P wave (electrocardiography) on:
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The P wave on the ECG represents atrial depolarization, which results in atrial contraction, or
Peaked P waves (> 0.25 mV) suggest right atrial enlargement, cor pulmonale, (''P pulmonale'' rhythm), but have a low predictive value (~20%).
A P wave with increased amplitude can indicate 
Bifid P waves (known as ''P mitrale'') indicate left-atrial abnormality - e.g. dilatation or hypertrophy.
If at least three different shaped P waves can be seen in a given ECG lead tracing, this implies that even if one of them arises from the SA node, at least two others are arising elsewhere. This is taken as evidence of multiple (i.e. at least two) ectopic foci, and is called multifocal (or more correctly, multiform) atrial rhythm if the rate is ≤100) or multifocal atrial tachycardia if the rate is over 100. This appears particularly commonly in exacerbations of chronic obstructive lung disease.
If the baseline has a totally irregular form, this suggests fibrillatory waves of atrial fibrillation or possibly artefact; a saw tooth shaped baseline suggests the flutter waves of
The P wave on the ECG represents atrial depolarization, which results in atrial contraction, or atrial systole
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, followin ...
.
Physiology
The P wave is a summation wave generated by the depolarization front as it transits the atria. Normally the right atrium depolarizes slightly earlier than left atrium since the depolarization wave originates in the sinoatrial node, in the high right atrium and then travels to and through the left atrium. The depolarization front is carried through the atria along semi-specialized conduction pathways includingBachmann's bundle
In the heart's conduction system, Bachmann's bundle (also called the Bachmann bundle or the interatrial band) is a branch of the anterior internodal tract that resides on the inner wall of the left atrium. It is a broad band of cardiac muscle t ...
resulting in uniform shaped waves. Depolarization originating elsewhere in the atria (atrial ectopics) result in P waves with a different morphology from normal.
Pathology
Peaked P waves (> 0.25 mV) suggest right atrial enlargement, cor pulmonale, (''P pulmonale'' rhythm), but have a low predictive value (~20%).
A P wave with increased amplitude can indicate hypokalemia
Hypokalemia is a low level of potassium (K+) in the blood serum. Mild low potassium does not typically cause symptoms. Symptoms may include feeling tired, leg cramps, weakness, and constipation. Low potassium also increases the risk of an abno ...
. It can also indicate right atrial enlargement
Rights are legal, social, or ethical principles of freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people according to some legal system, social convention, or ethical theory ...
.
A P wave with decreased amplitude can indicate hyperkalemia
Hyperkalemia is an elevated level of potassium (K+) in the blood. Normal potassium levels are between 3.5 and 5.0mmol/L (3.5 and 5.0mEq/L) with levels above 5.5mmol/L defined as hyperkalemia. Typically hyperkalemia does not cause symptoms. Occa ...
.
Bifid P waves (known as ''P mitrale'') indicate left-atrial abnormality - e.g. dilatation or hypertrophy.
If at least three different shaped P waves can be seen in a given ECG lead tracing, this implies that even if one of them arises from the SA node, at least two others are arising elsewhere. This is taken as evidence of multiple (i.e. at least two) ectopic foci, and is called multifocal (or more correctly, multiform) atrial rhythm if the rate is ≤100) or multifocal atrial tachycardia if the rate is over 100. This appears particularly commonly in exacerbations of chronic obstructive lung disease.
If the baseline has a totally irregular form, this suggests fibrillatory waves of atrial fibrillation or possibly artefact; a saw tooth shaped baseline suggests the flutter waves of atrial flutter
Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart. When it first occurs, it is usually associated with a fast heart rate and is classified as a type of supraventricular tachycardia. Atrial f ...
. With either of these rhythms, if the ventricular rate is fast, the fibrillatory or flutter waves can easily be misinterpreted as P waves.
Absence of the P wave with a flat baseline may indicate:
* Fine atrial fibrillation
* Sinoatrial arrest
Sinoatrial arrest is a medical condition wherein the sinoatrial node of the heart transiently ceases to generate the electrical impulses that normally stimulate the myocardial tissues to contract and thus the heart to beat. It is defined as lasti ...
(with a secondary escape rhythm)
If P waves are not clearly delineated in the surface ECG, a Lewis lead
A Lewis Lead (also called the S5 lead) is a modified ECG lead used to detect atrial flutter waves when atrial flutter is suspected clinically, based on signs and symptoms, but is not definitely demonstrated on the standard 12 lead ECG. In order to ...
may be used to better visualize P waves.
Atrial repolarization
This occurs a mean of 320 ms after the end of the P wave, with a duration of 2-3 times that of the P wave and a polarity always opposite to that of the P wave. It is represented on the surface ECG by a so-called Ta wave. The clinical relevance of this is that, although a normal phenomenon, the nadir of the Ta wave can occur just after the QRS complex and cause ST depression similar to (and easily mistaken with) that occurring with disease states such as cardiac ischaemia.Related pages
*Electrocardiography
Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
* PR interval
In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization); ...
* QRS complex
* QT interval
The QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart. It is calculated as the time from the start of the Q wave to the end of the T wave, and approximates to the time taken ...
* ST segment
In electrocardiography, the ST segment connects the QRS complex and the T wave and has a duration of 0.005 to 0.150 sec (5 to 150 ms).
It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the ...
* T wave
In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the ''absolute refractory period''. The last half of the T wave ...
* U wave
References
{{Cardiovascular physiology Cardiac electrophysiology