Electrocardiogram (ECG)

Definition and Purpose

The Electrocardiogram (ECG or EKG) is an instrument that measures the electrical activity of the heart using electrodes placed on the body. The recording of cardiac electrical activity provides valuable information about heart function and rhythm.

• Depolarization: The process by which cardiac muscle cells lose their electrical charge, leading to contraction.

• Repolarization: The process by which cardiac muscle cells restore their electrical charge, leading to relaxation.

• ECGs measure depolarization and repolarization of the atria and ventricles.

ECG Wave Components

An ECG tracing consists of several distinct waves and intervals, each representing specific electrical events in the heart:

• P Wave: Represents depolarization of the SA node and atria.

• P-R Interval: Time between atrial depolarization and ventricular depolarization; includes AV node delay.

• QRS Complex: Represents ventricular depolarization; atrial repolarization also occurs but is masked by the QRS complex.

• S-T Segment: Represents the plateau phase of ventricular contraction.

• T Wave: Represents ventricular repolarization.

Example: On an ECG, the P wave precedes the QRS complex, which is followed by the T wave. The intervals and segments help in diagnosing cardiac conduction abnormalities.

ECG Analysis and Interpretation

Analyzing an ECG involves identifying the waves and intervals and correlating them with cardiac events:

• Label the heart contraction with the letters P, Q, R, S, and T.

• Box the sections that measure activity of the ventricles (QRS complex and T wave).

• Circle the sections that measure activity of the atria (P wave).

• Count the number of times ventricular repolarization (T wave) is observed in a given readout.

Physiological Events in ECG Intervals

Different intervals on the ECG correspond to specific physiological events:

• The interval from the beginning of the QRS complex to the end of the T wave includes ventricular depolarization and repolarization.

• A prolonged PR interval may indicate delayed conduction through the AV node.

Electrocardiogram: Pathologies

Arrhythmias

Arrhythmias are abnormal heart rhythms detected by ECGs. They can be classified based on heart rate and rhythm:

• Bradycardia: Resting heart rate below 60 bpm (beats per minute).

• Tachycardia: Resting heart rate above 100 bpm.

• Fibrillation: Uncoordinated contraction; can be atrial or ventricular. Ventricular fibrillation is often fatal if not immediately treated.

• Heart Block: Disrupted signal due to issues with the heart's conduction circuit.

Example: A normal ECG shows regular P waves, QRS complexes, and T waves. Pathological ECGs may show missing or abnormal waves, irregular intervals, or erratic patterns.

ECG Pathology Table

The following table summarizes normal and pathological ECGs and the failing part of the conduction system:

Additional info: The table above is inferred from the provided example and typical ECG pathology analysis.

Fibrillation Types

Fibrillation can occur in either the atria or ventricles:

• Atrial fibrillation: Irregular, often rapid heart rhythm originating in the atria.

• Ventricular fibrillation: Disorganized electrical activity in the ventricles; life-threatening and requires immediate intervention.

Example: On ECG, ventricular fibrillation appears as rapid, erratic, and uncoordinated electrical activity without distinct QRS complexes.

Clinical Application

ECGs are used in clinical settings to diagnose heart rate abnormalities and conduction defects:

• Athletes with unusually low heart rates may be described as having bradycardia.

• Abnormal ECG patterns can indicate fibrillation, heart block, or tachycardia.

Example: Doctors may use ECG findings to determine if a patient requires further cardiac evaluation or emergency intervention.

Key Terms and Definitions

• Depolarization: Loss of electrical charge across the cell membrane, leading to muscle contraction.

• Repolarization: Restoration of electrical charge, leading to muscle relaxation.

• Arrhythmia: Abnormal heart rhythm.

• Bradycardia: Slow heart rate.

• Tachycardia: Fast heart rate.

• Fibrillation: Uncoordinated contraction of heart muscle.

• Heart Block: Disruption in the heart's electrical conduction system.

Formulas and Equations

• Heart Rate Calculation:

Additional info: The heart rate formula is commonly used in ECG analysis to determine beats per minute from the R-R interval.