ECG Fundamentals and Arrhythmias: Waveforms, Conduction & Rhythm Disorders

Describe the basic 3-lead ECG.

4 electrodes: both arms + left leg; the right leg is only a ground (records nothing)
Lead I: RA-LA; lead II: RA-LL; lead III: LA-LL
Together they form Einthoven's triangle

Is a lead the same as an electrode?

No. A lead is a particular view of the heart's electrical activity; an electrode is the sensor on the body

Describe the 12-lead ECG.

Gold standard; uses 10 electrodes
6 precordial (chest) → V1-V6
4 limb (RA, LA, RL, LL) → I, II, III (bipolar) + aVR, aVL, aVF (unipolar)
If something looks abnormal, focus on that one lead

Why can small ventricular abnormalities cause marked changes on individual chest leads?

The heart is close to the chest wall, so each chest electrode mainly records the muscle directly beneath it, especially the anterior ventricular wall

What do the P wave, QRS complex, and T wave represent?

"P: atrial depolarization
QRS: ventricular depolarization
T: ventricular repolarization
(The ECG records electrical activity, not contraction)"

Define the PQ segment and PR interval.

"PQ segment: impulse traveling through the AV node, bundle of His, and bundle branches
PR interval: onset of the P wave → middle of the QRS
"

Define the ST segment, ST interval, and QT interval.

"ST segment: end of QRS → onset of T
ST interval: end of QRS → end of T
QT interval: onset of QRS → end of T
"

What are the criteria for normal sinus rhythm in adults?

Rate 60-100 bpm; rhythm regular
P waves uniform, upright in lead II, one before each QRS
PR interval 0.12-0.20 s, constant
QRS ≤ 0.10 s

Overdrive suppression: what are latent pacemakers?

Latent pacemakers = AV node and Purkinje fibers: they have automaticity and can take over if the SA node is suppressed

Rank the intrinsic pacemaker rates and give each rate.

SA node (60-100 bpm) > AV node (40-60 bpm) > Purkinje fibers (15-40 bpm)
Phase 4 depolarization is fastest in the SA node and slowest in the Purkinje fibers

Describe the basic 3-lead ECG.

4 electrodes: both arms + left leg; the right leg is only a ground (records nothing)
Lead I: RA-LA; lead II: RA-LL; lead III: LA-LL
Together they form Einthoven's triangle

Is a lead the same as an electrode?

No. A lead is a particular view of the heart's electrical activity; an electrode is the sensor on the body

Describe the 12-lead ECG.

Gold standard; uses 10 electrodes
6 precordial (chest) → V1-V6
4 limb (RA, LA, RL, LL) → I, II, III (bipolar) + aVR, aVL, aVF (unipolar)
If something looks abnormal, focus on that one lead

Why can small ventricular abnormalities cause marked changes on individual chest leads?

The heart is close to the chest wall, so each chest electrode mainly records the muscle directly beneath it, especially the anterior ventricular wall

What do the P wave, QRS complex, and T wave represent?

"P: atrial depolarization
QRS: ventricular depolarization
T: ventricular repolarization
(The ECG records electrical activity, not contraction)
"

Define the PQ segment and PR interval.

"PQ segment: impulse traveling through the AV node, bundle of His, and bundle branches
PR interval: onset of the P wave → middle of the QRS
"

Define the ST segment, ST interval, and QT interval.

"ST segment: end of QRS → onset of T
ST interval: end of QRS → end of T
QT interval: onset of QRS → end of T"

What are the criteria for normal sinus rhythm in adults?

Rate 60-100 bpm; rhythm regular
P waves uniform, upright in lead II, one before each QRS
PR interval 0.12-0.20 s, constant
QRS ≤ 0.10 s

Overdrive suppression: what are latent pacemakers?

Latent pacemakers = AV node and Purkinje fibers: they have automaticity and can take over if the SA node is suppressed

Rank the intrinsic pacemaker rates and give each rate.

SA node (60-100 bpm) > AV node (40-60 bpm) > Purkinje fibers (15-40 bpm)
Phase 4 depolarization is fastest in the SA node and slowest in the Purkinje fibers

1. What are the four basic types of ECG abnormalities?

Abnormal sinus rates, escape rhythms, ectopic beats and rhythms, and atrioventricular (AV) blocks.

2. What is the normal pathway of electrical conduction through the heart?

The SA node initiates the impulse, which spreads through the atria, reaches the AV node, travels through the bundle of His, divides into right and left bundle branches, and spreads through the Purkinje fibers to activate the ventricles.

3. What is the SA node, and what is its function?

The sinoatrial node is the heart's natural pacemaker. It normally initiates electrical impulses at 60-100 beats per minute, causing atrial depolarization.

