Cardiac Electrophysiology: Key Concepts in Respiratory Care

Cardiac electrophysiology is the study of the heart’s electrical activity and how that activity controls the heartbeat. Every normal cardiac cycle depends on electrical impulses that form in specialized pacemaker cells, travel through the conduction system, and trigger coordinated contraction of the atria and ventricles.

When this electrical process is disrupted, the result may be an abnormal rhythm, impaired cardiac output, or a life-threatening emergency.

For respiratory therapists and other healthcare professionals, understanding cardiac electrophysiology is essential for interpreting ECG findings and recognizing when a patient may need urgent intervention.

What is Cardiac Electrophysiology?

Cardiac electrophysiology refers to the electrical events that allow the heart to generate and conduct impulses. These impulses stimulate myocardial contraction, helping the heart pump blood to the lungs, brain, and the rest of the body.

Although the heart is often described as a mechanical pump, it cannot function properly without electrical control. The myocardium must contract in a coordinated sequence. The atria contract first to help fill the ventricles, and then the ventricles contract to eject blood into the pulmonary and systemic circulation.

This timing depends on specialized cardiac cells that can create, conduct, and respond to electrical impulses. When the impulse begins in the correct location, follows the normal pathway, and produces an appropriate rate and rhythm, the result is normal sinus rhythm.

However, if the impulse starts in the wrong area, travels too slowly, travels too quickly, becomes blocked, or becomes chaotic, an arrhythmia may occur. Some arrhythmias are mild and may not require treatment, while others can quickly reduce cardiac output and become fatal.

Electrical Properties of Cardiac Cells

Cardiac cells have unique electrical properties that allow them to support the heartbeat. These properties include automaticity, excitability, conductivity, and contractility.

• Automaticity is the ability of certain cardiac cells to generate an electrical impulse spontaneously. This is especially important in pacemaker cells, such as those in the sinoatrial node. These cells do not need an outside nerve signal to initiate the heartbeat.
• Excitability is the ability of cardiac cells to respond to an electrical stimulus. When a cardiac cell receives an impulse, it can become electrically activated.
• Conductivity is the ability to pass an impulse from one cell or structure to another. This allows the electrical signal to spread through the heart in an organized pattern.
• Contractility is the ability of myocardial cells to shorten and generate force after electrical activation. Electrical activity does not pump blood by itself. Instead, it triggers mechanical contraction, which produces blood flow.

Note: These properties work together during each cardiac cycle. An impulse forms, cardiac cells respond, the signal spreads through the conduction system, and the myocardium contracts.

Depolarization and Repolarization

Two of the most important terms in cardiac electrophysiology are depolarization and repolarization.

• Depolarization is the electrical activation of a cardiac cell. During this process, the cell changes from its resting electrical state to an active state. This electrical activation prepares the cell for contraction.
• Repolarization is the recovery phase. During repolarization, the cardiac cell returns to its resting electrical condition so it can respond to another impulse.

These electrical changes occur because ions move across the cardiac cell membrane. Sodium, potassium, calcium, and chloride ions help create electrical differences between the inside and outside of the cell. When these ions shift, the electrical charge across the cell membrane changes.

A cardiac cell at rest is considered polarized. When it receives an impulse, it depolarizes. After activation, it repolarizes and returns to a resting state.

Note: This cycle of depolarization and repolarization happens repeatedly with every heartbeat. The electrocardiogram, or ECG, records the summed electrical activity of these processes as waves, intervals, and segments.

The Cardiac Conduction System

The cardiac conduction system is the pathway that carries electrical impulses through the heart. It allows the atria and ventricles to contract in the correct order.

The major structures of the conduction system include:

1. Sinoatrial node
2. Atrioventricular node
3. Bundle of His
4. Right and left bundle branches
5. Purkinje fibers

Note: Each structure plays a specific role in producing a coordinated heartbeat.

Sinoatrial Node

The sinoatrial node, or SA node, is the heart’s normal pacemaker. It is located in the right atrium near the opening of the superior vena cava. The SA node usually controls the heart rate because it has the fastest natural firing rate. In a healthy adult at rest, this produces a heart rate of about 60 to 100 beats/min.

When the SA node fires, the impulse spreads through both atria. This causes atrial depolarization, which appears on the ECG as the P wave.

Because the SA node initiates the impulse, a rhythm that begins there is called a sinus rhythm. Normal sinus rhythm means the impulse starts in the SA node, follows the normal conduction pathway, and produces a regular rhythm with an appropriate rate.

Note: If the SA node fails to fire properly or fires too slowly, another part of the conduction system may take over as a backup pacemaker. However, these backup pacemakers usually fire at slower rates.

