolarization. The normal P wave morphology is upright in leads I, II, and aVF, but it is inverted in lead aVR. The P wave is typically biphasic in lead V1 (positive-negative), but when the negative terminal component of the P wave exceeds 0.04 seconds in duration (equivalent to one small box), it is abnormal.

Left atrial enlargement should be suspected when the P wave duration is increased; it is associated with being more than one small box deep (>1 mm²) in lead V1 and a bifid P wave in lead II with a duration that is longer than 110 seconds.^[19] Right atrial enlargement is associated with a peaked P wave taller than 2.5 mm in the inferior leads and more than 1.5 mm tall in leads V1 and V2.^[20]

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Media Gallery

I, II and III represent the original three leads used by Einthoven. These describe the electrical vector between the right and left arms, right arm and left leg, and left arm and left leg, respectively. R, L, and F represent the augmented limb leads aVR, aVL, and aVF. They are calculated as follows: aVR = (I+II)/2 , aVL = (I-III)/2, aVF = (II+III)/2.
The hexagonal reference system is the standard method of determining the axis of electrical activation of the heart. As an example, depolarization that is parallel with lead II would yield a value of 60º, whereas a depolarization vector in the opposite direction would be -120º. Image courtesy of Wikipedia.
Proper placement of the precordial leads for standard 12-lead electrocardiography.
The hexagonal reference system is the standard method of determining the axis of electrical activation of the heart. As an example, depolarization parallel with lead II would yield a value of 60º, whereas a depolarization vector in the opposite direction would be -120º.
Electrocardiogram waves, intervals, and segments.
On standard calibration, each large box has sides of 0.5 cm. On the horizontal axis, each large box represents 0.2 seconds, and each smaller box represents 0.04 seconds. On the vertical axis, each small box is 1 mm in height; 10 mm = 1 mV.
Heart rates associated with each of the large boxes in the following order are 300, 150, 100, 75, 60, 50, 43, 37, 33 beats per minute (bpm).
Heart rate boxes.
ST depression.
ST elevation.
ST morphology.
Electrocardiographic axis. Normal axis is between -30º and 90º; left axis deviation (LAD) is between -30º and - 90º; right axis deviation (RAD) is between 90º and 180º; extreme axis deviation (EAD) (left or right) is between -90º and -180º.
R-wave progression.
A 12-lead electrocardiogram is divided into the following four main contiguous lead groups: inferior, lateral, septal, and anterior.
Typical normal 12-Lead electrocardiogram. Courtesy of Dr Bob McGraw, Queen’s University Technical Skills Program and Department of Emergency Medicine (https://meds.queensu.ca/central/assets/modules/ECG/index.html).

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