Cardiac Electrophysiology: Overview and Practice Questions

Cardiac electrophysiology is a specialized branch of cardiology involving the intricate electrical activities governing the heart’s rhythmic contractions and relaxations.

At its core, this field seeks to understand the genesis and propagation of electrical impulses within the heart, which ensures its synchronized function.

Any disruptions in this electrical system can lead to arrhythmias, conditions where the heart beats too quickly, too slowly, or irregularly, often requiring clinical intervention.

This article explains the foundational principles of cardiac electrophysiology, offering a comprehensive overview of its significance in diagnosing, treating, and managing various heart conditions.

What is Cardiac Electrophysiology?

Cardiac electrophysiology is the study of the heart’s electrical activity and associated disorders. It involves diagnosing and treating irregular heart rhythms or arrhythmias. Electrophysiologists use techniques like electrocardiograms and catheter ablations to monitor and correct abnormal electrical pathways in the heart, ensuring its proper and coordinated function.

What is an Arrhythmia?

An arrhythmia is a disorder of the heart’s rhythm, which means the heart beats irregularly. This can manifest in several ways:

• Too Fast (Tachycardia): This can occur in either the heart’s upper chambers (atria), leading to atrial tachycardia, or in the lower chambers (ventricles), leading to ventricular tachycardia.
• Too Slow (Bradycardia): While a slow heart rate can be normal for some individuals, especially athletes, it can be problematic if the heart doesn’t pump enough blood to meet the body’s needs.
• Irregular Rhythm: Even if the heart rate is normal, the heart can beat with an irregular rhythm. A common example is atrial fibrillation, where the atria quiver instead of beating in a coordinated manner.

Causes

Arrhythmias can arise due to various reasons, including:

• Structural heart disease
• High blood pressure
• Diabetes
• Smoking
• Excessive caffeine or alcohol consumption
• Certain medications and drugs
• Electrolyte imbalances in the blood
• Genetic predisposition

Note: Some arrhythmias might not cause any noticeable symptoms. However, when symptoms occur, they can range from palpitations, dizziness, respiratory failure, fainting (syncope), shortness of breath, and chest pain, to fatigue.

Diagnosis

Arrhythmias are typically diagnosed using an electrocardiogram (ECG or EKG) which records the heart’s electrical activity.

For intermittent arrhythmias, extended monitoring with devices like Holter monitors or event recorders might be used.

Treatment

Treatment depends on the type and severity of the arrhythmia. Options include:

• Medications to regulate heart rhythm
• Procedures like catheter ablation
• Devices like pacemakers or defibrillators
• Lifestyle changes and addressing underlying conditions

Note: It’s important to consult with a cardiologist or cardiac electrophysiologist if one suspects they have an arrhythmia, as untreated arrhythmias can lead to complications such as stroke or heart failure.

What is an Electrocardiogram?

An electrocardiogram (ECG or EKG) is a noninvasive diagnostic test that records the electrical activity of the heart over a period of time.

Using electrodes placed on the skin, it captures the timing and strength of heartbeats, helping identify irregularities in heart rhythms, structural abnormalities, or signs of heart damage.

The ECG provides a graphical representation with characteristic waves (P, QRS, and T) that correspond to specific heart actions.

By analyzing these waves, physicians can diagnose a variety of cardiac conditions, from arrhythmias to heart attacks. It’s a fundamental tool in cardiology, offering a quick and painless insight into heart function.

Cardiac Electrophysiology Practice Questions

1. What is the definition of cardiac electrophysiology?
Cardiac electrophysiology is the branch of cardiology that deals with the study and assessment of the electrical activities of the heart.

2. What is a cardiologist?
A medical doctor who specializes in diagnosing, treating, and preventing diseases and conditions of the heart and cardiovascular system.

3. What is an electrophysiologic?
A subspecialist within cardiology who focuses on diagnosing and treating irregular heart rhythms or arrhythmias, often using procedures like catheter ablation and device implantation. They deal specifically with the heart’s electrical system and its associated disorders.

4. What is the difference between a cardiologist and an electrophysiologist?
While both cardiologists and electrophysiologists focus on heart conditions, the primary distinction lies in their areas of expertise. A cardiologist addresses a broad range of cardiovascular issues, including coronary artery disease, heart failure, and valve disorders. In contrast, an electrophysiologist narrows in on the heart’s electrical system and treats arrhythmias.