Atrioventricular Node

After the impulse spreads through the atria, it reaches the atrioventricular node, or AV node. The AV node is located near the junction between the atria and ventricles. The AV node has two major functions. First, it conducts the impulse from the atria to the ventricles. Second, it briefly delays the impulse before allowing it to pass into the ventricular conduction system.

This delay is clinically important. It gives the ventricles time to fill with blood before they contract. Without this pause, the ventricles might contract too soon, which could reduce stroke volume and cardiac output.

The AV node also helps protect the ventricles from excessively fast atrial rhythms. For example, during atrial flutter or atrial fibrillation, the atria may generate impulses at very rapid rates. The AV node blocks some of these impulses, preventing every atrial impulse from reaching the ventricles.

Note: If AV conduction is delayed or blocked, heart block may occur.

Bundle of His, Bundle Branches, and Purkinje Fibers

After passing through the AV node, the impulse enters the bundle of His. From there, it travels into the right and left bundle branches.

The right bundle branch carries the impulse toward the right ventricle. The left bundle branch carries the impulse toward the left ventricle. These branches then divide into smaller fibers and connect with the Purkinje fiber network.

Purkinje fibers conduct impulses very rapidly through the ventricular myocardium. This rapid conduction allows both ventricles to depolarize and contract in a coordinated manner.

Ventricular contraction normally begins near the apex of the heart and moves upward toward the base. This pattern helps push blood out of the ventricles and into the pulmonary artery and aorta.

Note: When conduction through the bundle branches or Purkinje fibers is abnormal, ventricular depolarization may be delayed or widened. This can affect the appearance of the QRS complex on the ECG.

How the ECG Reflects Cardiac Electrophysiology

The electrocardiogram records the heart’s electrical activity from the body surface. It does not directly measure mechanical contraction, blood pressure, or cardiac output. Instead, it displays electrical changes produced by depolarization and repolarization.

Each waveform on the ECG represents a specific electrical event.

The P wave represents atrial depolarization. This occurs when the impulse spreads from the SA node through the atria. The QRS complex represents ventricular depolarization. This occurs when the impulse spreads through the ventricles and triggers ventricular contraction. The T wave represents ventricular repolarization. This is the recovery phase of the ventricles after depolarization.

Atrial repolarization also occurs, but it is usually hidden within the QRS complex because ventricular depolarization produces a much larger electrical signal.

Note: Understanding these basic waveforms is necessary before interpreting rhythm abnormalities. If the P wave, QRS complex, or T wave is abnormal, it may suggest a problem with impulse formation, impulse conduction, myocardial oxygen supply, electrolyte balance, or ventricular recovery.

ECG Paper and Timing

ECG paper uses a grid that allows clinicians to measure time and voltage. At a standard paper speed of 25 mm/sec, each small box represents 0.04 second. Each large box represents 0.20 second.

This timing is important because many rhythm abnormalities are identified by measuring intervals. For example, the PR interval reflects conduction from the atria through the AV node and into the ventricular conduction system.

Because one large box equals 0.20 second, a normal PR interval should not be longer than one large box. A PR interval longer than 0.20 second suggests delayed AV conduction.

The ECG grid also helps estimate heart rate and evaluate rhythm regularity. By measuring the spacing between QRS complexes, clinicians can determine whether the rhythm is regular or irregular.

The PR Interval

The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It represents the time from the start of atrial depolarization to the start of ventricular depolarization. This includes impulse formation in the SA node, conduction through the atria, delay in the AV node, and movement through the early ventricular conduction pathway.

A normal PR interval is generally no longer than 0.20 second. If the PR interval is prolonged, it means the impulse is taking too long to travel from the atria to the ventricles.

A prolonged PR interval is the defining feature of first-degree AV block. In this rhythm, every P wave is still followed by a QRS complex, but conduction is delayed.

Note: More severe AV blocks occur when some or all atrial impulses fail to reach the ventricles.

The QRS Complex

The QRS complex represents ventricular depolarization. Because the ventricles contain more muscle mass than the atria, the QRS complex is larger than the P wave.

A normal QRS complex is relatively narrow because ventricular depolarization occurs rapidly through the His-Purkinje system. If the QRS complex is widened, it may suggest delayed ventricular conduction.

A wide QRS complex may occur with bundle branch block, ventricular rhythms, premature ventricular contractions, ventricular tachycardia, or certain medication and electrolyte effects.

The appearance of the QRS complex can help determine where a rhythm originates. Rhythms that begin above the ventricles often have narrow QRS complexes because the impulse still travels through the normal ventricular conduction system. Rhythms that begin in the ventricles often have wide QRS complexes because conduction spreads more slowly through the ventricular myocardium.

The ST Segment

The ST segment extends from the end of the QRS complex to the beginning of the T wave. It represents the period between ventricular depolarization and ventricular repolarization. Normally, the ST segment is isoelectric, meaning it lies along the baseline. Significant ST elevation or ST depression may suggest myocardial injury or ischemia.

