ECG Module 1

Question 1

From an electrophysiological point of view, how many chambers does the heart have?

Answer 1

Electrically, the heart has two chambers: the atria and the ventricles, rather than four anatomical chambers.

Question 2

How are the atria and ventricles electrically related to each other?

Answer 2

They are electrically insulated from each other, except at a small junction area.

Question 3

What is the name of the junction that electrically connects the atria and ventricles?

Answer 3

The AV gap (atrioventricular gap).

Question 4

Why is the AV gap important in cardiac electrophysiology?

Answer 4

It is the only electrical connection between the atria and ventricles, allowing impulses to pass from one to the other.

Question 5

How is the heart electrically related to the rest of the body?

Answer 5

The heart is relatively electrically insulated from the rest of the body.

Question 6

Why must the heart generate its own electricity?

Answer 6

Because it is electrically insulated and must produce its own electrical impulses to stimulate (enervate) cardiac muscle

Question 7

What are the primary functions of myocardial cells?

Answer 7

They are contractile cells that can also generate and transmit electrical activity

Question 8

Why are myocardial cells important for heart rhythm?

Answer 8

Because they can both conduct electrical signals and contract, enabling coordinated heartbeats.

Question 9

Where are pacemaker cells primarily located in the heart?

Answer 9

Mainly in the SA node of the right atrium

Question 10

Besides the SA node, where else are pacemaker cells found?

Answer 10

In the AV node (at the atrioventricular junction) and the terminal ventricular Purkinje fibers.

Question 11

How do pacemaker cells differ from ordinary myocardial cells in activation?

Answer 11

They are not activated by electrical spread from cell to cell, but instead spontaneously depolarize.

Question 12

What causes spontaneous depolarization in pacemaker cells?

Answer 12

A relative permeability of the cell membrane to Na⁺, causing gradual depolarization.

Question 13

What is the term for the ability of cardiac pacemaker cells to spontaneously depolarize?

Answer 13

Automaticity

Question 14

What is meant by “drifting” in pacemaker cells?

Answer 14

The continuous depolarization–repolarization cycles occurring without external stimulation.

Question 15

What is the intrinsic firing rate of the SA node?

Answer 15

60–80 times per minute.

Question 16

Why is the SA node considered the primary pacemaker of the heart?

Answer 16

Because it has the fastest rate of automatic depolarization.

Question 17

How does the SA node determine normal heart rate?

Answer 17

Its intrinsic rate (60–80 bpm) sets the normal resting heart rate

Question 18

How is heart rate modified despite the intrinsic SA node rate?

Answer 18

Through parasympathetic and sympathetic fibers from the central nervous system.

Question 19

What is the intrinsic firing rate of Purkinje fibers?

Answer 19

About 30 beats per minute.

Question 20

What is the intrinsic firing rate of the AV node?

Answer 20

Approximately 40–60 beats per minute

Question 21

When do AV node and Purkinje fibers act as pacemakers?

Answer 21

Only if the SA node fails to fire.

Question 22

What are the AV node and Purkinje fibers called when acting as pacemakers?

Answer 22

Back-up pacemakers.

Question 23

What is an escape rhythm?

Answer 23

A rhythm generated by AV node or Purkinje fibers when they escape SA node control.

Question 24

Why are these rhythms called “escape rhythms”?

Answer 24

Because the pacemaker cells have escaped domination by the SA node.

Question 25

How are cardiac myocytes structurally connected to each other?

Answer 25

By cytoplasmic bridges, forming syncytia.

Question 26

What is the functional significance of myocytes forming a syncytium?

Answer 26

It allows electrical impulses to spread rapidly from one cell to adjacent cells.

Question 27

What happens once a single myocyte becomes depolarized

Answer 27

A wave of depolarization spreads to neighbouring myocytes.

Question 28

What is the term used to describe the ability of cardiac cells to transmit electrical impulses?

Answer 28

Conductivity

Question 29

Where does the electrical impulse originate in the heart?

Answer 29

In the SA node.

Question 30

How does the electrical impulse spread through the atria?

Answer 30

Relatively slowly, taking approximately 120–200 milliseconds.

Question 31

What happens to conduction when the impulse reaches the AV node?

