ECG Basics MCQ
P-QRS-T waves, intervals, the conduction system (SA → AV → His-Purkinje), the high-yield arrhythmias (AFib, VT, VF), AV blocks, and the rhythms that require defibrillation. 25 MCQs and 8 INBDE patient cases.
Concept summary and clinical relevance.
Quick-reference structure first, then detailed coverage. Mnemonics in amber, clinical pearls in blue.
ECG questions on the INBDE focus on the basics: which wave is which, normal interval ranges, the standard arrhythmia patterns, and which rhythms are emergencies. You won't be asked to diagnose subtle infarcts, but you must recognize ventricular fibrillation as a “shock now” rhythm and atrial fibrillation as “irregularly irregular.”
| Wave / interval | Represents | Normal value |
|---|---|---|
| P wave | Atrial depolarization | Upright in lead II |
| PR interval | AV node conduction delay (allows ventricular filling) | 0.12–0.20 sec |
| QRS complex | Ventricular depolarization | < 0.12 sec (narrow) |
| T wave | Ventricular repolarization | Upright in most leads |
| QT interval | Total ventricular depolarization + repolarization | Prolonged QT → torsades / VF risk |
| Structure | Role | Intrinsic rate |
|---|---|---|
| SA node | Primary pacemaker | 60–100 bpm |
| AV node | Conduction delay → atrial kick before ventricles fire | 40–60 bpm (escape) |
| Bundle of His → bundle branches → Purkinje fibers | Rapid conduction to ventricular myocardium | 20–40 bpm (escape) |
| Rhythm | ECG features | Clinical action |
|---|---|---|
| Atrial fibrillation (AF) | Irregularly irregular; no distinct P waves | Anticoagulation (stroke prevention); rate control |
| Atrial flutter | Sawtooth flutter waves; regular ventricular response | Anticoagulation; rate control |
| Ventricular tachycardia (VT) with pulse | Wide QRS, fast and regular; pulse present | Pharmacologic or synchronized cardioversion |
| Ventricular fibrillation (VF) | Chaotic baseline, no organized QRS; no pulse | CPR + defibrillation immediately |
| 1st-degree AV block | PR > 0.20 sec; every P conducted | Usually benign |
| 2nd-degree AV block | Some P waves not followed by QRS | May need monitoring or pacing |
| 3rd-degree (complete) AV block | Atria and ventricles beat independently | Requires pacemaker |
Conduction system
- SA node (right atrium): the natural pacemaker. Fires at 60–100 bpm.
- AV node: the only normal electrical connection between atria and ventricles; introduces a 0.12–0.20 second delay so atria contract before ventricles.
- His–Purkinje system: rapid conduction throughout the ventricular myocardium → narrow QRS.
Common arrhythmia patterns
- Atrial fibrillation: irregularly irregular, no distinct P waves; major stroke risk → anticoagulation.
- Atrial flutter: sawtooth flutter waves; usually regular ventricular response.
- Ventricular tachycardia: wide QRS, rapid; can be with pulse (cardiovert) or pulseless (CPR + defibrillation).
- Ventricular fibrillation: chaotic, no organized QRS, no pulse → CPR + defibrillation immediately.
AV blocks at a glance
- 1st-degree: PR > 0.20 sec, every P conducted. Generally benign.
- 2nd-degree: some P waves dropped (Mobitz I gradually lengthens PR before drop; Mobitz II drops without warning).
- 3rd-degree (complete): atria and ventricles fire independently: requires pacemaker.
Other ECG patterns to recognize
- ST elevation: acute MI (STEMI): call EMS immediately.
- ST depression: ischemia or digitalis effect.
- Tall, peaked T waves: hyperkalemia (commonly tested).
- Prolonged QT interval: predisposes to torsades de pointes and VF.
25 board-style MCQs.
Active recall is the highest-yield study method. Pick an answer, check it, and read why every distractor is wrong.
- Question 1EasyThe P wave on an ECG represents:
- Question 2EasyThe T wave represents:
- Question 3EasyWhich structure is the normal pacemaker of the heart?
- Question 4EasyThe AV node delays conduction in order to:
- Question 5EasyNormal sinus rhythm is defined as:
- Question 6ModerateAtrial fibrillation is best described as:
- Question 7ModerateIn third-degree (complete) AV block, the ECG shows:
- Question 8ModerateProlonged QT interval is concerning because:
- Question 9ModerateWhich rhythm requires immediate CPR and defibrillation?
- Question 10ModerateWhich ECG feature distinguishes ventricular tachycardia from supraventricular tachycardia?
- Question 11ModerateA dental patient becomes syncopal. ECG shows HR 40 bpm with a PR interval of 0.28 sec, but every P wave is followed by a QRS. The diagnosis is:
- Question 12EasyThe QRS complex on an ECG represents:
- Question 13EasyA normal PR interval measures:
- Question 14EasyThe normal sequence of cardiac electrical conduction is:
- Question 15ModeratePurkinje fibers are specialized to:
- Question 16ModerateST-segment elevation on an ECG most classically indicates:
- Question 17EasySinus bradycardia is defined as a sinus rhythm with a heart rate:
- Question 18ModerateSinus tachycardia (rate above 100 bpm) in a dental patient is most often caused by:
- Question 19HardMobitz type I (Wenckebach) second-degree AV block shows:
- Question 20ModerateWhich rhythms are shockable with defibrillation?
- Question 21ModerateThe major long-term risk of atrial fibrillation is:
- Question 22ModerateA normal QRS duration is:
- Question 23HardDefibrillation differs from synchronized cardioversion in that defibrillation:
- Question 24HardA premature ventricular contraction (PVC) appears on an ECG as:
- Question 25ModerateA permanent pacemaker is most commonly indicated for:
INBDE patient cases.
8 ADA INBDE-format patient cases on ecg basics. Each case is a shared patient box plus linked questions with full distractor explanations.
8 patient cases · 40 linked questions
Founder, KYT Dental Services. These MCQs are reviewed by a practicing clinician and offered as an educational reference for dental students.
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