ST depression is commonly associated with myocardial ischemia, such as angina. ST elevation may be seen in certain types of myocardial infarction. This is why the term STEMI stands for ST-elevation myocardial infarction.

ST-segment changes must be interpreted carefully and in the context of the patient’s symptoms. Chest pain, shortness of breath, diaphoresis, hypotension, nausea, or other signs of poor perfusion may increase concern for a serious cardiac event.

The T Wave

The T wave represents ventricular repolarization. It reflects the recovery phase of the ventricles after electrical activation.

T-wave abnormalities may be associated with ischemia, electrolyte disturbances, ventricular strain, medication effects, or other cardiac problems. For example, T-wave inversion may be seen with myocardial ischemia. Peaked T waves may occur with hyperkalemia.

Although the T wave is sometimes overlooked by beginners, it can provide important clinical information. Changes in T-wave shape, direction, or amplitude should be evaluated along with the rest of the ECG and the patient’s condition.

Normal Sinus Rhythm

Normal sinus rhythm is the expected rhythm in a healthy adult at rest. It begins in the SA node and follows the normal conduction pathway.

Normal sinus rhythm generally has these features:

1. Heart rate between 60 and 100 beats/min
2. Regular rhythm
3. P wave before every QRS complex
4. QRS complex after every P wave
5. Consistent PR interval
6. Narrow QRS complex
7. P waves with a normal appearance, especially upright in lead II

Normal sinus rhythm does not guarantee that the patient is stable, but it does show that the heart’s electrical rhythm is organized and originating from the SA node.

Note: Clinicians must always compare ECG findings with the patient’s clinical status. A patient can have an organized rhythm but still have poor perfusion, severe hypoxemia, hypotension, or other serious problems.

Sinus Bradycardia and Sinus Tachycardia

Sinus bradycardia occurs when the rhythm begins in the SA node but the rate is slower than normal, typically less than 60 beats/min in adults.

It may occur during sleep, in physically fit individuals, or as a response to certain medications. It may also be associated with hypothermia, increased intracranial pressure, myocardial infarction, or conduction system disease.

Treatment depends on the patient’s condition. A stable patient may not need treatment. However, if bradycardia causes hypotension, altered mental status, chest pain, or signs of shock, intervention may be required.

Sinus tachycardia occurs when the rhythm begins in the SA node but the rate is faster than normal, typically greater than 100 beats/min in adults.

It may be caused by fever, anxiety, pain, hypoxemia, anemia, dehydration, shock, pulmonary embolism, or increased metabolic demand. In respiratory care, sinus tachycardia may be seen in patients with respiratory distress, hypoxemia, ventilator weaning stress, or acute bronchospasm.

Note: Sinus tachycardia is usually treated by addressing the underlying cause rather than simply slowing the heart rate.

Sinus Arrhythmia

Sinus arrhythmia is a rhythm that begins in the SA node but varies with the respiratory cycle. The heart rate often increases during inspiration and decreases during expiration.

This occurs because changes in intrathoracic pressure and venous return influence heart rate. In many patients, sinus arrhythmia is a normal finding and does not require treatment.

In patients receiving positive-pressure ventilation, the pattern may differ because intrathoracic pressure changes are reversed compared with spontaneous breathing. This is one reason respiratory therapists should understand the relationship between breathing, venous return, and cardiac rhythm.

Atrial Arrhythmias

Atrial arrhythmias begin in the atria outside the SA node. Because the impulse usually still travels through the AV node and normal ventricular conduction system, the QRS complex is often narrow.

Atrial arrhythmias may affect heart rate, rhythm regularity, atrial emptying, and ventricular filling. In some cases, they can reduce cardiac output or increase the risk of clot formation.

Paroxysmal Atrial Tachycardia

Paroxysmal atrial tachycardia, also called paroxysmal supraventricular tachycardia, is a rapid rhythm that starts suddenly and originates above the ventricles.

It is often caused by an abnormal atrial focus or reentry pathway. The heart rate may be very fast, often between 140 and 250 beats/min. The QRS complexes are usually narrow because ventricular conduction remains normal.

Patients may report palpitations, chest discomfort, shortness of breath, weakness, dizziness, or anxiety. Some patients tolerate the rhythm well, while others develop hypotension or angina, especially if they have underlying coronary artery disease.

Note: Treatment depends on severity and patient stability. Vagal maneuvers, medications, or synchronized cardioversion may be used depending on the situation.

Atrial Flutter

Atrial flutter is caused by a rapid atrial rhythm, often with atrial rates around 250 to 350 beats/min. The ECG may show flutter waves, sometimes described as a sawtooth pattern.

