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CTB Block 2 > B2 W3 - Introduction to the Electrocardiogram (ECG) > Flashcards

B2 W3 - Introduction to the Electrocardiogram (ECG) Flashcards

1
Q

What key understanding about cardiac muscle predates the development of the electrocardiogram?

A

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.

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2
Q

Who are credited with pioneering the electrocardiogram?

A

Willem Einthoven and Augustus Waller are recognized as the pioneers of the electrocardiogram in the early 20th century.

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3
Q

How does the electrocardiogram work?

A

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.

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4
Q

How did Einthoven and Waller demonstrate the ECG concept?

A

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.

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5
Q

List at least four clinical applications of the electrocardiogram.

A

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)

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6
Q

Despite its age, how is the ECG viewed in contemporary clinical practice?

A

Even though the technology is over 100 years old, the ECG remains a highly valuable and routinely used tool in clinical practice.

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7
Q

What initiates the electrical impulses that propagate through the heart, and where is this located?

A

The pacemaker, also known as the sinoatrial (SA) node, initiates the electrical impulses. It is located in the right atrium.

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8
Q

Describe the sequence of the cardiac conduction pathway after the SA node.

A

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.

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9
Q

How does the spread of excitation in the heart relate to the ECG waveform?

A

The spread of excitation creates extracellular currents, which are detected by the ECG and recorded as the familiar waveform.

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10
Q

Outline the six key steps in the wave of excitation through the ventricles.

A

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.

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11
Q

What is an electrical dipole, and how does it relate to the heart?

A

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.

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12
Q

Define a vector and its relevance to the ECG.

A

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.

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13
Q

How does the ECG measure the electrical dipole in the heart?

A

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.

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14
Q

Explain how the vector of the depolarisation wave creates the characteristic upward deflection in the ECG waveform.

A

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.

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15
Q

What causes the ECG recording to return to the isoelectric line?

A

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.

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16
Q

Why does the ECG waveform show a small downward deflection after the main upward spike?

A

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.

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17
Q

How is the sequential depolarisation of the atria reflected in the P wave?

A

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.

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18
Q

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?

A

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.

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19
Q

What is the normal duration of the PR interval, and what does a prolonged PR interval suggest?

A

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.

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20
Q

Besides a prolonged PR interval, what does a shortened PR interval indicate?

A

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.

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21
Q

How does the morphology (shape) of the QRS complex provide information about the origin of the electrical impulse?

A

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.

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22
Q

What does the ST segment represent, and how does it change in a myocardial infarction?

A

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.

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23
Q

What does ST segment depression indicate?

A

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.

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24
Q

What does the T wave represent, and to what phase of the cardiac action potential is it related?

A

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.

