What is the physiology of electrocardiography (ECG) in cardiac function?

From the Guidelines

The electrocardiogram (ECG) is a non-invasive tool that records the electrical activity of the heart, providing valuable insights into cardiac physiology.

Key Components of the ECG

A normal ECG tracing consists of a P wave, QRS complex, and T wave, corresponding to atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively 1.

• The P wave represents the electrical signal that triggers atrial contraction.
• The QRS complex represents the electrical signal that triggers ventricular contraction.
• The T wave represents the electrical signal that triggers ventricular repolarization. The cardiac cycle is regulated by the autonomic nervous system, with sympathetic stimulation increasing heart rate and contractility, and parasympathetic stimulation, via acetylcholine, decreasing heart rate 1.

ECG Signal

The standard 12-lead ECG records potential differences between prescribed sites on the body surface that vary during the cardiac cycle; it reflects differences in transmembrane voltages in myocardial cells that occur during depolarization and repolarization within each cycle 1.

• The fundamental frequency for the QRS complex at the body surface is 10 Hz, and most of the diagnostic information is contained below 100 Hz in adults.
• The QRS of infants often contains important components as high as 250 Hz.
• The fundamental frequency of T waves is approximately 1 to 2 Hz.

Clinical Applications

Medications such as atropine, administered at a dose of 0.5-1.0 mg intravenously, can be used to increase heart rate in cases of symptomatic bradycardia, while beta blockers, such as metoprolol, dosed at 25-100 mg orally twice daily, can be used to slow heart rate in cases of tachyarrhythmias. The ECG is a crucial tool in clinical practice, allowing for the diagnosis and monitoring of various cardiac conditions, including arrhythmias, electrolyte imbalances, and cardiac structural abnormalities 1.

From the Research

Physiology of Electrocardiography (ECG) in Cardiac Function

The physiology of electrocardiography (ECG) in cardiac function is a complex process that involves the recording of electrical activity in the heart. Key aspects of this process include:

• The use of a multielectrode vest to record body-surface electrocardiograms, which are then reconstructed on the heart's surface using geometrical information from computed tomography (CT) and a mathematical algorithm 2
• The influence of clinical variables, such as heart rate, heart size, and ventricular wall thickness, on surface electrocardiogram (ECG) measurements and non-invasive cardiac mapping 3
• The ability of ECG to provide diagnostic insights regarding the mechanistic basis of arrhythmias and the location of arrhythmia origin, as well as guide therapeutic decision-making, including during cardiac resynchronization therapy (CRT) 4

Factors Affecting ECG Readings

Several factors can affect ECG readings, including:

• Heart rate: an increase in heart rate can induce order-dependent errors in the accuracy of reconstruction 3
• Ventricular wall thickness: an increase in ventricular wall thickness can affect the accuracy of ECG readings 3
• Body composition: changes in body composition can influence ECG measurements 3
• Anesthesia and stress: these can mask or compensate subtle cardiac phenotypes in non-invasive ECG methods 5

Applications of ECG

ECG has several applications in cardiac function, including:

• Diagnosis of arrhythmias: ECG can be used to diagnose and classify different types of arrhythmias, including life-threatening and non-life-threatening types 6
• Guiding therapeutic decision-making: ECG can provide valuable information to guide therapeutic decision-making, including during CRT 4
• Monitoring cardiac activity: ECG can be used to monitor cardiac activity in real-time, allowing for early detection of changes in cardiac function 5