The AV node usually blocks some atrial impulses, so not every atrial impulse reaches the ventricles. This can produce conduction ratios such as 2:1, 3:1, or 4:1.

The ventricular rate depends on how many atrial impulses pass through the AV node. A fast ventricular response can reduce ventricular filling time and lower cardiac output.

Note: Treatment may focus on slowing AV conduction, controlling the rhythm, preventing complications, or performing synchronized cardioversion if the patient is unstable.

Atrial Fibrillation

Atrial fibrillation is one of the most common clinically significant arrhythmias. It occurs when many abnormal electrical impulses fire within the atria, causing chaotic atrial activity.

Instead of organized P waves, the ECG shows fibrillatory activity or an irregular baseline. The QRS complexes are usually irregularly spaced because the AV node allows impulses through in an unpredictable pattern.

Atrial fibrillation prevents normal atrial contraction. This can reduce ventricular filling and increase the risk of blood clot formation within the atria. If a clot travels from the heart to the brain, it can cause a stroke.

Management may include rate control, rhythm control, anticoagulation, or cardioversion depending on the patient’s condition, duration of the rhythm, and risk factors.

Note: For respiratory therapists, atrial fibrillation is important because it may appear during hypoxemia, acute respiratory failure, sepsis, pulmonary embolism, postoperative stress, or severe cardiopulmonary disease.

Atrioventricular Blocks

AV blocks occur when conduction from the atria to the ventricles is delayed or interrupted. These blocks can occur at the AV node, bundle of His, or lower conduction system.

AV blocks are classified as first-degree, second-degree, or third-degree.

First-Degree AV Block

First-degree AV block is characterized by a prolonged PR interval. Every P wave is followed by a QRS complex, but the impulse takes longer than normal to reach the ventricles.

The PR interval is typically greater than 0.20 second. This rhythm is often not immediately dangerous and may not require treatment. However, it may indicate underlying conduction delay and should be evaluated in context. Causes may include medications, myocardial ischemia, increased vagal tone, or conduction system disease.

Second-Degree AV Block

Second-degree AV block occurs when some atrial impulses are conducted to the ventricles and others are blocked. There are two main types: Mobitz type I and Mobitz type II.

Mobitz type I, also known as Wenckebach, occurs when the PR interval gradually lengthens until one P wave is not followed by a QRS complex. After the dropped beat, the cycle repeats.

Mobitz type II occurs when some P waves are suddenly not conducted to the ventricles without progressive PR lengthening. This is usually more serious because it suggests disease in the lower conduction system.

Note: Mobitz type II may progress to complete heart block and often requires a pacemaker.

Third-Degree AV Block

Third-degree AV block, or complete heart block, occurs when atrial impulses do not conduct to the ventricles at all. The atria and ventricles beat independently.

The P waves occur at their own rate, while the QRS complexes occur at a slower escape rhythm. If the escape rhythm originates near the AV junction, the QRS complex may be narrow. If it originates in the ventricles, the QRS complex may be wide.

Complete heart block can significantly reduce cardiac output. Patients may experience weakness, fatigue, dizziness, syncope, hypotension, or signs of shock.

Note: A pacemaker is often required because the ventricles cannot rely on normal atrial conduction.

Ventricular Arrhythmias

Ventricular arrhythmias begin in the ventricles. They are often more dangerous than atrial arrhythmias because they can severely impair cardiac output.

Since ventricular impulses do not follow the normal His-Purkinje pathway, the QRS complexes are often wide and abnormal-looking.

Premature Ventricular Contractions

Premature ventricular contractions, or PVCs, are early beats that originate in the ventricles. They usually appear as wide, abnormal QRS complexes that occur before the next expected beat.

PVCs may be isolated or frequent. They may occur in healthy individuals, but they can also be associated with hypoxemia, myocardial ischemia, electrolyte abnormalities, stimulant use, medication effects, or cardiac disease.

In respiratory care, PVCs may appear during low oxygen levels, acid-base disturbances, suctioning, ventilator changes, or acute cardiopulmonary stress. Occasional PVCs may not require treatment, but frequent, multifocal, or repetitive PVCs should be evaluated carefully.

Ventricular Tachycardia

Ventricular tachycardia is a rapid rhythm that originates in the ventricles. It is usually defined as a series of three or more consecutive ventricular beats.

The ECG typically shows a fast rhythm with wide QRS complexes. Because the ventricles are beating rapidly and inefficiently, cardiac output may fall. Some patients with ventricular tachycardia have a pulse, while others are pulseless. This distinction is critical.

Ventricular tachycardia with a pulse may cause chest pain, shortness of breath, hypotension, altered mental status, or shock. Pulseless ventricular tachycardia is a cardiac arrest rhythm and requires immediate defibrillation.

