B2 W3 - Introduction to the Electrocardiogram (ECG) Flashcards
What key understanding about cardiac muscle predates the development of the electrocardiogram?
Scientists already knew that cardiac muscle generated its own electrical activity, which initiated contraction. This discovery paved the way for the development of the ECG.
Who are credited with pioneering the electrocardiogram?
Willem Einthoven and Augustus Waller are recognized as the pioneers of the electrocardiogram in the early 20th century.
How does the electrocardiogram work?
The ECG detects and records the small voltage differences generated on the body surface due to the spread of electrical excitation through the heart muscle.
How did Einthoven and Waller demonstrate the ECG concept?
They used Einthoven’s dog, “Jemmy,” placing his feet in saline solutions connected to electrodes, demonstrating that the heart’s electrical activity could be recorded from the skin surface.
List at least four clinical applications of the electrocardiogram.
The ECG is used to assess:Arrhythmias (irregular heart rhythms)Conduction abnormalities (issues with the electrical signals within the heart)Myocardial infarction (heart attack)Chamber hypertrophy (enlargement of the heart chambers)
Despite its age, how is the ECG viewed in contemporary clinical practice?
Even though the technology is over 100 years old, the ECG remains a highly valuable and routinely used tool in clinical practice.
What initiates the electrical impulses that propagate through the heart, and where is this located?
The pacemaker, also known as the sinoatrial (SA) node, initiates the electrical impulses. It is located in the right atrium.
Describe the sequence of the cardiac conduction pathway after the SA node.
From the SA node, the electrical activity spreads through the atria to the atrioventricular (AV) node, then down the bundle of His and bundle branches, finally reaching the Purkinje fibers that penetrate the ventricular muscle.
How does the spread of excitation in the heart relate to the ECG waveform?
The spread of excitation creates extracellular currents, which are detected by the ECG and recorded as the familiar waveform.
Outline the six key steps in the wave of excitation through the ventricles.
SA node depolarizes, spreading to the atria and AV node.Excitation travels down the bundle of His and bundle branches, depolarizing the septum from left to right.Excitation spreads to the anterior septal myocardium towards the apex.The bulk of the ventricular myocardium depolarizes from endocardium to epicardium (inside to outside).The posterior base of the left ventricle (the thickest part) is the last to depolarize.The entire ventricular myocardium is depolarized.
What is an electrical dipole, and how does it relate to the heart?
An electrical dipole is created when there are separated positive and negative charges. In the heart, the moving wave of excitation creates dipoles as different regions become depolarized.
Define a vector and its relevance to the ECG.
A vector represents a force with both magnitude and direction. The electrical dipole in the heart can be represented as a vector, and the ECG records changes in this vector as the wave of excitation moves.
How does the ECG measure the electrical dipole in the heart?
The ECG uses electrodes placed on the body to measure the extracellular negative charge and how it moves relative to a positive electrode. This captures the changes in the dipole vector.
Explain how the vector of the depolarisation wave creates the characteristic upward deflection in the ECG waveform.
As the depolarisation wave moves towards a positive electrode, the vector points in the same direction, causing a positive deflection in the ECG recording. The larger the magnitude of the vector (more tissue depolarizing), the greater the positive deflection.
What causes the ECG recording to return to the isoelectric line?
When the entire myocardium is in the same electrical state (either fully depolarized or repolarized), there is no dipole, hence no vector to measure, resulting in a zero voltage reading, represented as the isoelectric line.
Why does the ECG waveform show a small downward deflection after the main upward spike?
This downward deflection occurs as the final portion of the heart (the base of the left ventricle) depolarizes. The vector now points away from the positive electrode, producing a small negative deflection.
How is the sequential depolarisation of the atria reflected in the P wave?
As the SA node is located in the right atrium, the P wave represents the sequential depolarisation of both atria, progressing from right to left.
What does an absent P wave on the ECG indicate about the heart rhythm, and what are some examples of conditions that can cause this?
An absent P wave suggests that the heartbeat is not originating from the SA node, meaning the heart is not in sinus rhythm. This can occur in conditions like atrial fibrillation, where ectopic pacemakers arise in the atrial tissue, or when rhythms are initiated in the ventricle or the conduction system.
What is the normal duration of the PR interval, and what does a prolonged PR interval suggest?
A healthy PR interval is between 120 and 200 milliseconds. A prolonged PR interval (greater than 200 milliseconds) is indicative of AV block, meaning there is a delay or complete blockage of conduction at the AV node.
Besides a prolonged PR interval, what does a shortened PR interval indicate?