4. What is the function of the AV node?

The atrioventricular node briefly delays electrical conduction, allowing the ventricles time to fill with blood before they contract.

5. What are the heart's three main pacemaker rates?

SA node: 60-100 bpm; AV junction: 40-60 bpm; ventricular pacemakers: 20-40 bpm.

6. What are the characteristics of normal sinus rhythm (NSR)?

Regular rhythm, rate of 60-100 bpm, one P wave before each QRS, PR interval of 0.12-0.20 seconds, and QRS duration of 0.04-0.10 seconds.

7. What do the P wave, QRS complex, and T wave represent?

P wave: atrial depolarization; QRS complex: ventricular depolarization; T wave: ventricular repolarization.

8. What does the PR interval measure, and what is its normal duration?

It measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. Normal duration is 0.12-0.20 seconds.

9. What does the QRS duration measure, and what is its normal value?

It measures the time required for ventricular depolarization. Normal duration is approximately 0.04-0.10 seconds.

10. What does the R-R interval measure, and how is it used?

It measures the time between consecutive ventricular depolarizations and helps determine heart rate and rhythm regularity.

11. What is sinus tachycardia, and what are its ECG characteristics?

A sinus rhythm faster than 100 bpm. It has normal P waves before every QRS complex, generally normal PR and QRS intervals, and shorter R-R intervals.

12. Why is sinus tachycardia still considered a sinus rhythm?

Because the electrical impulse continues to originate from the SA node, even though the heart rate is faster than normal.

13. What are two pathological and two physiological causes of sinus tachycardia?

Pathological: fever and anemia. Physiological: exercise and emotional stress. These increase metabolic demand or sympathetic stimulation.

14. How does the sympathetic nervous system affect heart rate?

It increases heart rate and cardiac activity through sympathetic stimulation and the release of catecholamines such as epinephrine.

15. What is sinus bradycardia, and what are its ECG characteristics?

A sinus rhythm slower than 60 bpm. It has normal P waves before each QRS, generally normal PR and QRS intervals, and longer R-R intervals.

16. Why is sinus bradycardia still considered a sinus rhythm?

Because the SA node still initiates the electrical impulses, but it fires at a slower rate.

17. What are two pathological and two physiological causes of sinus bradycardia?

Pathological: hypothyroidism and SA node dysfunction. Physiological: sleep and athletic conditioning.

18. How does the parasympathetic nervous system affect heart rate?

It slows the heart rate primarily through the vagus nerve, which decreases SA node firing and slows AV node conduction.

19. What is an escape rhythm, and why does it occur?

A rhythm generated by a backup or latent pacemaker when the SA node fails to initiate impulses or impulses fail to reach the ventricles. It allows the heart to continue beating.

20. What is a junctional escape rhythm, and what are its ECG characteristics?

The AV junction takes over as the pacemaker. The rate is approximately 40-60 bpm. P waves may be absent, inverted, or occur before or after the QRS. QRS complexes are usually narrow.

21. Why can P waves be absent or inverted during a junctional escape rhythm?

The impulse may travel backward toward the atria, producing retrograde atrial depolarization, or atrial and ventricular activation may occur nearly simultaneously.

22. What is a ventricular escape rhythm, and what are its ECG characteristics?

A ventricular pacemaker takes over. The rate is approximately 20-40 bpm, P waves may be absent or independent, and QRS complexes are wide and abnormal.

23. Why is the QRS complex wide during a ventricular escape rhythm?

The impulse originates in the ventricles instead of following the normal rapid His-Purkinje conduction pathway, causing slower ventricular depolarization.

24. What is the difference between junctional and ventricular escape rhythms?

Junctional: 40-60 bpm, AV junction acts as pacemaker, usually narrow QRS. Ventricular: 20-40 bpm, ventricular tissue acts as pacemaker, wide QRS.

25. How does a pacemaker correct an escape rhythm?

It delivers electrical impulses through electrodes to stimulate the heart when the natural pacemakers fail or produce an inadequate heart rate.

26. What is an ectopic beat, and what causes it?

An abnormal heartbeat originating outside the SA node due to increased excitability or abnormal electrical activity in a group of cardiac cells.

27. What is the difference between an ectopic beat and an ectopic rhythm?

An ectopic beat is an isolated abnormal heartbeat. An ectopic rhythm occurs when abnormal electrical activity produces repeated or consistent abnormal beats.

28. What is a premature atrial complex (PAC), and what are its ECG characteristics?

An early heartbeat originating from an ectopic focus in the atria. The ECG shows an early, abnormally shaped P wave, usually followed by a normal narrow QRS.