Answer 31

Conduction slows down even further.

Question 32

Why is conduction slowed at the AV node?

Answer 32

Due to parasympathetic inhibition (vagal tone) and the complex structure of the AV node.

Question 33

What is meant by “vagal tone” at the AV node?

Answer 33

The inhibitory effect of parasympathetic fibers acting on the AV node.

Question 34

How does the structure of the AV node affect conduction velocity?

Answer 34

Its labyrinthine, interweaving strands slow electrical transmission.

Question 35

How does the structure of the bundle of His and bundle branches differ from the AV node?

Answer 35

They have parallel fibers, allowing faster conduction.

Question 36

Why is slowed AV nodal conduction physiologically important?

Answer 36

It allows time for atrial contraction and ventricular filling before ventricular depolarization.

Question 37

What is the physiological purpose of slowing conduction at the AV node?

Answer 37

To facilitate passive filling of the ventricles before ventricular contraction.

Question 38

Why is the AV node described as a “gatekeeper”?

Answer 38

It moderates the number of atrial impulses that reach the ventricles.

Question 39

How does the AV node protect the ventricles during atrial arrhythmias?

Answer 39

Fast chaotic atrial impulses are partially blocked from entering the ventricles.

Question 40

What mechanical maneuvers slow AV nodal conduction?

Answer 40

Vagal stimulation and carotid sinus massage.

Question 41

Which drugs can slow AV nodal conduction?

Answer 41

Beta-blockers (β-blockers).

Question 42

Why is the AV node considered a “one-way” pathway?

Answer 42

It normally conducts only from atria to ventricles, unless a congenital abnormality is present.

Question 43

Where does the electrical impulse go after passing through the AV node?

Answer 43

Into the bundle of His.

Question 44

Into which structures does the bundle of His divide?

Answer 44

The right bundle branch (RBB) and left bundle branch (LBB).

Question 45

What is the function of the right bundle branch?

Answer 45

It conducts the depolarization wave to the right ventricle.

Question 46

How does the left bundle branch divide?

Answer 46

Into anterior and posterior fascicles.

Question 47

What areas do the left bundle branch fascicles supply?

Answer 47

The anterior and posterior regions of the left ventricle.

Question 48

Why can 2D diagrams of the LBB be misleading?

Answer 48

Because the so-called “proximal” branch runs on the posterior surface and the “distal” branch on the anterior surface of the left ventricle.

Question 49

How fast does ventricular depolarization occur after entering the bundle of His?

Answer 49

Rapidly, in less than 120 ms

Question 50

Why do both ventricles contract simultaneously?

Answer 50

Because depolarization spreads simultaneously to the left and right ventricles.

Question 51

In which direction does the interventricular septum depolarize?

Answer 51

From left to right.

Question 52

What is the role of Purkinje fibers?

Answer 52

They form a fine network that rapidly distributes electricity across the entire endocardium.

Question 53

How does depolarization spread through the ventricular wall?

Answer 53

From the endocardium outward through the ventricular myocardium.

Question 54

What happens to cardiac tissue immediately after depolarization?

Answer 54

It becomes refractory and unresponsive to further stimulation.

Question 55

Which ECG components represent depolarization?

Answer 55

The P wave (atrial depolarization) and QRS complex (ventricular depolarization).

Question 56

Which ECG components represent repolarization?

Answer 56

The ST segment and T wave.

Question 57

What occurs during repolarization at the cellular level?

Answer 57

Electrolytes are actively pumped back to their pre-depolarization concentrations

Question 58

What does the isoelectric line before the P wave represent?

Answer 58

The resting membrane potential.

Question 59

Why is the refractory period essential for normal cardiac function?

Answer 59

It prevents tetany and ensures orderly, rhythmic contractions.

Question 60

Which two fundamental electrophysiological processes guide ECG interpretation?

Answer 60

Automaticity and conductivity

Question 61

What analogy can be used to explain cardiac electrical activity?

Answer 61

A row of dominoes, where knocking over the first domino triggers a sequential chain reaction.

Question 62

In the domino analogy, what does the first domino represent?

Answer 62

The origin of the electrical impulse (the pacemaker).

