Electrocardiogram — Revision Notes

⚡ 30-Second Revision

P Wave: — Atrial depolarization.
QRS Complex: — Ventricular depolarization.
T Wave: — Ventricular repolarization.
PR Interval: — Start of P to start of QRS. Normal: $0.12 - 0.20,\text{s}$. Reflects AV nodal delay.
ST Segment: — End of S to start of T. Isoelectric. Elevation/depression indicates ischemia/injury.
QT Interval: — Start of Q to end of T. Total ventricular electrical activity.
Heart Rate: — Normal $60-100,\text{bpm}$. Tachycardia $>100,\text{bpm}$. Bradycardia $<60,\text{bpm}$.
Atrial repolarization is masked by the QRS complex.

2-Minute Revision

The Electrocardiogram (ECG) graphically records the heart's electrical activity. This activity originates in the SA node, the heart's natural pacemaker, and spreads through a specialized conduction system.

The P wave represents atrial depolarization, leading to atrial contraction. The electrical impulse then travels to the AV node, where it's delayed (reflected in the PR interval, normally $0.12-0.20,\text{s}$), allowing atrial emptying.

Next, the impulse rapidly spreads through the Bundle of His, bundle branches, and Purkinje fibers, causing ventricular depolarization, which is depicted as the large QRS complex (normally $<0.12,\text{s}$).

This leads to ventricular contraction. Atrial repolarization occurs during ventricular depolarization and is thus masked by the QRS complex. Finally, the ventricles electrically recover, known as ventricular repolarization, which forms the T wave.

The ST segment, between the QRS and T wave, is crucial for detecting myocardial ischemia (ST depression) or injury (ST elevation). The overall heart rate and rhythm are derived from the regularity and frequency of these complexes.

5-Minute Revision

An Electrocardiogram (ECG) is a vital tool for assessing cardiac electrical function. It records the sum of action potentials from myocardial cells. The sequence begins with the SA node generating an impulse, causing atrial depolarization, which is seen as the P wave.

This electrical event precedes atrial contraction. The impulse then reaches the AV node, where a physiological delay occurs, represented by the PR interval (normal: $0.12-0.20,\text{s}$). A prolonged PR interval suggests an AV block.

From the AV node, the impulse travels rapidly through the Bundle of His, bundle branches, and Purkinje fibers to depolarize the ventricles. This rapid, powerful event creates the prominent QRS complex (normal: $<0.

12, ext{s}$), which precedes ventricular contraction. A widened QRS can indicate a bundle branch block. Notably, atrial repolarization occurs during the QRS complex and is typically masked by its larger electrical signal.

Following ventricular depolarization, the ventricles undergo electrical recovery, known as ventricular repolarization, which is represented by the T wave. The ST segment, the flat line between the QRS and T wave, is crucial for diagnosing myocardial ischemia (ST depression) or acute myocardial infarction (ST elevation).

The QT interval measures the total duration of ventricular electrical activity. Analyzing the rate, rhythm, and morphology of these waves and intervals allows for the diagnosis of arrhythmias, conduction abnormalities, and myocardial damage.

For example, a heart rate consistently above $100,\text{bpm}$ is tachycardia, while below $60,\text{bpm}$ is bradycardia. Understanding these components is fundamental for NEET.

Prelims Revision Notes

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ECG Definition: — Graphical recording of heart's electrical activity (depolarization and repolarization).
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Conduction Pathway: — SA node (pacemaker) $ightarrow$ Atria $ightarrow$ AV node (delay) $ightarrow$ Bundle of His $ightarrow$ Bundle Branches $ightarrow$ Purkinje fibers $ightarrow$ Ventricles.
3
P Wave:

* Represents: Atrial depolarization. * Precedes: Atrial contraction. * Normal: Small, rounded, upright.

1
PR Interval:

* Represents: Time from start of atrial depolarization to start of ventricular depolarization. * Includes: Atrial conduction, AV nodal delay, His-Purkinje conduction. * Normal Duration: $0.12 - 0.20,\text{s}$ (3-5 small squares on ECG paper). * Clinical Significance: Prolonged PR indicates AV block.

1
QRS Complex:

* Represents: Ventricular depolarization. * Precedes: Ventricular contraction. * Normal Duration: $<0.12,\text{s}$ (less than 3 small squares). * Components: Q (first negative), R (first positive), S (negative after R). * Clinical Significance: Widened QRS indicates bundle branch block or ventricular hypertrophy; pathological Q wave indicates past MI.

1
ST Segment:

* Represents: Isoelectric period when ventricles are fully depolarized (plateau phase). * Normal: Flat, at baseline. * Clinical Significance: Elevation (acute MI/injury); Depression (ischemia).

1
T Wave:

* Represents: Ventricular repolarization. * Normal: Rounded, usually upright, broader than P wave. * Clinical Significance: Inverted T waves (ischemia); peaked T waves (hyperkalemia).

1
QT Interval:

* Represents: Total time for ventricular depolarization and repolarization. * Normal: Varies with heart rate (corrected QT, QTc, is often used). * Clinical Significance: Prolonged QT interval increases risk of arrhythmias.

1
Atrial Repolarization: — Occurs during QRS complex, masked by its larger electrical activity, hence not visible.
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Heart Rate Calculation: — Count R-R intervals in 6 seconds, multiply by 10. Or, $300 / \text{number of large squares between R-R}$.
3
Normal Heart Rate: — $60-100,\text{bpm}$.
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Tachycardia: — Heart rate $>100,\text{bpm}$.
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Bradycardia: — Heart rate $<60,\text{bpm}$.

Vyyuha Quick Recall

Pump Quietly, Really Slowly, Then Recover.

Pump: P wave = Atrial Pumping (depolarization).
Quietly, Really Slowly: QRS complex = Ventricular Quick Rush Systole (depolarization).
Then Recover: T wave = Ventricular Recovery (repolarization).

(The 'Quietly, Really Slowly' part is a bit of a stretch for QRS, but the 'P' for pumping and 'T' for recovery are strong associations.)