5. What are the components of the cardiac electrical conduction system?
Sinoatrial node, atrioventricular node, bundles of his, and Purkinje fibers.

6. Where is the sinoatrial (SA) node located?
It is located at the right atria next to the superior vena cava entrance.

7. Where is the atrioventricular (AV) node located?
It is located at the bottom of the right atria between the atria and ventricle.

8. Where is the bundle of his located?
It is located at the interventricular septum.

9. Where are the Purkinje fibers located?
They are innervated throughout the ventricles from the apex towards the atrioventricular valves.

10. What is the primary function of the SA node?
It acts as a pacemaker.

11. What is the function of the AV node?
It relays SA node impulses to ventricles.

12. What is the function of the bundle of his?
It propagates electrical impulses to the ventricular apex.

13. What is the function of the Purkinje fibers?
They propagate cardiac action potentials throughout the ventricles.

14. How are the components of the cardiac electrical conduction system connected?
They are all electrically coupled.

15. What are the conduction pathways between the SA node and the AV node?
Posterior tract, middle tract (faster than posterior), and anterior tract.

16. What is interesting about the anterior tract between the SA node and AV node?
It bifurcates to Bachmann’s bundle.

17. What is the function of Bachmann’s bundle?
It carries SA node impulses to the left atrium.

18. Is the conduction velocity of electrical impulses in the heart equal?
No

19. What is the purpose of a refractory period in the heart?
It keeps action potentials from going backward.

20. What factor affects the width of the QRS complex?
It depends on the velocity of depolarization of the ventricles.

21. In pacemaker cells of the SA node, the action potential is due to the opening of what channels?
Long-lasting (L) Ca2+ channels

22. Pacemaker potential in pacemaker cells of the SA node is due to what?
Decreasing outflow of potassium ions and the opening of transient (T) Ca2+ channels.

23. What cardiac sound does the QRS interval make?
S1 sound

24. What ECG segments represent the conduction of an action potential through AV-node and AV-bundle?
P-Q segment and P-R segment

25. What wave represents atrial repolarization?
The beginning of the Q wave to the peak of the R wave.

26. What ECG segment represents completely depolarized ventricles?
S-T segment

27. What effect on the ventricles can be seen on an ECG?
One or more P waves may occur without initiating a QRS complex. The atria can depolarize regularly, while the ventricles contract at a much slower pace.

28. How do you look for heart rate on an ECG?
Beginning of one P wave to the beginning of the next P wave, or from the peak of one R wave to the peak of the next R wave.

29. Tachycardia can be seen on ECG as?
A closer distance between two P waves or between two T waves.

30. Bradycardia can be seen on ECG as?
A further distance between two P waves or between two T waves.

31. What is the area that connects cardiac cells?
Intercalated disks

32. What is the area between cardiac cells where permeability is the greatest, which allows the electrical signal to pass quickly between the cells?
Gap junctions

33. What allows an electrical current to easily flow into individual cardiac cells?
Transverse tubules

34. What substance enters the cell when it is electrically activated?
Calcium

35. What is the resting membrane potential value?
-90 mV

36. What accounts for the resting membrane potential being a negative value?
The cell is highly permeable to potassium, so it leaves the cell to enter a lower concentration gradient.

37. What can result from having a lower-than-normal concentration gradient?
Decreased electrical conduction velocity

38. Do the SA/AV nodes have higher or lower threshold potentials than the rest of the cardiac cells?
Lower

39. What is happening during stage 0 of the cardiac cell action potential?
Depolarization; the fast-acting sodium channels are open.

40. What is happening during stage 1 of the cardiac cell action potential?
Initial repolarization; the fast-acting potassium channels are open and sodium channels are closed.

41. What is happening during stage 2 of the cardiac cell action potential?
A leveling off at 0 charge; slow-acting calcium channels are open, and potassium channels are still open.

42. What is happening during stage 3 of the cardiac cell action potential?
Late repolarization; calcium channels close, but sodium channels are still open.

43. What is happening during stage 4 of the cardiac cell action potential?
Resting state; all channels are closed, and only the sodium/potassium pump is working.