Ventricular Fibrillation

Ventricular fibrillation is a chaotic rhythm in which the ventricles quiver instead of contracting effectively. There is no organized ventricular depolarization, no effective stroke volume, and no meaningful cardiac output.

On the ECG, ventricular fibrillation appears as irregular, chaotic electrical activity without identifiable P waves, QRS complexes, or T waves. This rhythm is immediately life-threatening. The patient is pulseless and requires rapid defibrillation and cardiopulmonary resuscitation.

Note: Ventricular fibrillation may occur with myocardial infarction, severe ischemia, electrolyte disturbances, hypoxemia, drug toxicity, or other serious cardiac conditions.

Pulseless Electrical Activity

Pulseless electrical activity, or PEA, occurs when the ECG shows organized electrical activity, but the patient has no palpable pulse.

This is an important reminder that electrical activity does not always mean mechanical contraction is effective. The ECG may look organized, but the heart may not be producing enough force to generate circulation.

PEA is not treated with defibrillation because the rhythm is not a shockable rhythm. Management focuses on high-quality CPR, medications according to emergency protocols, and identifying reversible causes.

Potential causes include hypoxemia, hypovolemia, hydrogen ion excess, hyperkalemia, hypokalemia, hypothermia, tension pneumothorax, tamponade, toxins, thrombosis, and other life-threatening problems.

Note: Respiratory therapists may play an important role in identifying and correcting hypoxemia, ventilation problems, and tension pneumothorax during resuscitation.

Why Cardiac Electrophysiology Matters in Respiratory Care

Respiratory therapists often care for patients with cardiopulmonary instability. Because the heart and lungs work together, respiratory problems can directly affect cardiac electrical activity.

Hypoxemia can irritate the myocardium and trigger dysrhythmias. Hypercapnia and respiratory acidosis can affect cardiac function and vascular tone. Electrolyte disturbances, especially potassium abnormalities, can change depolarization and repolarization. Myocardial ischemia may produce ST-segment and T-wave changes. Positive-pressure ventilation can alter venous return and influence heart rate, blood pressure, and rhythm.

During ventilator weaning, increased work of breathing can raise oxygen demand and stress the cardiovascular system. A patient who develops tachycardia, arrhythmias, chest pain, or ECG changes during weaning may not be tolerating the process.

Respiratory therapists may also be involved in performing or assisting with ECG monitoring, recognizing dangerous rhythms, responding to cardiac arrest, and supporting oxygenation and ventilation during emergencies.

Note: Because of this, ECG interpretation should never be viewed as only a nursing or cardiology responsibility. Respiratory therapists must be able to recognize when cardiac electrical activity suggests a serious problem.

Systematic ECG Interpretation

A systematic approach helps prevent missed findings. When reviewing an ECG rhythm strip, clinicians should evaluate the same features each time.

Important features include heart rate, rhythm regularity, P waves, PR interval, QRS duration, QRS appearance, ST segment, T wave, QT interval, and the relationship between atrial and ventricular activity.

The first step is often to determine the heart rate. Next, assess whether the rhythm is regular or irregular. Then look for P waves and determine whether each P wave is followed by a QRS complex.

The PR interval should be measured to assess AV conduction. The QRS complex should be evaluated to determine whether ventricular conduction is normal or delayed. The ST segment and T wave should be checked for signs of ischemia, injury, or repolarization abnormalities.

Finally, the rhythm must be interpreted in relation to the patient. A stable patient with a mild rhythm abnormality may require monitoring, while an unstable patient with the same rhythm may need urgent treatment.

Clinical Signs That Increase Concern

ECG findings are most meaningful when combined with clinical assessment. Certain signs and symptoms increase concern for a serious cardiac problem. These include chest pain, shortness of breath, hypotension, syncope, altered mental status, diaphoresis, cyanosis, severe weakness, pulmonary edema, low oxygen saturation, and signs of shock.

A rhythm disturbance should be taken more seriously if the patient is symptomatic. For example, sinus bradycardia may be harmless in one patient but dangerous in another if it causes hypotension or altered mental status.

Similarly, atrial fibrillation may be stable in one patient but poorly tolerated in another if the ventricular response is very fast or the patient has limited cardiac reserve.

Note: The ECG provides electrical information, but patient assessment determines urgency.

Cardiac Electrophysiology Practice Questions

1. What is cardiac electrophysiology?
Cardiac electrophysiology is the study of the heart’s electrical activity, including how electrical impulses are generated, conducted, and interpreted on an ECG.

2. What does a cardiac electrophysiologist specialize in?
A cardiac electrophysiologist is a cardiology specialist who diagnoses and treats heart rhythm disorders, also known as arrhythmias.

3. What is the difference between a cardiologist and an electrophysiologist?
A cardiologist treats a broad range of heart and vascular conditions, while an electrophysiologist focuses specifically on the heart’s electrical system and rhythm disturbances.

