The P wave

Meet the P wave

The P wave is a deflection above or below the isoelectric line (baseline) of an ECG. It iscaused by electrical activation of the atrial chambers.
The atria can be activated from various sites:

  • the sinoatrial node (SAN)
  • single or multiple electrical foci in the atria that are outside the SAN
  • electrical foci in the atrioventricular node (AVN)  
  • electrical foci that began in the ventricles and cause an electrical impulse to travel from the ventricle through the AVN into the atria    

The presence of P waves is evidence ofelectrical activation of the atria, but does not necessarily prove that the P wave has originated from the SAN.

Components of the P Wave
Under normal conditions theelectrical activation of the atria

  • starts in the SAN region of the right atrium
  • extends to involve the rest of the right atrium, including the AVN (located in the lower part of the right atrium)
  • extends to involve the left atrium

The electrical activation of the atria involvesdepolarization followed byrepolarization.

The depolarization of the atria produces deflections (or waves) in the ECG tracing that we call P waves.  Depolarization is the stimulus for contraction of the atrial muscle.

Repolarization of the (normal) atria does not produce any changes in the ECG tracing, but pericarditis or atrial injury can cause depression or elevation of the PQ segment of the ECG.   

Weconclude that the normal P waveis acombination of

  • initial depolarization of the right atrium
  • simultaneous depolarization of the right and left atria
  • depolarization of the left atrium.

Figure 1. Contribution of right atrial (RA) depolarization and left atrial (LA) depolarization to the formation of the P wave in Lead II

P Waves in the Normal ECG
P waves are described according to:

  • The direction of the wave above or below the isoelectric line. They are called upright or positive P waves when the deflections are above the isoelectric line, or are called downward or negative or inverted P waves  when the deflections are below the isoelectric line.  P waves can have both an upward and a downward deflection, and are called biphasic P waves.  A biphasic P wave usually has a upright (positive) deflection followed by a downward (negative) deflection.
  • Their shape, using terms such as round, peak shaped or notched
  • Their size i.e. their horizontal and vertical measurements.

P waves are upright in the inferior leads, with Lead II usually having the greatest P wave amplitude (and thus Lead II is commonly used in monitoring rhythms).

Figure 2.  Normal P wave in Lead II. The P wave is upright with a horizontal width less than 0.12 seconds (i.e less than the width of three small squares) and a vertical height less than 2.5 mm (i.e. less than the height of two and a half small squares).

Figure 3. Single P - QRS complex showing a normal P wave (upright,  width 0.10 seconds, height less than 1 mm) that precedes a qR complex with a normal ST segment and a normal T wave. The PR interval is about 0.18 seconds, and the PR segment is horizontal.

In Figure 3 we have introduced the term “PR” segment”.  We are using the same convention to name the PR segment that we use for the “PR interval”. The term “PR interval” describes the interval between the start of the P wave and the start of the QRS complex, irrespective of whether or not the first deflection in the QRS is a Q wave or a R wave or S wave.  The  normal PR segment between the end of the P wave and the start of the QRS complex is flat i.e.  neither elevated or depressed (relative to the isoelectric line). 

P waves are inverted in Lead aVR
The next two figures show normally inverted P waves in lead aVR.  Note that the QRS complex can also be inverted relative to the isoelectric line (Figure 4) or have an upright configuration relative to the isoelectric line (Figure 5). This difference is caused by changes in the frontal plane QRS axis from a normal axis (Figure 4) to a extreme axis (Figure 5).

Figure 4. Single P - QRS complex in Lead aVR showing an inverted P wave, a deep Q wave and an inverted T wave.     

Figure 5. Single P - QRS complex in Lead aVR showing an inverted P wave, a qR complex, and an inverted T wave.

P waves in Lead V1 are often biphasic
The P wave in V1 is often biphasic, with an initial positive deflection due to right atrial depolarization followed by a negative deflection due to left atrial depolarization.

Figure 6. This shows the contribution of right atrial depolarization (Arrow 1 in RA) and left atrial depolarization (Arrow 2 in LA) to the formation of a biphasic P wave in Lead V1

Figure 7. Normal P wave in Lead V1 with a small amplitude biphasic P wave (upward deflection followed by downward deflection), a PR interval of about 0.14 seconds, a QRS duration of 0.14 seconds, a deep rS complex (vertical amplitude of the S wave below the isoelectric line is about 20 mm) and a positive T wave

Summary of P Waves in Normal ECG
Upright in the inferior leads, usually most prominent in Lead II.  Horizontal width is ≤ 0.12 seconds, and vertical height is ≤ 2.5 mm.
Inverted in Lead aVR
Biphasic in Lead V1 with duration of the downward deflection ≤ 0.04seconds and the depth ≤ 1 mm

More Than Meets the Eye: Introducing P Wave Abnormalities
We will start by considering abnormalities of size or shape or orientation
Different P waves shapes
The P waves will have different shapes in the same ECG lead if there are multiple electrical foci stimulating the atria. We will discuss this further when we discuss ectopic beats.
 Inverted P wave in Lead II
The presence of inverted P waves in Lead II indicates that the P waves are arising from a region of the atrium near the AVN.

