Electrocardiography

Electrocardiography is the process of recording the electrical activity of the heart over a period of time using electrodes placed on the skin surface. It provides a non-invasive means to assess the heart's rhythm, conduction, and myocardial integrity by capturing the composite electrical signals generated by cardiac muscle depolarization and repolarization. These signals are graphically represented as an electrocardiogram (ECG), which forms the basis for diagnosing a wide range of cardiac disorders.

Origin of Body-Surface Cardiac Potentials

The electrical activity of the heart originates from the orderly depolarization and repolarization of cardiac myocytes. Each heartbeat generates electrical currents that propagate through the myocardial tissue and spread through the conductive fluids and tissues of the body, including blood, lungs, muscles, and skin, before reaching the body surface. The resultant potentials detected at the skin are the summation of multiple dipolar sources within the heart, modulated by the volume conduction properties of the torso.

Cardiac Electrical Sources and Equivalent Dipoles

The fundamental cardiac electrical event is the action potential generated by individual cardiac cells. When these cells depolarize synchronously, they form a wavefront of electrical activity that can be represented as an equivalent dipole vector. This vector has both magnitude and direction, reflecting the net electrical charge separation and its orientation in three-dimensional space at any instant. The heart’s electrical activity can be modeled as a time-varying dipole whose projection onto various lead axes produces the signals recorded in the ECG.

Volume Conduction Through the Torso

The torso acts as a volume conductor, transmitting the cardiac electrical signals from the heart to the skin surface electrodes. This conduction is influenced by the heterogeneous electrical properties of different tissues, including muscle, fat, bone, and lungs. The body’s volume conductor properties attenuate and spatially disperse the electrical signals, affecting the amplitude and morphology of the recorded potentials. Understanding torso volume conduction is essential for accurate interpretation and lead placement in electrocardiography.

Electrocardiographic Leads and Reference Systems

Electrocardiographic leads are specific electrode configurations that record the electrical potential differences between two or more points on the body surface. Common lead systems include the standard limb leads (I, II, III), augmented limb leads (aVR, aVL, aVF), and precordial (chest) leads (V1-V6). These leads provide different spatial perspectives of the heart’s electrical activity, enabling comprehensive assessment of cardiac function. The reference system, often the Wilson central terminal, serves as a baseline potential for unipolar leads.

Lead Vectors and Electrical Projection

Each ECG lead can be conceptualized as a vector projection of the heart’s electrical dipole onto a specific axis defined by the electrode placement. The recorded waveform amplitude and polarity depend on the angle between the cardiac dipole vector and the lead vector. When the dipole vector aligns with the lead axis, maximal positive deflection is recorded; when it opposes the axis, a negative deflection occurs. This vector projection principle underlies the interpretation of waveforms and electrical axis determination.

Cardiac Electrical Axis

The cardiac electrical axis represents the predominant direction of ventricular depolarization in the frontal plane. It is determined by analyzing the net QRS complex vector across limb leads. The axis provides information about the orientation of the heart and can indicate pathological conditions such as ventricular hypertrophy, conduction blocks, or myocardial infarction. Normal axis ranges from approximately -30° to +90°, with deviations classified as left or right axis deviation.

P Wave

The P wave corresponds to atrial depolarization, representing the electrical activation of the atria. It is typically a small, positive deflection preceding the QRS complex. The morphology, duration, and amplitude of the P wave provide information about atrial size, conduction pathways, and potential atrial abnormalities such as enlargement or atrial arrhythmias.

Atrioventricular Conduction in the Electrocardiogram

The atrioventricular (AV) conduction system transmits electrical impulses from the atria to the ventricles. The PR interval on the ECG reflects the time from atrial depolarization onset to ventricular depolarization onset, encompassing AV nodal delay. Abnormalities in AV conduction manifest as prolonged PR intervals (first-degree AV block), dropped beats (second-degree block), or complete dissociation (third-degree block), each with distinct ECG patterns.

QRS Complex

The QRS complex represents ventricular depolarization, a rapid electrical activation of the ventricular myocardium. It consists of three components: Q (initial negative deflection), R (positive deflection), and S (subsequent negative deflection). The duration, amplitude, and morphology of the QRS complex reflect the integrity of ventricular conduction pathways and myocardial mass. Abnormalities can indicate bundle branch blocks, ventricular hypertrophy, or infarction.

ST Segment

The ST segment follows the QRS complex and corresponds to the early phase of ventricular repolarization when the ventricles are electrically neutral. Normally isoelectric, deviations in the ST segment can indicate myocardial ischemia or injury. Elevation or depression of the ST segment relative to baseline are key diagnostic markers in acute coronary syndromes and other myocardial pathologies.

T Wave and Ventricular Repolarization

The T wave represents the process of ventricular repolarization, where ventricular myocytes return to their resting electrical state. Its shape, amplitude, and polarity provide insights into the repolarization sequence and can be altered in electrolyte disturbances, ischemia, or ventricular hypertrophy. Abnormal T wave morphology often precedes or accompanies cardiac pathology.

QT Interval and Ventricular Electrical Duration

The QT interval encompasses the total time for ventricular depolarization and repolarization, measured from the beginning of the QRS complex to the end of the T wave. It reflects the duration of ventricular electrical systole. Prolongation or shortening of the QT interval can predispose to arrhythmias such as torsades de pointes and is influenced by heart rate, medications, and electrolyte imbalances.

Activation-Recovery Relationships in the ECG

The temporal relationship between ventricular activation (depolarization) and recovery (repolarization) governs the electrophysiological stability of the myocardium. The ECG reflects this through the sequence and timing of QRS complexes and T waves. Disruptions in activation-recovery patterns can lead to heterogeneity in refractoriness, promoting arrhythmogenesis.

Electrocardiographic Amplitude and Morphology

Amplitude and morphology of ECG waveforms depend on the underlying electrical activity, electrode placement, and tissue conduction properties. Variations in size and shape of waves and complexes help differentiate normal from pathological states. Precise assessment of waveform characteristics is essential for accurate diagnosis of conduction abnormalities, chamber enlargement, and ischemic changes.

Electrocardiographic Acquisition and Sampling

Accurate ECG recording requires appropriate electrode placement, high-fidelity amplifiers, and adequate sampling rates to capture the fast-changing cardiac signals. Modern digital ECG systems sample signals at rates typically exceeding 500 samples per second to prevent loss of information and enable detailed waveform analysis. Signal quality depends on skin preparation, electrode-skin contact, and minimization of artifacts.

ECG Filtering, Noise, and Artifacts

Electrocardiographic signals are often contaminated by noise sources including muscle activity, electrical interference, baseline wander, and motion artifacts. Filters are employed to enhance signal-to-noise ratio by attenuating frequencies outside the physiological range of cardiac signals. However, inappropriate filtering can distort waveform morphology and obscure critical diagnostic features, necessitating careful filter design and selection.

Vectorcardiography

Vectorcardiography is a technique that represents the magnitude and direction of the heart's electrical forces in three-dimensional space, producing loop diagrams of cardiac electrical activity. It complements traditional ECG by providing spatial information about the cardiac vector trajectory during depolarization and repolarization phases. Vectorcardiography aids in detailed analysis of complex conduction abnormalities and myocardial pathology.