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Notching in electrocardiography
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Notching in electrocardiography refers to the presence of distinct deflections or irregularities in the waveform of an electrocardiogram (ECG or EKG), particularly within the P wave, QRS complex (fragmented QRS (fQRS)), or T wave.[1] These notches appear as abrupt changes in the direction or slope of the waveform and can provide critical diagnostic information about cardiac conditions.[2][3]
Notching in different components of the ECG waveform is associated with various cardiac conditions, ranging from benign variants to serious pathologies, such as conduction delays, atrial fibrillation,[4] myocardial ischemia,[5] or structural heart disease ('crochetage sign' in atrial septal defect (ASD)).[3][6][7][8][9][10]

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Definition, characteristics
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Notching is identified as an abrupt change in the direction of an ECG waveform, resulting in a "notch" or dip that creates a bimodal or M-shaped appearance. It is distinct from slurring, which involves a smooth transition or slowing in the slope of the waveform without a clear change in direction.[11] Notching can occur in the following ECG components:

- P Wave notching: A notched P wave typically appears as a double-peaked or M-shaped wave in lead II, often reflecting delayed atrial conduction or left atrial enlargement. A notched P wave is defined by a peak-to-peak distance of ≥20 ms or ≥40 ms, depending on the diagnostic criteria used.[12][13]
- QRS complex notching: Notching in the QRS complex is characterized by additional deflections or peaks within the QRS waveform, often in the form of multiple R waves or notches in the R or S waves. It is commonly associated with conduction abnormalities, such as bundle branch blocks, early repolarization. It is also associated with cardiac resynchronization therapy, atrial septal defect, myocardial infarction, or myocardial scarring.[14]
- T wave notching: A notched T wave appears as a biphasic or double-peaked T wave, often linked to conditions like long QT syndrome type 2 (LQTS2) or electrolyte imbalances.[15]
Notching is typically assessed using a standard 12-lead ECG, with modern systems employing digital analysis at high sampling rates (e.g., 500 Hz) to detect subtle notches that may be missed in conventional recordings. High-frequency ECG techniques can enhance the visibility of notching, particularly in the QRS complex.
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Mechanisms
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Notching in ECG waveforms arises from disruptions in the normal sequence of cardiac depolarization or repolarization. Specific mechanisms include:
- Atrial conduction delay: Notched P waves result from delayed or asynchronous atrial activation, often due to left atrial enlargement or fibrosis, which slows intra-atrial conduction.[16]
- Ventricular conduction abnormalities: QRS notching in bundle branch blocks is caused by delayed conduction through the Purkinje system, leading to asynchronous ventricular depolarization.[17] In LBBB, the left ventricular lateral wall is depolarized last, producing notching in lateral leads.[18]
- Myocardial scarring: Notching in ischemic heart disease reflects local conduction delays due to myocardial scarring or fibrosis, altering the QRS contour. HF ECGs can detect subtle notches obscured in standard recordings.[19]
- Ion channel dysfunction: T-wave notching in LQTS2 is linked to reduced potassium currents (e.g., IKr), which prolong repolarization and create a biphasic T-wave morphology.[20]
- Structural heart disease: Conditions like ASD cause notching (e.g., crochetage sign) due to altered ventricular activation patterns secondary to volume overload.[21]
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References
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