Electrocardiography (ECG): A Detailed Overview

I. Introduction to Electrocardiography

An Electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the electrical activity of the heart over a period of time. It detects the tiny electrical changes on the skin that arise from the heart muscle's electrophysiological pattern of depolarization and repolarization during each heartbeat. The ECG provides valuable information about the heart's rhythm, rate, and the condition of its conduction system and muscle.

II. The Cardiac Electrical Cycle: Depolarization and Repolarization

To understand the ECG, it's crucial to grasp the basic electrical events within cardiac cells:

1. Depolarization: This is the electrical activation of cardiac cells, caused by the rapid influx of positively charged ions (primarily Na+ and Ca2+) into the cell. This makes the inside of the cell more positive, leading to muscle contraction (systole).
2. Repolarization: This is the electrical recovery of cardiac cells, caused by the efflux of positively charged ions (primarily K+) out of the cell. This restores the negative charge inside the cell, leading to muscle relaxation (diastole).

These cellular electrical events, when coordinated across millions of heart cells, generate electrical currents that can be detected on the body surface by electrodes.

III. The Heart's Conduction System

The heart's electrical activity originates and propagates through a specialized conduction system:

                                  SA Node (Pacemaker)
                                       │
                                       ▼
                                 Atrial Pathways
                                       │
                                       ▼
                                  AV Node (Delay)
                                       │
                                       ▼
                                Bundle of His
                                       │
                     ┌─────────────────┴─────────────────┐
                     ▼                                   ▼
               Left Bundle Branch                  Right Bundle Branch
                     │                                   │
                     ▼                                   ▼
                 Purkinje Fibers                     Purkinje Fibers
                     │                                   │
                     ▼                                   ▼
                 Ventricular Myocardium

• Sinoatrial (SA) Node: The primary pacemaker, initiating the electrical impulse that spreads across the atria.
• Atrioventricular (AV) Node: Delays the impulse, allowing atrial contraction to complete before ventricular contraction begins.
• Bundle of His, Bundle Branches, Purkinje Fibers: Rapidly conduct the impulse throughout the ventricles, ensuring synchronized ventricular contraction.

IV. Basic ECG Waveforms, Segments, and Intervals

A typical ECG tracing consists of several waves, segments, and intervals, each representing a specific electrical event in the cardiac cycle.

      P Wave   PR Segment   QRS Complex   ST Segment   T Wave
     ┌───┐   ┌───────────┐ ┌─┬─┐ ┌───────────┐ ┌───┐
     │   │   │           │ │ │ │ │           │ │   │
     │   │   │           │ │ │ │ │           │ │   │
     └─┬─┘───┴───────────┴─┘ └─┘─┴───────────┴─┘ └─┬─┘
       │                     │                     │
       └─────────────────────┴─────────────────────┘
             PR Interval           QT Interval

A. Waves

1. P Wave:

   • Represents: Atrial depolarization (electrical activation of the atria, leading to atrial contraction).
   • Normal: Small, rounded, upright deflection.
   • Why: The SA node fires, and the electrical impulse spreads through the atrial muscle. This is a relatively small muscle mass, so the electrical signal is small.

2. QRS Complex:

   • Represents: Ventricular depolarization (electrical activation of the ventricles, leading to ventricular contraction).
   • Normal: A rapid, large deflection. The Q wave is the first negative deflection, R is the first positive, and S is the negative deflection after R.
   • Why: The impulse rapidly spreads through the large ventricular muscle mass via the Purkinje fibers, generating a strong electrical signal. Atrial repolarization also occurs during this time but is masked by the larger ventricular depolarization.

3. T Wave:

   • Represents: Ventricular repolarization (electrical recovery of the ventricles).
   • Normal: Upright, rounded, and slightly asymmetrical.
   • Why: The ventricular muscle cells are repolarizing. This is a slower process than depolarization, resulting in a broader wave.

4. U Wave (Optional):

   • Represents: Thought to be repolarization of papillary muscles or Purkinje fibers, or after-depolarizations.
   • Normal: Small, positive wave following the T wave, often not visible.

B. Segments

1. PR Segment:

   • Represents: The delay of the electrical impulse at the AV node.
   • Normal: Isoelectric (flat line).
   • Why: The impulse is held briefly at the AV node to allow the atria to fully contract and empty blood into the ventricles before ventricular contraction begins.

2. ST Segment:

   • Represents: The period between ventricular depolarization and repolarization (the plateau phase of the ventricular action potential).
   • Normal: Isoelectric.
   • Why: During this phase, the ventricular muscle cells are uniformly depolarized, and there is no net electrical current flowing, hence a flat line.

C. Intervals

1. PR Interval:

   • Represents: Time from the beginning of atrial depolarization to the beginning of ventricular depolarization.
   • Normal: 0.12 - 0.20 seconds (3-5 small squares).
   • Why: Includes the P wave and the PR segment, reflecting the time it takes for the impulse to travel from the SA node through the atria and AV node to the ventricles.

2. QT Interval:

   • Represents: Total time for ventricular depolarization and repolarization.
   • Normal: Varies with heart rate, typically < 0.44 seconds.
   • Why: Reflects the entire electrical activity of the ventricles, from the start of contraction to the end of relaxation.

V. ECG Anomalies: What They Mean and Why They Occur

Changes in the ECG waveforms, segments, and intervals can indicate various cardiac abnormalities, often linked to specific diseases or physiological changes.

A. P Wave Abnormalities

B. PR Interval Abnormalities

C. QRS Complex Abnormalities

D. ST Segment Abnormalities

E. T Wave Abnormalities

F. QT Interval Abnormalities

VI. Conclusion

The ECG is an indispensable tool in cardiology, offering a window into the heart's electrical health. By understanding the normal physiological basis of each wave and interval, and recognizing the deviations from normal, clinicians can diagnose a wide array of cardiac conditions, from subtle rhythm disturbances to acute myocardial infarctions. The changes observed on an ECG are direct manifestations of underlying cellular electrical events (depolarization and repolarization) and the structural or functional integrity of the heart's conduction system and muscle. This detailed understanding is critical for accurate diagnosis and appropriate patient management.