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25
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
26
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.
27
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.
28
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.
29
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.
30
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).
31
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.
32
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.
33
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).
34
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.
35
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.
36
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).
37
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.
38
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.
39
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.
40
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.
41
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.
42
What does a depressed ST segment indicate?
A depressed ST segment suggests acute myocardial ischaemia, where the heart muscle isn't receiving enough oxygen.
43
What does the T wave represent?
The T wave represents ventricular repolarisation, the process of the ventricles returning to their resting electrical state.
44
What does a large T wave indicate?
A large T wave can be a sign of hyperkalaemia (high potassium levels).
45
What does a small T wave indicate?
A small T wave can be a sign of hypokalaemia (low potassium levels).
46
What does an inverted T wave indicate?
An inverted T wave can be associated with myocardial ischaemia and infarction.
47
What does the QT interval represent?
The QT interval represents the total time for both ventricular depolarisation and repolarisation.
48
How does the QT interval relate to heart rate?
The QT interval shortens as heart rate increases and lengthens as heart rate decreases.
49
What is the corrected QT interval (QTc), and why is it used?
The QTc adjusts the QT interval to a standard heart rate of 60 beats per minute. This allows for comparisons between individuals and accounts for heart rate variations.
50
What can cause a prolonged QT interval?
A prolonged QT interval can be caused by certain medications, genetic mutations affecting cardiac ion channels, or heart conditions.
51
What is the significance of a prolonged QT interval?
A prolonged QT interval increases the risk of dangerous arrhythmias, especially Torsades de Pointes, a type of ventricular fibrillation that can be fatal.
52
How many leads does a 12-lead ECG record? What does each lead produce?
A 12-lead ECG records 12 different ECG waveforms. Each lead produces a different view of the heart's electrical activity.
53
How many electrodes are used to create a 12-lead ECG? What lead groups do these electrodes create?
Ten electrodes are used in a 12-lead ECG. These are arranged to create six limb leads and six chest (precordial) leads.
54
What is a rhythm strip, and which lead is it usually derived from?
A rhythm strip is a longer recording, most often of lead II, allowing for closer examination of the heart rhythm.
55
Where are the limb lead electrodes placed?
Limb lead electrodes are placed on the patient's arms and legs.
56
What do the different positions of the limb lead electrodes provide?
Different positions of the electrodes provide different views of the heart in the vertical plane.
57
What is Einthoven's Triangle used for?
Einthoven's Triangle helps determine which limb leads correspond to which views of the heart.
58
What configuration of electrodes does Lead I use? In what direction does it record?
Lead I has a negative electrode on the right arm and a positive electrode on the left arm. It records from right to left across the chest.
59
What configuration of electrodes does Lead II use? In what direction does it record?
Lead II has a negative electrode on the right arm and a positive electrode on the left leg. It records downwards and to the left, from the right arm towards the left leg.
60
What configuration of electrodes does Lead III use? In what direction does it record?
Lead III has a negative electrode on the left arm and a positive electrode on the left leg. It records downwards, from left to right.
61
How does electrode placement impact the ECG waveform?
The direction of the electrical vector in the heart, in relation to the position of the electrodes, determines the shape of the ECG waveform.
62
What is the difference between standard limb leads and augmented limb leads?
Standard limb leads use one positive and one negative electrode. Augmented limb leads combine two negative electrodes with one positive, creating a stronger signal and larger ECG voltage.
63
What do augmented limb leads create due to averaging two negative electrodes?
Augmented limb leads create an imaginary negative electrode between the two averaged electrodes. This produces a new view of the heart.
64
How many views of the electrical activity of the heart do the limb leads provide?
The limb leads provide six different views of the electrical activity of the heart in the vertical plane.
65
Which lead is most aligned with the normal vector of cardiac excitation? Why does it have the largest R wave?
Lead II is most aligned with the normal vector of cardiac excitation because the electrical vector during ventricular depolarisation moves towards Lead II's positive electrode, resulting in the largest R wave.
66
What regions of the heart do leads II, III, and aVF correspond to?
Leads II, III, and aVF provide information about the inferior region of the heart.
67
How many electrodes are used in the precordial leads, and where are they positioned?
Six electrodes are used in the precordial leads, positioned across the chest.
68
In what plane do the precordial leads provide a view of the heart's electrical activity?
The precordial leads provide views of the heart's electrical activity in the horizontal plane.
69
Why do the chest leads record different ECG waveforms?
Each chest lead records a different waveform because, as the vector of electrical excitation progresses through the myocardium, each electrode is in a different orientation to that vector.
70
What is R wave progression, and why is it important?
R wave progression refers to the increasing amplitude of the R wave across chest leads V1 to V5. It's important because changes can indicate cardiac pathologies.
71
Why is a 12-lead ECG used clinically, as opposed to a single lead?
A 12-lead ECG is used because it provides 12 different views of the heart in both vertical and horizontal planes. This allows for a more comprehensive examination of the heart's electrical activity and the identification of abnormalities that might be missed with a single lead.
72
In the context of the 12-lead ECG, how can different leads help in understanding electrical activity in specific heart regions?
Having 12 leads with varying views allows examination of electrical activity in different heart regions. For example, leads II, III, and aVF provide information about the inferior region of the heart.
73
What information can the ECG provide even when there's no direct access to the patient?
The ECG can be used to calculate heart rate, even when the patient is not physically present.
74
How does the standardisation of ECG paper aid in heart rate calculation?
