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Conceptual Overview

The P wave represents atrial depolarization - the electrical activation of both atria. In normal sinus rhythm, the impulse originates from the sinoatrial (SA) node in the high right atrium and spreads through the atrial myocardium to reach the AV node.

Understanding P wave morphology is fundamental to rhythm interpretation. The shape, size, and direction of the P wave in different leads reveals:

  • Origin of impulse: Sinus node, ectopic atrial focus, or AV junction
  • Atrial chamber abnormalities: Enlargement or hypertrophy
  • Conduction pathways: Normal interatrial conduction or abnormal patterns
  • Rhythm classification: Supraventricular vs ventricular

The Atrial Waveform - Understanding P Wave Formation

Atrial depolarization proceeds sequentially from right to left:

  • The right atrium is activated before the left atrium
  • The first 1/3 of the P wave corresponds to right atrial activation
  • The final 1/3 corresponds to left atrial activation
  • The middle 1/3 is a combination of both atrial waveforms

In most leads (e.g., lead II), the right and left atrial waveforms move in the same direction, forming a monophasic P wave. However, in lead V1, the waveforms move in opposite directions, producing a biphasic P wave with:

  • Initial positive deflection = right atrial activation
  • Subsequent negative deflection = left atrial activation
Key concept: The P wave is the "signature" of atrial activity. No P wave or abnormal P waves change your entire differential diagnosis for the rhythm. Lead V1's biphasic nature makes it the best lead for identifying individual atrial abnormalities.
Normal P wave formation showing sequential right-to-left atrial depolarization
Figure 1: Normal P wave formation in lead II. The right atrial depolarization wave precedes the left atrium, combining to form a monophasic P wave <0.12 seconds wide and <2.5 mm tall.
Normal P Wave Characteristics

Sinus P Wave Criteria

  • Upright in leads I, II, and aVF: Indicates depolarization traveling inferiorly and leftward from SA node
  • Inverted in aVR: As impulse travels away from right shoulder
  • Variable in III, aVL: Depending on heart axis
  • Biphasic in V1: Initial positive deflection (right atrium) followed by negative (left atrium)
  • Upright in V2-V6: Anterior and leftward spread

P Wave Axis

  • Normal P wave axis: 0° to +75°
  • Clinical significance: P wave axis outside this range suggests an ectopic atrial rhythm (impulse not originating from SA node)
  • Quick check: P wave should be upright in leads I and II, inverted in aVR

Morphology Patterns

  • Smooth contour: Normal P waves have a rounded, smooth appearance
  • Monophasic in lead II: Single positive deflection
  • Biphasic in V1: Initial positive (RA) + terminal negative (LA) deflections of similar size
Normal biphasic P wave morphology in lead V1
Figure 2: Normal P wave morphology in lead V1. Note the biphasic pattern with initial positive deflection (right atrial activation) followed by terminal negative deflection (left atrial activation), both components approximately equal in size.
P Wave Abnormalities

Right Atrial Enlargement (P Pulmonale)

In right atrial enlargement, right atrial depolarization lasts longer than normal and its waveform extends to the end of left atrial depolarization. Although the amplitude of the right atrial current remains unchanged, its peak now falls on top of the left atrial wave, producing a taller P wave while width remains normal.

  • Lead II/III/aVF criteria: P wave amplitude >2.5 mm (tall, peaked)
  • Lead V1 criteria: Increased height (>1.5 mm) of the initial positive deflection
  • Duration: Remains normal (<0.12 seconds)
  • Mechanism: Increased right atrial muscle mass and delayed RA depolarization
  • Morphology: Tall, peaked, "Gothic arch" P waves best seen in inferior leads

Common causes:

  • Pulmonary hypertension (any cause)
  • Chronic obstructive pulmonary disease (COPD)
  • Cor pulmonale
  • Pulmonary embolism (acute or chronic)
  • Tricuspid stenosis or regurgitation
  • Pulmonary stenosis
  • Congenital heart disease with right-sided volume/pressure overload
ECG strip showing P pulmonale
Figure 3: P pulmonale (right atrial enlargement) on ECG. Note the tall, peaked P waves >2.5 mm in the inferior leads, characteristic of right atrial hypertrophy. Duration remains normal.
Ectopic P Waves & Non-Sinus Rhythms