44. What is happening during the p wave of the ECG complex?
Depolarization of the atria beginning with the SA node.

45. What is happening during the QRS complex of the ECG complex?
Ventricular depolarization

46. What is happening during the T wave of the ECG complex?
Ventricular repolarization

47. What is happening during the U wave of the ECG complex?
Purkinje fiber repolarization

48. What is happening during the P-R segment of the ECG complex?
The ventricles are filling with blood from the atria.

49. What happens at the J-point of the ECG complex?
This is when ventricular polarization ends.

50. What happens during the S-T segment of the ECG complex?
This is the time between the end of ventricular depolarization and the start of repolarization.

51. What is needed for the heart to pump enough blood to meet the needs of the body?
Contraction must be strong enough to force an appropriate volume of blood at adequate pressure; cardiac muscle cells must contract regularly, at an appropriate rate, and synchronously; valves must have enough diameter when open, and they must close fully; adequate filling must occur during diastole.

52. What triggers an action potential?
Depolarization of the plasma membrane and a shift in the membrane potential to a less negative value.

53. What is the threshold potential?
It is the critical level at which a membrane potential must be depolarized to initiate an action potential.

54. During an action potential, which ions temporarily dominate?
Na+ and Ca+

55. What are the pacemakers of the heart?
SA node, AV node, and Purkinje fibers.

56. What is a pacemaker?
It is the spontaneous time-dependent depolarization of the cell membrane that leads to an action potential.

57. What is the sinoatrial node?
Located in the right atrium; the fastest pacemaker; the primary site of origin of an electrical signal that sets the heart rate in normal conditions.

58. What is the atrioventricular node?
It is located just above the atrioventricular ring; it does not set the heart rate in normal conditions, only when the SA node fails.

59. What are Purkinje fibers?
They originate in the AV node; they conduct an electrical signal from the atria to the right and left ventricles; they are tertiary pacemakers that only become the primary pacemaker when the SA and AV nodes fail.

60. What is the normal initiation source of an electrical cycle?
Sinoatrial node

61. What causes depolarization and repolarization of the heart?
Once a cell generates an electrical impulse, this electrical impulse causes the ions to cross the cell membrane, resulting in an action potential. This is known as depolarization. Repolarization is the return of the ions to their previous resting state, which corresponds with the relaxation of the myocardial muscle.

62. What are the normal values of the electrolytes inside the cardiac cell?
Sodium (Na) = 5 mEq/L; Potassium (K) = 145 mEq/L; Calcium (Ca) = 1 mEq/L

63. What are the normal values of the electrolytes outside the cardiac cell?
Sodium (Na) = 145 mEq/L; Potassium (K) = 5 mEq/L; Calcium (Ca) = 4 mEq/L

64. Where is the origination of a nodal rhythm?
AV node

65. What are some of the causes of a nodal rhythm?
Hypoxemia, caffeine, nicotine, electrolyte imbalance, and some drugs.

66. What is cardiac reserve?
The difference between resting cardiac output and maximum cardiac output.

67. What happens during a myocardial infarction?
Cardiac cell death due to ischemia.

68. How do we treat a myocardial infarction?
Angioplasty, coronary bypass surgery, and stent.

69. What causes CHF?
Congestive heart failure (CHF) occurs when one ventricle is not as efficient as the other ventricle.

70. What characteristics do primary cardiac cells have?
Automaticity, excitability, conductivity, and contractility.

71. What is a sinus rhythm?
Normal heart rhythm

72. What is the lubb sound?
The closing of the AV valve during ventricular diastole.

73. What is the dubb sound?
The closing of the semilunar valve during ventricular systole.

74. What is the cardiac cycle?
It involves one complete contraction and relaxation of the heart.

75. What are the two refractory periods?
(1) Absolute – the time in which the cells cannot respond to a stimulus. (2) Relative – the time in which repolarization is almost complete and a strong stimulus may cause depolarization.

Final Thoughts

Cardiac electrophysiology offers profound insights into the heart’s electrical system, shedding light on the complexities of its regular rhythms and potential abnormalities.

The continuous advancements in this field not only deepen our comprehension of cardiac functionality but also provide the foundation for improved therapeutic interventions.

As arrhythmias continue to affect a significant portion of the global population, the importance of mastering cardiac electrophysiology remains paramount for ensuring optimal cardiovascular health outcomes.