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Unit 3: Normal ECG Waveforms and Measurements

Prepare for Unit 3: Normal ECG Waveforms and Measurements with practice questions covering 8 topics. Part of CET — Certified EKG Technician (NHA) — build your knowledge and track your progress with AH Prep.

Questions
120
Topics
8
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What’s in it.

8 topics
  • Topic 01

    The P Wave — Atrial Depolarisation, Duration, and Morphology

    15 questions
  • Topic 02

    The PR Interval — Normal Range and Clinical Significance

    15 questions
  • Topic 03

    The QRS Complex — Ventricular Depolarisation and Duration

    15 questions
  • Topic 04

    The ST Segment — Isoelectric Line, Elevation, and Depression

    15 questions
  • Topic 05

    The T Wave — Ventricular Repolarisation and Inversion

    15 questions
  • Topic 06

    The QT Interval — Corrected QT and Long QT Syndrome

    15 questions
  • Topic 07

    Rate Calculation — Regular and Irregular Rhythms (6-Second Method, Large Box Method)

    15 questions
  • Topic 08

    Axis Determination — Normal, Left, and Right Axis Deviation

    15 questions

Sample questions

3 of many

A few questions from this unit, with the answer and a full explanation. The complete bank is available when you start practising.

  1. A patient taking sotalol has a QTc of 520 ms. What is the immediate clinical concern and what is the specific arrhythmia risk?

    • The prolonged QTc indicates that sotalol is working as intended; the drug target is QTc >500 ms to suppress atrial fibrillation
    • The QTc is markedly prolonged, raising high concern for Torsades de Pointes — a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation
      Correct answer
    • The immediate risk is atrial fibrillation, which is triggered by QT prolongation in patients on sotalol
    • The prolonged QTc indicates complete heart block caused by sotalol's AV nodal blocking effect rather than an arrhythmia risk
    Explanation

    Sotalol is a Class III antiarrhythmic with beta-blocking properties that prolongs ventricular repolarisation. A QTc of 520 ms is markedly above the threshold of 440–460 ms and is considered high risk. The specific arrhythmia danger is Torsades de Pointes (TdP) — a polymorphic ventricular tachycardia characterised by QRS complexes that twist around the isoelectric baseline, typically self-terminating but able to degenerate into ventricular fibrillation. The FDA considers QTc >500 ms a high-risk threshold in drug trials. Key takeaway: QTc >500 ms on sotalol (or any QT-prolonging drug) carries high risk of Torsades de Pointes and requires urgent clinical review.

  2. What is the normal range of the cardiac electrical axis?

    • −30° to +90°
      Correct answer
    • −30° to +120°
    • 0° to +90°
    • −60° to +60°
    Explanation

    The AHA and NHA CET define the normal cardiac axis as −30° to +90°. Within this range, both lead I and aVF show a positive (upright) QRS, and the dominant depolarisation vector points inferiorly and to the left — the expected direction given normal left ventricular dominance. Values more negative than −30° constitute left axis deviation, and values more positive than +90° constitute right axis deviation. Key takeaway: normal axis = −30° to +90°.

  3. A patient's ECG shows negative QRS complexes in both lead I and aVF. Which of the following conditions is most consistent with this finding?

    • Normal sinus rhythm in a trained athlete
    • Severe hyperkalemia causing a ventricular conduction disturbance with a northwest axis vector
      Correct answer
    • Left bundle branch block producing typical left axis deviation
    • Right ventricular hypertrophy
    Explanation

    A negative QRS in both lead I and aVF places the axis in the upper-right quadrant — the extreme or northwest axis (−90° to ±180°). This is produced when forces are directed superiorly and to the right, away from both leads. True causes include severe hyperkalemia (which inactivates sodium channels and produces bizarre conduction vectors), ventricular rhythms originating near the cardiac apex, and artificial pacing from certain ventricular sites. LAFB alone causes LAD (I +, aVF −), not extreme deviation. LAFB combined with LPFB produces effectively complete LBBB, which causes LAD not extreme axis. RVH causes RAD (I −, aVF +). Key takeaway: negative QRS in both lead I and aVF = extreme (northwest) axis deviation; true causes include severe hyperkalemia and ventricular rhythms from the apex.