Question 63

What must happen before the domino game can be repeated?

Answer 63

The dominoes must be restacked, analogous to repolarization.

Question 64

What question does automaticity answer in ECG interpretation?

Answer 64

Where did the impulse originate?

Question 65

What structures can act as pacemakers in the heart?

Answer 65

The SA node, AV node, or ventricular pacemakers (Purkinje system).

Question 66

Why is identifying the pacemaker important on an ECG?

Answer 66

It helps determine whether the rhythm is normal or abnormal.

Question 67

What question does conductivity address in ECG interpretation?

Answer 67

How did the impulse travel through the heart?

Question 68

What aspects of impulse travel are assessed under conductivity?

Answer 68

Whether the pathway, timing, and sequence are normal.

Question 69

What abnormalities may affect conductivity?

Answer 69

Delays, obstructions, or interference with impulse transmission.

Question 70

How do conduction abnormalities typically appear on an ECG?

Answer 70

As prolonged intervals, abnormal waveforms, or altered sequences.

Question 71

How is one full depolarization–repolarization cycle represented on an ECG?

Answer 71

By the P wave, QRS complex, ST segment, and T wave.

Question 72

What does the P wave represent?

Answer 72

Atrial depolarization.

Question 73

What does the PR interval reflect?

Answer 73

Atrial depolarization and AV nodal delay.

Question 74

What does the QRS complex represent?

Answer 74

Ventricular depolarization.

Question 75

What does the ST segment represent?

Answer 75

The early phase of ventricular repolarization.

Question 76

What does the T wave represent?

Answer 76

Ventricular repolarization.

Question 77

Why is repolarization essential before the next impulse?

Answer 77

It restores the resting membrane potential, allowing the next cycle to occur.

Question 78

What is the simplest structured approach to ECG interpretation?

Answer 78

Identify automaticity (origin of impulse) Assess conductivity (pathway and timing) Correlate findings with ECG waveforms

Question 79

Why does the term “vector” often seem difficult in ECG learning?

Answer 79

Because it sounds complex, but only a basic understanding is needed for beginners.

Question 80

What is a vector in simple ECG terms?

Answer 80

The predominant direction and size of electrical current spreading through the heart.

Question 81

How does electrical conduction spread through myocardial cells?

Answer 81

From cell to cell, starting at the site of activation (pacemaker) and spreading in all directions.

Question 82

If electrical activity spreads in all directions, what determines the main direction of spread?

Answer 82

The direction of the greatest available muscle mass.

Question 83

In which direction does an impulse originating in the SA node predominantly spread?

Answer 83

To the left and somewhat downward

Question 84

Why does SA node activation spread mainly leftward?

Answer 84

Because there is more atrial muscle mass to the left of the SA node.

Question 85

Are both ventricles activated at the same time?

Answer 85

Yes, they are activated simultaneously.

Question 86

Despite simultaneous activation, why is the ventricular vector directed leftward?

Answer 86

Because the left ventricle has greater muscle mass than the right ventricle.

Question 87

What is the predominant direction of ventricular depolarization?

Answer 87

Downwards and to the left.

Question 88

How does ventricular muscle mass influence ECG appearance?

Answer 88

Greater muscle mass produces a stronger electrical signal in that direction.

Question 89

What ECG concept is based on the principle of vectors?

Answer 89

The cardiac axis.

Question 90

How can the cardiac axis be visualized simply?

Answer 90

As a line running across a wall clock from 11 o’clock to 5 o’clock.

Question 91

What does the cardiac axis represent?

Answer 91

The overall direction of ventricular depolarization.

Question 92

What does the length of an arrow in a vector diagram represent?

Answer 92

The amount of muscle mass available for depolarization.

Question 93

What do the different angles of arrows in a vector diagram represent?

Answer 93

Electrical current flowing in all available directions

Question 94

What do all the arrows combined represent?

Answer 94

What do all the arrows combined represent?

Question 95

When we say an impulse is moving “towards” or “away” from a lead, what are we referring to?

Answer 95

The vector (direction and size of the electrical current).

Question 96

Why are vectors important for interpreting ECG leads?

Answer 96

Because they determine whether a lead records a positive, negative, or biphasic deflection.