29. What are common causes of premature atrial complexes?

Stress, caffeine, alcohol, nicotine, lack of sleep, stimulants, and underlying heart disease.

30. What is atrial fibrillation (A-fib), and what are its ECG characteristics?

Disorganized electrical activity in the atria. Normal P waves disappear and are replaced by chaotic atrial activity. The R-R intervals are irregularly irregular.

31. Why does atrial fibrillation increase the risk of stroke?

The atria do not contract effectively, causing blood to pool, especially in the left atrial appendage. Blood clots can form and travel to the brain.

32. What is a premature ventricular complex (PVC), and what are its ECG characteristics?

An early heartbeat originating from an ectopic focus in the ventricles. It produces a premature, wide, abnormal QRS complex, usually without a preceding P wave.

33. Why is the QRS complex wide during a PVC?

The electrical impulse starts in the ventricular muscle instead of traveling through the normal rapid conduction pathway, causing slower ventricular depolarization.

34. What are common causes of PVCs?

Stress, caffeine, nicotine, electrolyte imbalances, myocardial ischemia, heart attacks, and underlying heart disease.

35. What is ventricular fibrillation (V-fib), and what does it look like on an ECG?

A chaotic electrical rhythm in the ventricles that prevents effective contraction. The ECG shows disorganized electrical activity with no identifiable normal P waves, QRS complexes, or T waves.

36. Why is ventricular fibrillation more dangerous than atrial fibrillation?

In V-fib, the ventricles cannot pump blood effectively, causing cardiac arrest. In A-fib, the ventricles generally continue pumping blood despite the abnormal atrial rhythm.

37. How does an AED treat ventricular fibrillation?

It analyzes the heart rhythm and delivers a shock if a shockable rhythm is detected. The shock depolarizes a critical mass of cardiac cells, allowing organized electrical activity to potentially resume.

38. Which rhythms are shockable with an AED, and which are not?

Shockable: ventricular fibrillation and pulseless ventricular tachycardia. Not shockable: asystole and pulseless electrical activity (PEA). Atrial fibrillation is not an AED shockable rhythm.

39. What is an AV block, and what causes it?

An impairment or delay in electrical conduction between the atria and ventricles, commonly involving the AV node, bundle of His, or other conduction pathways.

40. What is first-degree AV block, and what are its ECG characteristics?

Delayed AV conduction in which every atrial impulse still reaches the ventricles. The PR interval is longer than 0.20 seconds, and every P wave is followed by a QRS complex.

41. What is happening to conduction during first-degree AV block, and what acts as the pacemaker?

Conduction is slowed but not completely blocked. The SA node remains the pacemaker.

42. What is second-degree AV block, and what are its ECG characteristics?

Some atrial impulses fail to reach the ventricles. The ECG shows P waves that are not followed by QRS complexes, also called dropped beats.

43. What are the two types of second-degree AV block, and how do their PR intervals differ?

Mobitz I (Wenckebach): PR intervals progressively lengthen until a QRS is dropped. Mobitz II: PR intervals remain constant in conducted beats, but some P waves suddenly fail to produce QRS complexes.

44. Which is more dangerous, first-degree or second-degree AV block, and why?

Second-degree AV block is generally more concerning because some impulses fail to reach the ventricles. Mobitz II is particularly dangerous because it can progress to complete heart block.

45. What acts as the pacemaker during second-degree AV block?

The SA node continues to pace the atria. Ventricles respond to impulses that successfully conduct; if conduction fails, a junctional or ventricular escape pacemaker may take over.

46. What is third-degree AV block, also called complete heart block?

A condition in which no electrical impulses from the atria reach the ventricles. The atria and ventricles beat independently.

47. What are the ECG characteristics of third-degree AV block?

P waves and QRS complexes occur independently, with no consistent PR interval. The atrial rate is usually faster than the ventricular escape rate.

48. What acts as the pacemaker during third-degree AV block, and why is it dangerous?

The SA node controls the atria, while a junctional or ventricular escape pacemaker controls the ventricles. It is dangerous because the ventricular rate may be extremely slow, decreasing cardiac output and potentially causing fainting or cardiac arrest.

49. How can a pacemaker treat third-degree AV block?

It delivers electrical impulses to stimulate ventricular contractions because the normal atrial impulses cannot reach the ventricles.

50. What is the difference between defibrillation and pacemaker treatment?

Defibrillation delivers a high-energy shock to treat shockable rhythms such as V-fib. A pacemaker delivers timed electrical impulses to maintain an adequate heart rate or support conduction when the heart beats too slowly.