4. What is the main purpose of the cardiac conduction system?
The cardiac conduction system coordinates the electrical impulses that cause the atria and ventricles to contract in an organized sequence.

5. What are the major components of the cardiac conduction system?
The major components include the sinoatrial node, atrioventricular node, bundle of His, right and left bundle branches, and Purkinje fibers.

6. What is the sinoatrial (SA) node?
The SA node is the heart’s natural pacemaker and normally initiates each electrical impulse that begins the cardiac cycle.

7. Where is the SA node located?
The SA node is located in the upper right atrium near the entrance of the superior vena cava.

8. Why is the SA node considered the primary pacemaker of the heart?
The SA node has the fastest intrinsic rate of spontaneous depolarization, allowing it to normally control the heart rate.

9. What is the normal intrinsic firing rate of the SA node?
The normal intrinsic firing rate of the SA node is approximately 60 to 100 impulses per minute.

10. What is the atrioventricular (AV) node?
The AV node is a specialized area of conduction tissue that receives impulses from the atria and delays them before transmitting them to the ventricles.

11. Where is the AV node located?
The AV node is located in the lower portion of the right atrium near the interatrial septum and the opening of the coronary sinus.

12. What is the main function of the AV node?
The AV node delays electrical conduction long enough to allow the ventricles to fill with blood before ventricular contraction begins.

13. What is the normal intrinsic firing rate of the AV junction?
The AV junction typically has an intrinsic firing rate of approximately 40 to 60 impulses per minute.

14. What is the bundle of His?
The bundle of His is a conduction pathway that carries impulses from the AV node into the interventricular septum.

15. What happens after the impulse travels through the bundle of His?
The impulse divides into the right and left bundle branches, which carry the signal toward the ventricles.

16. What are the bundle branches?
The right and left bundle branches are conduction pathways that transmit electrical impulses through the interventricular septum toward each ventricle.

17. What are Purkinje fibers?
Purkinje fibers are specialized conduction fibers that rapidly distribute electrical impulses throughout the ventricular myocardium.

18. Where are Purkinje fibers located?
Purkinje fibers are located throughout the ventricular walls, especially near the endocardial surfaces of the ventricles.

19. What is the normal intrinsic firing rate of the Purkinje fibers?
Purkinje fibers typically have an intrinsic firing rate of approximately 20 to 40 impulses per minute.

20. What is automaticity?
Automaticity is the ability of cardiac pacemaker cells to spontaneously generate electrical impulses without external stimulation.

21. What is excitability?
Excitability is the ability of cardiac cells to respond to an electrical stimulus by generating an action potential.

22. What is conductivity?
Conductivity is the ability of cardiac cells to transmit electrical impulses from one cell to another.

23. What is contractility?
Contractility is the ability of cardiac muscle cells to shorten and generate force in response to electrical stimulation.

24. What structure allows cardiac cells to communicate electrically?
Gap junctions allow ions to pass between cardiac cells, permitting rapid electrical communication.

25. What are intercalated discs?
Intercalated discs are specialized connections between cardiac muscle cells that contain gap junctions and desmosomes.

26. Why are gap junctions important in cardiac electrophysiology?
Gap junctions help cardiac muscle behave as a functional syncytium by allowing electrical impulses to spread quickly between cells.

27. What is a resting membrane potential?
The resting membrane potential is the electrical charge difference across the cell membrane when the cardiac cell is not actively depolarizing.

28. What is the approximate resting membrane potential of ventricular muscle cells?
The approximate resting membrane potential of ventricular muscle cells is about -90 mV.

29. Why is the inside of a resting cardiac cell negatively charged?
The inside of a resting cardiac cell is negative mainly because potassium tends to leave the cell, creating an electrical gradient across the membrane.

30. Which ion is most important for maintaining the resting membrane potential?
Potassium is the main ion involved in maintaining the resting membrane potential of cardiac muscle cells.

31. What is depolarization?
Depolarization is the process in which the inside of the cardiac cell becomes less negative, allowing an electrical impulse to occur.

32. What is repolarization?
Repolarization is the process in which the cardiac cell returns toward its resting electrical state after depolarization.

33. What is threshold potential?
Threshold potential is the membrane voltage that must be reached to trigger an action potential.

34. What is an action potential?
An action potential is a rapid change in membrane voltage that allows cardiac cells to transmit electrical signals and trigger contraction.

35. What ion is primarily responsible for rapid depolarization in ventricular muscle cells?
Sodium is primarily responsible for rapid depolarization in ventricular muscle cells.

36. What ion is important during the plateau phase of the ventricular action potential?
Calcium is important during the plateau phase because calcium entry helps sustain depolarization and contributes to contraction.

