SCST-D1 SCST Diploma in ECG
Interpretation: Questions and Answers
with Rationales
1. What is the most common lethal arrhythmia following a myocardial infarction
that requires immediate defibrillation?
Answer: Ventricular Fibrillation (VF).
Rationale: VF produces a chaotic, disorganized electrical activity with no effective
cardiac output; defibrillation is the only definitive treatment.
2. A patient presents with a wide QRS tachycardia at 220 bpm, regular, with no
visible P waves and a history of previous heart attacks. What is the most likely
diagnosis?
Answer: Monomorphic Ventricular Tachycardia (VT).
Rationale: In a patient with structural heart disease, a regular wide-complex
tachycardia is VT until proven otherwise; AV dissociation or fusion beats confirm
it.
3. Which ECG finding is most specific for acute pericarditis?
Answer: Diffuse ST-segment elevation with PR-segment depression.
Rationale: Unlike the regional ST elevation of infarction, pericarditis causes
widespread inflammation, leading to ST elevation in most leads and reciprocal PR
depression.
4. What is the "R-on-T" phenomenon and why is it dangerous?
Answer: A premature ventricular complex (PVC) falling on the T wave of the
preceding beat; it can trigger Ventricular Fibrillation.
Rationale: The T wave represents the vulnerable period of repolarization; an early
PVC during this window may initiate malignant arrhythmias.
5. In a patient with suspected inferior MI, which additional lead should be recorded
to detect right ventricular involvement?
Answer: Lead V4R (right-sided V4).
*Rationale: Right ventricular infarction complicates up to 40% of inferior MIs; ST
elevation in V4R (right-sided) is highly sensitive and specific.*
,6. What is the "intrinsic" firing rate of the Sinoatrial (SA) Node?
Answer: 60 to 100 beats per minute.
Rationale: The SA node is the primary pacemaker because it has the fastest rate of
spontaneous depolarization in a healthy heart.
7. A PR interval shorter than 0.12 seconds is a hallmark of:
Answer: Pre-excitation (e.g., Wolff-Parkinson-White Syndrome).
Rationale: This suggests the electrical impulse bypassed the AV node via an
accessory pathway, reaching the ventricles early.
8. "Sawtooth" baseline waves (F-waves) occurring at a rate of 300 bpm are seen in:
Answer: Atrial Flutter.
Rationale: This is caused by a macro-reentrant circuit in the atria, creating a
rapid, organized atrial rate.
9. What does a QRS duration of 0.11 seconds indicate?
Answer: A normal or slightly borderline QRS width.
Rationale: A truly "wide" QRS is defined as 0.12 seconds (3 small squares) or
greater, indicating a bundle branch block or ventricular origin.
10. The "J-point" on the ECG marks the junction between which two segments?
Answer: The end of the QRS complex and the start of the ST segment.
Rationale: Measuring the elevation or depression of the ST segment at the J-point
is critical for diagnosing ischemia or infarction.
11. Which lead is placed at the fourth intercostal space, left sternal border?
Answer: Lead V2.
Rationale: Proper lead placement is essential for accurate precordial (chest) lead
interpretation.
12. In a "Sinus Arrhythmia," how does the heart rate typically change with
breathing?
Answer: The rate increases during inspiration and decreases during
expiration.
Rationale: This is a normal physiological response caused by changes in vagal
tone during the respiratory cycle.
13. What is the hallmark of a First-Degree AV Block?
Answer: A consistent PR interval greater than 0.20 seconds.
Rationale: In this block, every atrial impulse is conducted to the ventricles, but it is
delayed at the AV node longer than normal.
, 14. What is the firing rate of a "Junctional Escape Rhythm"?
Answer: 40 to 60 beats per minute.
Rationale: If the SA node fails, the AV junction takes over as the secondary
pacemaker at its inherent slower rate.
15. Which ECG leads are the "Inferior Leads"?
Answer: Leads II, III, and aVF.
Rationale: These leads view the bottom surface of the heart, which is usually
supplied by the Right Coronary Artery (RCA).
16. What does a "Pathological Q-wave" signify?
Answer: Previous Myocardial Infarction (Tissue Necrosis).
*Rationale: A Q-wave >0.04s wide or >25% of the R-wave height indicates a
permanent loss of electrical activity in that portion of the muscle.*
17. Which electrolyte abnormality is most likely to cause tall, peaked T-waves?
Answer: Hyperkalemia.
Rationale: High potassium levels accelerate ventricular repolarization, leading to
the characteristic "tented" T-wave shape.
18. A "Delta Wave" is associated with which condition?
Answer: Wolff-Parkinson-White (WPW) Syndrome.
Rationale: The Delta wave is the slurred upstroke of the QRS complex caused by
early ventricular activation through an accessory pathway.
19. What is the "Triplicate" method for calculating heart rate?
Answer: 300-150-100-75-60-50.
Rationale: This is a quick way to estimate rate by counting the number of large
squares between consecutive R-waves.
20. A patient has a regular narrow-complex tachycardia at 180 bpm with no visible
P waves. After carotid sinus massage, the rhythm suddenly slows then resumes.
What is the diagnosis?
Answer: AV Nodal Reentrant Tachycardia (AVNRT).
Rationale: Vagal maneuvers terminate AVNRT by blocking the slow pathway; the
abrupt onset and offset are characteristic.
21. Which ECG pattern is described as "tombstone" ST elevation?
Answer: Convex (dome-shaped) ST elevation with loss of R-wave and a large
Q wave.