A shortened PR interval can indicate pre-excitation of the ventricles, where the electrical impulse bypasses the AV node, leading to early ventricular contraction and potentially affecting ventricular filling time. It can also indicate that the AV node has become the pacemaker, disrupting sinus rhythm.
How does the morphology (shape) of the QRS complex provide information about the origin of the electrical impulse?
A narrow QRS complex (less than 120 milliseconds) indicates that the electrical impulse is originating above the ventricles (supraventricular) and is being conducted normally. A broad QRS complex (greater than 120 milliseconds) suggests an ectopic pacemaker within the ventricles or a problem with the ventricular conduction system.
What does the ST segment represent, and how does it change in a myocardial infarction?
The ST segment represents the period between ventricular depolarisation and repolarisation. In a myocardial infarction, the ST segment can become elevated from the isoelectric line.
What does ST segment depression indicate?
ST segment depression, where the ST segment is lower than the isoelectric line, suggests acute myocardial ischemia, a condition where the heart muscle is not receiving enough oxygen.
What does the T wave represent, and to what phase of the cardiac action potential is it related?
The T wave represents ventricular repolarisation, the process of the ventricles returning to their resting state. It is related to the relative refractory period of the cardiac action potential.
What are some examples of T wave abnormalities, and what can they indicate?
T wave abnormalities can include:A large T wave: indicative of hyperkalemia (high potassium levels)A small T wave: indicative of hypokalemia (low potassium levels)Inverted T wave: associated with myocardial ischemia and infarction
What is the QT interval, and how does it change with heart rate?
The QT interval represents the total time for both ventricular depolarisation and repolarisation. It shortens as heart rate increases and lengthens as heart rate decreases.
Why is the corrected QT interval (QTc) important in clinical practice?
The QTc adjusts the QT interval to a standard heart rate of 60 beats per minute, allowing for comparisons between individuals and accounting for variations in heart rate.
Besides arrhythmias, what other factors can cause a prolonged QT interval?
Certain medications that interact with cardiac ion channels, or mutations in these channels, can lead to QT interval prolongation.
What are the clinical implications of a prolonged QT interval?
A prolonged QT interval is associated with an increased risk of arrhythmias, particularly Torsades de Pointes, a potentially fatal type of ventricular fibrillation.
What does the P wave represent, and what is its normal duration?
The P wave represents atrial depolarisation, leading to atrial contraction. In a healthy heart, it lasts less than 120 milliseconds (3 small squares on the ECG strip).
Why does the P wave show the sequential depolarisation of both atria?
Because the SA node, the pacemaker of the heart, is in the right atrium, the P wave captures the electrical activity moving from the right atrium to the left atrium.
What does an absent P wave indicate?
An absent P wave suggests the heartbeat is not initiated by the SA node, meaning the heart is not in sinus rhythm. This can occur in conditions like atrial fibrillation or when rhythms originate in the ventricles or the conduction system.
What does the PR interval represent, and what is its normal duration?
The PR interval represents the time it takes for the electrical impulse to travel from the SA node through the AV node and the rest of the conduction system to the ventricles. It should be between 120 and 200 milliseconds (3 to 5 small squares).
What does a prolonged PR interval indicate?
A prolonged PR interval (greater than 200 milliseconds) suggests AV block, meaning there’s a delay or blockage of conduction at the AV node.
What does a shortened PR interval indicate?
A shortened PR interval can indicate pre-excitation, where the impulse bypasses the AV node, causing early ventricular contraction. It can also mean the AV node has become the pacemaker.
What does the QRS complex represent, and what is its normal duration?
The QRS complex represents ventricular depolarisation, leading to ventricular contraction. It normally lasts between 70 and 120 milliseconds (less than 3 small squares).
Why is the QRS complex typically the largest deflection on the ECG?
The QRS complex is larger than the P wave because the ventricles have a greater muscle mass, generating stronger electrical currents.
What does a narrow QRS complex indicate?
A narrow QRS complex (less than 120 milliseconds) indicates the impulse is coming from above the ventricles (supraventricular) and is being conducted normally.
What does a broad QRS complex indicate?
A broad QRS complex (greater than 120 milliseconds) suggests an ectopic pacemaker within the ventricle or a problem with the ventricular conduction system, leading to unsynchronised contraction.
What does the ST segment represent?
The ST segment represents the interval between ventricular depolarisation and repolarisation, where the ventricles have fully contracted and are preparing to relax.
What does an elevated ST segment indicate?
An elevated ST segment is a sign of myocardial infarction (heart attack), where cell death disrupts normal conduction.
What does a depressed ST segment indicate?
A depressed ST segment suggests acute myocardial ischaemia, where the heart muscle isn’t receiving enough oxygen.