37. What ion is primarily responsible for repolarization?
Potassium is primarily responsible for repolarization as it exits the cardiac cell.

38. What occurs during phase 0 of the ventricular action potential?
Phase 0 is rapid depolarization caused mainly by the opening of fast sodium channels.

39. What occurs during phase 1 of the ventricular action potential?
Phase 1 is early repolarization, caused by sodium channel inactivation and a brief outward movement of potassium.

40. What occurs during phase 2 of the ventricular action potential?
Phase 2 is the plateau phase, caused by calcium moving into the cell while potassium continues to move out.

41. What occurs during phase 3 of the ventricular action potential?
Phase 3 is repolarization, caused mainly by potassium leaving the cell as calcium channels close.

42. What occurs during phase 4 of the ventricular action potential?
Phase 4 is the resting phase, during which the cell maintains its resting membrane potential until the next impulse arrives.

43. How is the pacemaker action potential different from the ventricular action potential?
Pacemaker cells depolarize spontaneously and rely more on calcium influx, while ventricular muscle cells rely heavily on fast sodium influx for rapid depolarization.

44. What causes spontaneous depolarization in SA node pacemaker cells?
Spontaneous depolarization occurs due to gradual changes in ion movement, including funny current activity, reduced potassium outflow, and calcium entry.

45. Which ion is mainly responsible for phase 0 depolarization in SA and AV nodal cells?
Calcium is mainly responsible for phase 0 depolarization in SA and AV nodal cells.

46. What is the funny current?
The funny current is a pacemaker current that contributes to spontaneous depolarization in SA node cells.

47. What is the refractory period?
The refractory period is the time during which a cardiac cell cannot respond, or responds only with difficulty, to another stimulus.

48. What is the absolute refractory period?
The absolute refractory period is the time during which a cardiac cell cannot generate another action potential, regardless of stimulus strength.

49. What is the relative refractory period?
The relative refractory period is the time during which a stronger-than-normal stimulus may trigger another action potential.

50. Why is the refractory period important?
The refractory period helps prevent sustained tetanic contraction and allows the heart to relax and refill between beats.

51. What is an ECG?
An ECG, or electrocardiogram, is a recording of the heart’s electrical activity from electrodes placed on the body.

52. What does the P wave represent on an ECG?
The P wave represents atrial depolarization.

53. What does the PR interval represent?
The PR interval represents the time from the start of atrial depolarization to the start of ventricular depolarization.

54. What is the normal PR interval?
The normal PR interval is approximately 0.12 to 0.20 seconds.

55. What does a prolonged PR interval suggest?
A prolonged PR interval may suggest delayed conduction through the AV node, such as first-degree AV block.

56. What does the QRS complex represent?
The QRS complex represents ventricular depolarization.

57. What is the normal QRS duration?
The normal QRS duration is usually less than 0.12 seconds.

58. What does a widened QRS complex suggest?
A widened QRS complex may suggest delayed ventricular conduction, such as bundle branch block or ventricular rhythm.

59. What does the T wave represent?
The T wave represents ventricular repolarization.

60. What does the ST segment represent?
The ST segment represents the time between ventricular depolarization and ventricular repolarization, when the ventricles are normally fully depolarized.

61. What is the J point on an ECG?
The J point is the point where the QRS complex ends and the ST segment begins.

62. What does the QT interval represent?
The QT interval represents the total time for ventricular depolarization and repolarization.

63. Why is a prolonged QT interval clinically important?
A prolonged QT interval can increase the risk of dangerous ventricular arrhythmias, including torsades de pointes.

64. What does the U wave represent?
The U wave is a small deflection that may be related to repolarization of Purkinje fibers or ventricular repolarization abnormalities.

65. What is atrial repolarization usually hidden by?
Atrial repolarization is usually hidden within the QRS complex.

66. How can heart rate be estimated on an ECG?
Heart rate can be estimated by measuring the distance between consecutive R waves or by counting QRS complexes over a defined time period.

67. What is normal sinus rhythm?
Normal sinus rhythm is a regular rhythm that originates in the SA node, has a rate of 60 to 100 beats/min, and includes a P wave before each QRS complex.

68. What is sinus bradycardia?
Sinus bradycardia is a sinus rhythm with a heart rate less than 60 beats/min.

69. What is sinus tachycardia?
Sinus tachycardia is a sinus rhythm with a heart rate greater than 100 beats/min.

70. What is an arrhythmia?
An arrhythmia is an abnormal heart rhythm caused by a problem with impulse formation, impulse conduction, or both.

71. What is an ectopic focus?
An ectopic focus is an abnormal pacemaker site outside the SA node that generates electrical impulses.

72. What is a premature atrial contraction?
A premature atrial contraction is an early beat that originates in the atria outside the SA node.

