The electrical activity of the heart, as recorded on an electrocardiogram (ECG or EKG), is a critical diagnostic tool used to assess the heart's rhythm and electrical conduction system. The ECG captures the electrical impulses generated by the heart as it beats, providing a visual representation of the heart's electrical activity over time. This activity is divided into distinct phases, each corresponding to specific events in the cardiac cycle. Below is a detailed explanation of the electrical activity of the heart as recorded on an ECG.

1. Overview of the Heart's Electrical System

The heart's electrical system is responsible for initiating and coordinating the contraction of the heart muscle, ensuring efficient pumping of blood throughout the body. This system consists of specialized cells that generate and conduct electrical impulses. The key components include:

• Sinoatrial (SA) node: The natural pacemaker of the heart, located in the right atrium. It initiates the electrical impulse.
• Atrioventricular (AV) node: Located between the atria and ventricles, it delays the impulse to allow the atria to contract before the ventricles.
• Bundle of His and Purkinje fibers: Conduct the impulse rapidly through the ventricles, ensuring coordinated contraction.

The ECG records the summation of these electrical events as they propagate through the heart.

2. The ECG Waveform

The ECG waveform is composed of several distinct components, each representing a specific phase of the heart's electrical activity. These components are labeled as P, Q, R, S, T, and sometimes U waves. Below is a breakdown of each:

P Wave

• What it represents: The P wave corresponds to the depolarization (electrical activation) of the atria. This occurs when the SA node fires, causing the atria to contract and push blood into the ventricles.
• Shape and duration: The P wave is typically small and rounded, lasting about 0.08–0.12 seconds.

PR Interval

• What it represents: The PR interval measures the time from the onset of atrial depolarization (P wave) to the onset of ventricular depolarization (QRS complex). It includes the delay at the AV node, allowing the atria to fully contract before the ventricles begin to depolarize.
• Duration: Normally 0.12–0.20 seconds.

QRS Complex

• What it represents: The QRS complex corresponds to the depolarization of the ventricles. This is the most prominent part of the ECG, as the ventricles have a larger muscle mass and generate a stronger electrical signal.
  • Q wave: The first downward deflection, representing depolarization of the interventricular septum.
  • R wave: The first upward deflection, representing depolarization of the main ventricular mass.
  • S wave: The downward deflection following the R wave, representing the final phase of ventricular depolarization.
• Duration: Normally 0.06–0.10 seconds.

ST Segment

• What it represents: The ST segment corresponds to the period when the ventricles are fully depolarized and contracting. It is an isoelectric (flat) line between the end of the QRS complex and the beginning of the T wave.
• Clinical significance: Elevation or depression of the ST segment can indicate myocardial ischemia or infarction.

T Wave

• What it represents: The T wave corresponds to the repolarization (recovery) of the ventricles. This is when the ventricles reset their electrical state in preparation for the next heartbeat.
• Shape and duration: The T wave is typically rounded and asymmetrical, lasting about 0.10–0.25 seconds.

QT Interval

• What it represents: The QT interval measures the total time for ventricular depolarization and repolarization. It extends from the beginning of the QRS complex to the end of the T wave.
• Duration: Varies with heart rate but is typically 0.36–0.44 seconds.

U Wave (if present)

• What it represents: The U wave is a small, often subtle wave that may follow the T wave. Its exact origin is debated, but it is thought to represent repolarization of the Purkinje fibers or late ventricular repolarization.
• Clinical significance: Prominent U waves can be associated with electrolyte imbalances or certain medications.

3. The Cardiac Cycle and ECG Correlation

The ECG provides a snapshot of the heart's electrical activity during one complete cardiac cycle, which includes:

1. Atrial depolarization (P wave): The atria contract, pushing blood into the ventricles.
2. Ventricular depolarization (QRS complex): The ventricles contract, pumping blood to the lungs and body.
3. Ventricular repolarization (T wave): The ventricles relax and reset for the next cycle.

The ECG does not directly show atrial repolarization, as it is overshadowed by the larger QRS complex.

4. Clinical Significance of ECG

The ECG is a vital tool for diagnosing a wide range of cardiac conditions, including:

• Arrhythmias: Abnormal heart rhythms, such as atrial fibrillation, ventricular tachycardia, or bradycardia.
• Ischemia and infarction: Changes in the ST segment or T wave can indicate reduced blood flow to the heart muscle.
• Electrolyte imbalances: Abnormalities in the QT interval or T wave morphology can signal issues with potassium, calcium, or magnesium levels.
• Structural abnormalities: Enlarged atria or ventricles can alter the shape and amplitude of the P wave or QRS complex.

5. Limitations of the ECG

While the ECG is a powerful diagnostic tool, it has some limitations:

• It provides only a brief snapshot of the heart's electrical activity and may miss transient abnormalities.
• It does not directly measure mechanical function (e.g., how well the heart is pumping blood).
• Some conditions, such as intermittent arrhythmias, may require continuous monitoring (e.g., Holter monitor) for detection.

6. Conclusion

The ECG is a fundamental tool for assessing the heart's electrical activity, providing critical insights into its rhythm and function. By analyzing the P wave, QRS complex, T wave, and other components, clinicians can diagnose a wide range of cardiac conditions and guide appropriate treatment. Understanding the electrical activity of the heart as recorded on the ECG is essential for both medical professionals and students of cardiology.