ECG Cheat Sheet 2026

The 30 highest-yield ECG facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. The 'vulnerable period' of the cardiac cycle, when an R-on-T phenomenon can trigger ventricular fibrillation, corresponds to which ECG feature? → The ascending limb of the T wave (relative refractory period)
2. During ECG acquisition, the technician notices the calibration pulse measures 5 mm instead of the standard 10 mm. What does this indicate? → The gain is set to half standard (5 mm/mV)
3. Which finding on a 12-lead ECG is most suggestive of left ventricular hypertrophy (LVH)? → S wave in V1 plus R wave in V5 or V6 exceeding 35 mm
4. Why is a baseline ECG important before starting certain medications? → To detect pre-existing QT prolongation or other abnormalities
5. A technician notices 60-Hz electrical interference on the ECG tracing. The most appropriate first step is to: → Check and improve electrode-skin contact and grounding
6. What is first-degree AV block characterized by? → PR interval greater than 0.20 seconds with all P waves conducted
7. A patient's ECG shows PR interval of 320 ms that is constant across all beats. How is this classified? → First-degree AV block
8. ST segment elevation most commonly indicates: → Acute myocardial injury
9. What is the recommended minimum number of leads monitored continuously in a telemetry unit? → 2
10. What is the function of the coronary sinus? → Collects deoxygenated blood from cardiac veins
11. Which AAMI/IEC standard governs ECG electrode performance requirements in the US? → AAMI EC12
12. What is the most common cause of pseudo-Q waves (QS pattern) in leads V1-V2 in an otherwise healthy patient? → Incorrect precordial electrode placement (too high)
13. On an ECG, the QT interval measures 480 ms at a heart rate of 60 bpm. What is the corrected QT (QTc) and its significance? → QTc = 480 ms; prolonged, risk of Torsades de Pointes
14. What is the primary function of an EKG technician during an ECG procedure? → Set up the equipment and ensure accurate lead placement
15. Which patient activity should be specifically avoided during standard Holter monitoring? → Showering or immersing in water
16. What is the significance of lead reversal on a 12-lead ECG? → It can mimic pathological conditions and lead to misdiagnosis
17. What does the P wave on an ECG represent? → Atrial depolarization
18. What is the difference between depolarization and repolarization? → Depolarization is electrical activation; repolarization is electrical recovery
19. An ECG technician observes a 60-cycle (60 Hz) interference pattern on the tracing. Which corrective measure is most appropriate? → Check electrode contact and ensure leads are not crossing electrical equipment
20. What does the QRS complex represent? → Ventricular depolarization
21. Which lead is considered the ground lead in a standard 12-lead ECG setup? → Right leg (RL)
22. Which of the following workplaces is most common for ECG-certified professionals? → Diagnostic laboratories
23. Where is the V1 precordial lead electrode placed? → 4th intercostal space, right sternal border
24. Lead II records electrical activity between which electrodes? → Right arm and left leg
25. What is the conduction velocity of the His-Purkinje system, and why is it important? → 4 m/s; enables near-simultaneous ventricular depolarization for effective contraction
26. A 60-year-old male presents with chest pain. The ECG shows ST depression in leads V1-V3. Which condition should be ruled out first? → Posterior MI (reciprocal changes)
27. Which ECG finding is characteristic of atrial fibrillation? → Irregularly irregular rhythm with no P waves
28. A prolonged QT interval increases the risk of: → Torsades de pointes
29. What is the antidote for warfarin overdose? → Vitamin K
30. A patient with chest pain has ST elevation in leads V1–V4. Which coronary artery territory is most likely occluded? → Left anterior descending artery (LAD)