The standardised ECG paper runs at 25 mm per second with small squares representing 40 milliseconds and large squares 200 milliseconds. This standardisation allows for accurate measurements of the component waves and intervals, making heart rate calculation easy and accurate.
75
What is the purpose of measuring heart rate from an ECG?
Measuring heart rate helps determine if a patient has tachycardia (fast heart rate) or bradycardia (slow heart rate).
76
What is the RR interval, and why is it examined for heart rate measurement?
The RR interval is the time between two successive ventricular depolarisations (i.e., between two consecutive heartbeats). It is examined because it represents the time between two heartbeats, thus providing information about heart rate.
77
Why is it recommended to average the values from several RR intervals when measuring heart rate?
Averaging multiple RR intervals is best because heartbeats may not be perfectly regular, and some variation can occur between each RR interval.
78
What is sinus rhythm, and what is its origin?
Sinus rhythm is the default heart rhythm, originating from the sinoatrial (SA) node. The electrical impulse from the SA node travels to the ventricles through the AV node and the Purkinje system.
79
List the characteristics of a normal sinus rhythm ECG.
A normal sinus rhythm ECG has: a regular rhythm (equidistant QRS complexes); a rate of 60-100 beats per minute in adults; each QRS complex preceded by a P wave; a normal P wave deflection (upright in leads I and II, inverted in aVR); a constant PR interval; and narrow QRS complexes (less than 120 milliseconds).
80
What is sinus tachycardia?
Sinus tachycardia is sinus rhythm with a rate exceeding 100 beats per minute.
81
What is sinus bradycardia?
Sinus bradycardia is sinus rhythm with a rate slower than 60 beats per minute.
82
Describe sinus arrhythmia and its characteristics.
Sinus arrhythmia is sinus rhythm with beat-to-beat variation in the P-to-P interval, leading to an irregular ventricular rhythm. It can be associated with breathing and is commonly observed in healthy athletic individuals, especially at lower heart rates.
83
Why is understanding sinus rhythm crucial in ECG interpretation?
Knowing sinus rhythm is important because it serves as a baseline for identifying abnormalities in the ECG.
84
What is cardiac axis, and what does it represent?
Cardiac axis describes the overall direction of the heart's electrical activity. It represents the sum of the depolarisation vectors generated by all the individual cardiomyocytes in the heart.
85
Why does the left ventricle contribute significantly to the cardiac axis?
The left ventricle, having the most myocardial mass in a healthy heart, contributes most significantly to the cardiac axis.
86
How is cardiac axis determined from the ECG?
Cardiac axis is determined by examining the QRS complexes in the ECG, as they reflect the direction of ventricular depolarisation.
87
What is considered the normal range for cardiac axis?
The normal cardiac axis lies between -30 and 90 degrees.
88
What do deviations from the normal cardiac axis often suggest?
Deviation from the normal cardiac axis often indicates a cardiac pathology that has disrupted the normal conduction of depolarisation through the heart.
89
What does cardiac axis describe, and what does it not describe?
Cardiac axis describes the axis of electrical activity in the heart. It does not describe the anatomical orientation of the heart within the thoracic cavity.
90
What does cardiac axis represent?
Cardiac axis represents the sum of the depolarisation vectors generated by all the individual cardiomyocytes in the heart.
91
How is the cardiac axis reflected, and why?
The cardiac axis is reflected by the direction of electrical activity through the ventricular myocardium. This is because the left ventricle makes up the majority of the heart muscle, therefore contributing most to the cardiac axis.
92
How can the cardiac axis be determined from an ECG?
Cardiac axis can be determined by examining the QRS complexes in the ECG.
93
What is the relationship between the QRS complex and ventricular depolarisation?
The QRS complex reflects the direction of ventricular depolarisation. An upward deflection of the R wave signifies depolarisation moving towards a positive electrode.
94
What does it mean if the QRS complex is positive, negative, or equiphasic in relation to the cardiac axis?
If the QRS is positive, the axis is pointing towards the lead; if the QRS is negative, the axis is moving away from the lead; if the QRS is equiphasic (R and S waves of equal amplitude), the axis is 90 degrees to the lead.
95
What is the normal cardiac axis range, and what does a deviation from this range often indicate?
Normal cardiac axis lies between -30 and 90 degrees. Deviation often indicates a cardiac pathology affecting the normal conduction of depolarisation.
96
What are the three main types of cardiac axis deviation?
The three types are right axis deviation, left axis deviation, and extreme axis deviation.
97
What are the two methods for estimating cardiac axis described in the sources?
The sources describe the three-lead analysis and the isoelectric lead analysis.
98
What leads are used in the three-lead analysis method, and what does the QRS complex in each lead reveal about the cardiac axis?
Leads I, II, and aVF are used. The QRS complex in each lead, whether positive, negative, or equiphasic, helps determine the overall direction of the cardiac axis.
99
Can you give an example of how the three-lead analysis method is used to determine normal cardiac axis?
If Lead I, Lead II, and Lead aVF all show a positive QRS complex, then the combined direction of the cardiac axis must be between 0 and 90 degrees, indicating a normal cardiac axis.
100
Can you give an example of how the three-lead analysis method is used to determine left axis deviation?
If Lead I shows a positive QRS complex, while Leads II and aVF show negative QRS complexes, the combined direction of the cardiac axis must be between -30 and -90 degrees, indicating left axis deviation.
101
Can you give an example of how the three-lead analysis method is used to determine right axis deviation?
If Lead I shows a negative QRS complex, while Leads II and aVF show positive QRS complexes, the combined direction of the cardiac axis must be between 90 and 180 degrees, indicating right axis deviation.
102
What is the first step in the isoelectric lead method for determining cardiac axis?
The first step is to identify the isoelectric lead, which is the lead with an equiphasic QRS complex.
103
In the isoelectric lead method, what does the isoelectric lead tell us about the cardiac axis?
The cardiac axis must be 90 degrees to the isoelectric lead.
104
What is the second step in the isoelectric lead method?
The second step is to identify the positive leads, which indicate the direction the electrical vector is traveling.
105
Can you provide an example of how the isoelectric lead method can be used to determine the cardiac axis?
If Lead II is the isoelectric lead, the axis must be either 150 or -30 degrees. If Leads III and aVF (rightward facing leads) are positive and Lead aVL (leftward facing lead) is negative, the vector is traveling towards Leads III and aVF and away from aVL. This places the vector at approximately 150 degrees, indicating right axis deviation.

CTB Block 2 (19 decks)

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