Identifying Ectopic Atrial Activity

When the P wave doesn't match normal sinus morphology, consider an ectopic atrial rhythm:

Swipe to see more
P Wave Finding Rhythm Interpretation
Inverted P in II, III, aVF; upright in aVR Low atrial or junctional rhythm (retrograde atrial activation)
Varying P wave morphology (≥3 forms) Multifocal atrial tachycardia (MAT) or wandering atrial pacemaker
Sawtooth flutter waves Atrial flutter (typically 2:1, 3:1, or variable block)
Irregular, chaotic baseline without discrete P waves Atrial fibrillation
No P waves, narrow QRS Junctional rhythm or atrial fibrillation with controlled rate
P waves following QRS (retrograde) Junctional tachycardia or AVNRT

Premature Atrial Complexes (PACs)

  • P wave morphology: Different from sinus P waves; shape depends on ectopic focus location
  • Timing: Occurs earlier than expected sinus P
  • PR interval: May be shorter, normal, or longer than sinus PR; very premature PACs may not conduct (blocked PAC)
  • QRS: Typically narrow and identical to sinus beats; aberrantly conducted PACs can be wide
ECG showing ectopic atrial rhythm
Figure 8: Ectopic atrial rhythm. Note the inverted P waves in the inferior leads (II, III, aVF) with PR interval ≥120 ms, indicating an atrial origin below the SA node but above the AV junction.
Quick Reference
  • Normal P wave duration: <0.12 seconds (less than 3 small boxes)
  • Normal P wave amplitude: <2.5 mm in limb leads
  • Normal sinus P wave: Upright in leads I, II, aVF; inverted in aVR
  • P wave represents: Atrial depolarization from SA node through atrial myocardium
  • First half: Right atrial activation
  • Second half: Left atrial activation
  • Key pearl: P wave morphology tells you where the impulse originated
Clinical Pearls
"No P wave, no problem... sometimes": Absence of clear P waves can indicate atrial fibrillation, junctional rhythm, or artifact obscuring P waves. Context matters - irregular narrow-complex tachycardia without P waves = Afib; regular narrow complex without P = junctional.
"V1 is your friend": Lead V1 is the best lead to evaluate atrial activity. Its proximity to the atria makes P waves and flutter waves most prominent here. When in doubt about the rhythm, examine V1 carefully or use a Lewis lead. The biphasic nature of V1 allows you to distinguish between right and left atrial abnormalities that might be masked in other leads.
"The Morris Index": For left atrial enlargement in V1, the terminal negative deflection must be both deep (≥1mm) AND wide (≥0.04 sec). Calculate: width (sec) × depth (mm) ≥ 0.04 mm-sec. This specificity helps avoid false positives from normal variants.
"P wave axis unlocks ectopic rhythms": A negative P in lead II (where it should be positive) immediately tells you the impulse is NOT from the SA node - think low atrial or junctional origin.
Don't miss flutter waves: Atrial flutter can be subtle, especially with 2:1 conduction at rates around 150 bpm. The "flutter" portion may hide in the T wave or QRS. Use vagal maneuvers or calipers to unmask the rhythm.
Biatrial enlargement = advanced disease: When you see both tall AND wide P waves, think severe, chronic structural heart disease. These patients often have significant valvular pathology or cardiomyopathy.
References
  1. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
  2. Khan, M. G. (2007). Rapid ECG Interpretation. Humana.
  3. Sigg, D. C., Iaizzo, P. A., Xiao, Y.-F., Bin He, & Springerlink (Online Service). (2010). Cardiac Electrophysiology Methods and Models. Springer Us.
  4. Wang, K. (2012). Atlas of Electrocardiography. JP Medical Ltd.
  5. ECG Library • LITFL • ECG Library Basics. (2018). Life in the Fast Lane • LITFL • Medical Blog. https://litfl.com/ecg-library/
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