73. What is a premature ventricular contraction?
A premature ventricular contraction is an early beat that originates in the ventricles before the next expected sinus beat.

74. What is atrial fibrillation?
Atrial fibrillation is an irregular rhythm caused by chaotic electrical activity in the atria, often producing an irregularly irregular ventricular response.

75. What is atrial flutter?
Atrial flutter is a rapid atrial rhythm commonly associated with sawtooth flutter waves on ECG.

76. What is supraventricular tachycardia?
Supraventricular tachycardia is a rapid rhythm that originates above the ventricles, often producing a narrow QRS complex.

77. What is ventricular tachycardia?
Ventricular tachycardia is a rapid rhythm that originates in the ventricles and typically produces wide QRS complexes.

78. Why is ventricular tachycardia dangerous?
Ventricular tachycardia can reduce cardiac output and may deteriorate into ventricular fibrillation or cardiac arrest.

79. What is ventricular fibrillation?
Ventricular fibrillation is a chaotic ventricular rhythm that prevents effective ventricular contraction and requires immediate defibrillation.

80. What is asystole?
Asystole is the absence of detectable ventricular electrical activity, often described as a flatline rhythm.

81. What is pulseless electrical activity?
Pulseless electrical activity is organized electrical activity on the ECG without a palpable pulse or effective mechanical contraction.

82. What is first-degree AV block?
First-degree AV block is delayed AV conduction with a prolonged PR interval, but every P wave is followed by a QRS complex.

83. What is second-degree AV block?
Second-degree AV block occurs when some atrial impulses fail to conduct to the ventricles.

84. What is Mobitz type I AV block?
Mobitz type I, or Wenckebach, is characterized by progressive PR interval prolongation until a QRS complex is dropped.

85. What is Mobitz type II AV block?
Mobitz type II is characterized by intermittent dropped QRS complexes without progressive PR interval prolongation.

86. What is third-degree AV block?
Third-degree AV block is complete heart block, in which atrial impulses do not conduct to the ventricles and the atria and ventricles beat independently.

87. Why can third-degree AV block be dangerous?
Third-degree AV block can cause severe bradycardia, low cardiac output, syncope, and hemodynamic instability.

88. What is a bundle branch block?
A bundle branch block is delayed or blocked conduction through one of the ventricular bundle branches, resulting in delayed ventricular activation.

89. What ECG finding is commonly associated with bundle branch block?
Bundle branch block commonly produces a widened QRS complex.

90. How does sympathetic stimulation affect cardiac electrophysiology?
Sympathetic stimulation increases heart rate, conduction velocity, and myocardial excitability.

91. How does parasympathetic stimulation affect cardiac electrophysiology?
Parasympathetic stimulation slows the SA node firing rate and slows conduction through the AV node.

92. How can hypoxemia affect the heart’s electrical activity?
Hypoxemia can irritate the myocardium, impair conduction, and increase the risk of arrhythmias.

93. How can potassium abnormalities affect cardiac electrophysiology?
Potassium abnormalities can alter resting membrane potential and repolarization, increasing the risk of conduction problems and arrhythmias.

94. What ECG changes may occur with hyperkalemia?
Hyperkalemia may cause peaked T waves, PR prolongation, QRS widening, and severe arrhythmias if untreated.

95. What ECG changes may occur with hypokalemia?
Hypokalemia may cause flattened T waves, ST depression, prominent U waves, and increased risk of arrhythmias.

96. How can calcium abnormalities affect the QT interval?
Hypocalcemia can prolong the QT interval, while hypercalcemia can shorten the QT interval.

97. What is an electrophysiology study?
An electrophysiology study is an invasive test that maps the heart’s electrical activity to identify the source of arrhythmias.

98. What is catheter ablation?
Catheter ablation is a procedure that destroys or isolates abnormal electrical pathways or tissue responsible for certain arrhythmias.

99. What is an artificial pacemaker?
An artificial pacemaker is a device that delivers electrical impulses to help maintain an appropriate heart rate or rhythm.

100. What is the main goal of studying cardiac electrophysiology?
The main goal is to understand how the heart generates and conducts electrical impulses so clinicians can recognize, interpret, and manage rhythm disturbances.

Final Thoughts

Cardiac electrophysiology explains how the heart generates, conducts, and recovers from electrical impulses during each heartbeat. This process begins with pacemaker activity, moves through the SA node, AV node, bundle branches, and Purkinje fibers, and appears on the ECG as waves, intervals, and segments.

When impulse formation or conduction becomes abnormal, patients may develop arrhythmias, heart blocks, ischemic changes, or cardiac arrest rhythms.

For respiratory therapists, this knowledge is especially important because oxygenation, ventilation, acid-base balance, and cardiopulmonary stress can all influence rhythm. Understanding cardiac electrophysiology supports faster recognition, better assessment, and safer patient care.