Causes of Biphasic P Wave
A biphasic P wave is most commonly caused by left atrial abnormality, which results from delayed left atrial activation creating temporal separation between right and left atrial depolarization peaks. 1, 2
Primary Mechanism: Left Atrial Abnormality
The biphasic P wave morphology—particularly the characteristic notched or "M-shaped" appearance—occurs when left atrial activation is delayed relative to right atrial activation. 1 This temporal separation causes the normally fused right and left atrial peaks to become distinct, creating the biphasic pattern. 2
Specific Pathophysiological Contributors
Conduction delay is the dominant mechanism, specifically:
- Delay in Bachmann's bundle (the specialized interatrial pathway) is the most common cause 1, 2
- Intraatrial conduction delay within the left atrial myocardium itself 1
- These conduction abnormalities prolong total atrial activation time, manifesting as P-wave duration ≥120 ms 1, 2
Elevated left atrial pressure independently affects P-wave morphology:
- Isolated or combined left atrial pressure elevation is strongly associated with abnormal (type 2) P-wave morphology 3
- In contrast, isolated right atrial pressure elevation typically maintains normal P-wave morphology 3
Left atrial myocardial hypertrophy (wall thickening) has distinct effects:
- Consistently increases P-wave terminal force in lead V1 (PTF-V1) through enlarged amplitude 4
- Does not significantly affect P-wave duration 4
- This contrasts with pure dilation, which has variable and inconsistent effects on P-wave criteria 4
Anatomical and Structural Factors
Multiple structural changes contribute, though their effects cannot always be distinguished:
Important caveat: The term "left atrial abnormality" is preferred over older terminology like "left atrial enlargement" or "atrial hypertrophy" because P-wave changes reflect a combination of factors that may appear together and are not individually distinguishable. 1, 2
Anatomical Variability Effects
The site of earliest right atrial activation significantly influences biphasic P-wave characteristics:
- Mid-septal earliest activation sites yield the highest PTF-V1 5
- More anterior/superior or inferior sites can alter PTF-V1 by up to a factor of 2.0 5
- The intactness of posterior interatrial connections affects PTF-V1 by up to 150% for inferior activation sites 5
Specific Clinical Conditions
Underlying cardiac diseases that commonly cause biphasic P waves include:
Post-surgical changes: Atrial compartment operations create characteristic biphasic P waves with initial positive and terminal slurred negative deflections, particularly in leads II, III, and aVF, due to altered conduction pathways. 6
Diagnostic Correlation
The biphasic P wave in lead V1 (increased P terminal force) correlates strongly with:
- P-wave duration ≥120 ms (interatrial block) 7
- These two findings are significantly correlated (P ≤ 0.001) and should be expected together in the majority of cases 7
Clinical Pitfall to Avoid
Do not assume that a biphasic P wave always indicates left atrial enlargement by size alone—computational modeling demonstrates that isolated left atrial dilation has only moderate and variable effects on P-wave criteria. 4 The biphasic pattern more reliably indicates conduction delay or myocardial hypertrophy rather than pure chamber enlargement. 1, 4