Figure 8. Lead II showing inverted P waves preceding the QRS complexes. The PR interval is about 0.20 seconds, the ST segments are elevated (nearly 3 mm above the isoelectric line) and the T waves are tall. These changes are consistent with an acute inferior STEMI and a junctional (nodal) tachycardia (the heart rate is about 100 beats per minute).

Figure 9.  Rhythm strips (Lead II) in a case of supraventricular tachycardia (SVT) during and after the tachycardia. (A): Inverted P waves are seen immediately after every QRS complex during the SVT;  (B): P waves precede every QRS complex during sinus rhythm

Right Atrial Enlargement or Right Atrial Abnormality (RAA)
Stretching or hypertrophy of the right atrium can produce changes in P wave size and shape.  The usual change is an increase in the size and shape of the P wave, which becomes taller and has a narrow apex. There is usually no change in the width of the P wave.

These changes are best seen in the inferior leads, and also sometimes in V1. The changes are often called p pulmonale or p congenitale,  because the P wave changes are commonly seen in chronic lung disease or certain types of congenital heart disease.
An alternative term is right atrial abnormality (RAA).

Summary of Right Atrial Enlargement: P wave amplitude > 2.5 mm in Lead II or > 1.5 mm in V1 or both

Figure 10. P wave changes of right atrial abnormality (p pulmonale) in Lead II

Figure 11.  P wave changes of right atrial abnormality (p pulmonale)  in Lead V1

Figure 12.  P wave changes of right atrial enlargement are seen in Lead II. The isoelectric line slopes slightly upwards to the right.  The second P wave has a width of about 0.10 seconds, a height of about 3.5 mm, and a pointed tip. The PR interval is about 0.18 seconds. The rS shape of the QRS complexes suggests the presence of left axis deviation.

Figure 13. P wave changes of right atrial enlargement are seen in Lead II. The P wave has a width of about 0.12 seconds,  a height of nearly 3 mm and a pointed tip.  The PR interval is about 0.16 seconds. The QRS complex has a RS shape, but the complex is small (the vertical height is about 6 mm) compared to the P wave amplitude. The J point is depressed, and there is upward sloping ST depression and a flattened T wave.

Figure 14. P wave changes of right atrial enlargement are seen in V1. The P waves have an initial positive deflection that is 2 mm in amplitude and are followed by a very shallow negative deflection. The PR interval is about 0.24 seconds,  the QRS duration is about 0.14 seconds, and the QRS complexes have a rS configuration with a deep S wave (amplitude of at least 25 mm below the isoelectric line).

Left Atrial Enlargement or Left Atrial Abnormality (LAA)
Stretching or hypertrophy of the left atrium can increase the duration of left atrial depolarization. The effect of this in the frontal leads is to increase the P wave duration to greater than 0.12 seconds. The P wave in the frontal leads may also have a notched appearance, with the second hump caused by the prolonged left atrial depolarization. The inter-peak duration of the notch is equal to or greater than 0.04 seconds (1 small square). This humped appearance is sometimes called p mitrale, because it was commonly seen in the ECGs of persons with mitral stenosis who were in sinus rhythm.  P wave changes in the frontal leads similar to those described above can occur in persons with no evidence of left atrial enlargement, and are presumed to be caused by a delay in atrial conduction. The term left atrial abnormality (LAA) may be a better general term than left atrial enlargement or p mitrale.
LAA can alter the the P wave in Lead V1, with an increase in the width and amplitude of the second (downward) component of the P wave. This terminal P wave negativity is longer than 0.04 seconds in duration or 1.0 mm or more in depth.  
Summary of Left Atrial Enlargement: Limb leads have a P wave duration ≥ 0.12s (usually seen in lead II), with or without notching of the P wave with an inter-peak duration ≥ 0.04s
Lead V1 has increased terminal P negativity with the duration of the downward deflection ≥ 0.04s and the depth ≥ 1 mm.

Figure 15. P wave changes of left atrial abnormality in Lead II

Figure 16. P wave changes of left atrial abnormality in Lead V1

Figure 17. P wave changes of left atrial enlargement are seen in Lead II. The P wave duration is 0.14 seconds, with a notched peak and an inter-peak distance of about 0.06 seconds.  The PR interval is 0.20 seconds, with a rS configuration and a QRS duration of 0.16 seconds.

Figure 18. P wave changes of left atrial enlargement are seen in Lead V1. There is an inverted P wave with a width of about 0.06 seconds and a depth of 1 mm.

PR Segment Changes
Pericarditis can be associated with depression of the PR segment in the inferior and precordial leads, and elevation of the PR segment in Lead aVR.

Figure 19. ECG of 43 year old man with chest pain. There is ST elevation in Leads II, III, aVF and V6. PR segment depression is present in the inferior leads, and PR segment elevation is seen in Lead aVR

Figure 20. Lead aVR and Lead II from the ECG in Figure 19, with arrows showing the PR segment changes.

Figure 21. PR segment depression in Lead II in another patient with pericarditis

Figure 22. PR segment elevation in Lead aVR in the same patient in Figure 21.