Interpretation: Questions and Answers
with Rationales
1. What is the most common lethal arrhythmia following a myocardial infarction
that requires immediate defibrillation?
Answer: Ventricular Fibrillation (VF).
Rationale: VF produces a chaotic, disorganized electrical activity with no effective
cardiac output; defibrillation is the only definitive treatment.
2. A patient presents with a wide QRS tachycardia at 220 bpm, regular, with no
visible P waves and a history of previous heart attacks. What is the most likely
diagnosis?
Answer: Monomorphic Ventricular Tachycardia (VT).
Rationale: In a patient with structural heart disease, a regular wide-complex
tachycardia is VT until proven otherwise; AV dissociation or fusion beats confirm
it.
3. Which ECG finding is most specific for acute pericarditis?
Answer: Diffuse ST-segment elevation with PR-segment depression.
Rationale: Unlike the regional ST elevation of infarction, pericarditis causes
widespread inflammation, leading to ST elevation in most leads and reciprocal PR
depression.
4. What is the "R-on-T" phenomenon and why is it dangerous?
Answer: A premature ventricular complex (PVC) falling on the T wave of the
preceding beat; it can trigger Ventricular Fibrillation.
Rationale: The T wave represents the vulnerable period of repolarization; an early
PVC during this window may initiate malignant arrhythmias.
5. In a patient with suspected inferior MI, which additional lead should be recorded
to detect right ventricular involvement?
Answer: Lead V4R (right-sided V4).
*Rationale: Right ventricular infarction complicates up to 40% of inferior MIs; ST
elevation in V4R (right-sided) is highly sensitive and specific.*
,6. What is the "intrinsic" firing rate of the Sinoatrial (SA) Node?
Answer: 60 to 100 beats per minute.
Rationale: The SA node is the primary pacemaker because it has the fastest rate of
spontaneous depolarization in a healthy heart.
7. A PR interval shorter than 0.12 seconds is a hallmark of:
Answer: Pre-excitation (e.g., Wolff-Parkinson-White Syndrome).
Rationale: This suggests the electrical impulse bypassed the AV node via an
accessory pathway, reaching the ventricles early.
8. "Sawtooth" baseline waves (F-waves) occurring at a rate of 300 bpm are seen in:
Answer: Atrial Flutter.
Rationale: This is caused by a macro-reentrant circuit in the atria, creating a
rapid, organized atrial rate.
9. What does a QRS duration of 0.11 seconds indicate?
Answer: A normal or slightly borderline QRS width.
Rationale: A truly "wide" QRS is defined as 0.12 seconds (3 small squares) or
greater, indicating a bundle branch block or ventricular origin.
10. The "J-point" on the ECG marks the junction between which two segments?
Answer: The end of the QRS complex and the start of the ST segment.
Rationale: Measuring the elevation or depression of the ST segment at the J-point
is critical for diagnosing ischemia or infarction.
11. Which lead is placed at the fourth intercostal space, left sternal border?
Answer: Lead V2.
Rationale: Proper lead placement is essential for accurate precordial (chest) lead
interpretation.
12. In a "Sinus Arrhythmia," how does the heart rate typically change with
breathing?
Answer: The rate increases during inspiration and decreases during
expiration.
Rationale: This is a normal physiological response caused by changes in vagal
tone during the respiratory cycle.
13. What is the hallmark of a First-Degree AV Block?
Answer: A consistent PR interval greater than 0.20 seconds.
Rationale: In this block, every atrial impulse is conducted to the ventricles, but it is
delayed at the AV node longer than normal.
, 14. What is the firing rate of a "Junctional Escape Rhythm"?
Answer: 40 to 60 beats per minute.
Rationale: If the SA node fails, the AV junction takes over as the secondary
pacemaker at its inherent slower rate.
15. Which ECG leads are the "Inferior Leads"?
Answer: Leads II, III, and aVF.
Rationale: These leads view the bottom surface of the heart, which is usually
supplied by the Right Coronary Artery (RCA).
16. What does a "Pathological Q-wave" signify?
Answer: Previous Myocardial Infarction (Tissue Necrosis).
*Rationale: A Q-wave >0.04s wide or >25% of the R-wave height indicates a
permanent loss of electrical activity in that portion of the muscle.*
17. Which electrolyte abnormality is most likely to cause tall, peaked T-waves?
Answer: Hyperkalemia.
Rationale: High potassium levels accelerate ventricular repolarization, leading to
the characteristic "tented" T-wave shape.
18. A "Delta Wave" is associated with which condition?
Answer: Wolff-Parkinson-White (WPW) Syndrome.
Rationale: The Delta wave is the slurred upstroke of the QRS complex caused by
early ventricular activation through an accessory pathway.
19. What is the "Triplicate" method for calculating heart rate?
Answer: 300-150-100-75-60-50.
Rationale: This is a quick way to estimate rate by counting the number of large
squares between consecutive R-waves.
20. A patient has a regular narrow-complex tachycardia at 180 bpm with no visible
P waves. After carotid sinus massage, the rhythm suddenly slows then resumes.
What is the diagnosis?
Answer: AV Nodal Reentrant Tachycardia (AVNRT).
Rationale: Vagal maneuvers terminate AVNRT by blocking the slow pathway; the
abrupt onset and offset are characteristic.
21. Which ECG pattern is described as "tombstone" ST elevation?
Answer: Convex (dome-shaped) ST elevation with loss of R-wave and a large
Q wave.
