ECG Rhythm Analysis 3 — Questions and Answers
Question 1: What is the intrinsic escape rate of the ventricular (His-Purkinje) pacemaker cells?
- 20–40 bpm (Correct answer)
- 40–60 bpm
- 60–80 bpm
- 80–100 bpm
Correct answer: 20–40 bpm
Ventricular escape pacemakers fire at 20–40 bpm, which is the slowest intrinsic rate in the conduction hierarchy.
Question 2: On telemetry, a patient's rhythm suddenly shows fine, chaotic baseline deflections with no identifiable P waves, QRS complexes, or T waves. What is this rhythm?
- Ventricular fibrillation (Correct answer)
- Atrial fibrillation
- Artifact
- Asystole
Correct answer: Ventricular fibrillation
Ventricular fibrillation (VF) produces disorganized chaotic waveforms with no discernible cardiac complexes and requires immediate defibrillation.
Question 3: A junctional rhythm with a rate of 60–100 bpm is best described as:
- Accelerated junctional rhythm (Correct answer)
- Junctional escape rhythm
- Junctional tachycardia
- Normal sinus rhythm
Correct answer: Accelerated junctional rhythm
When the AV junction fires at 60–100 bpm (faster than its intrinsic 40–60 bpm but not tachycardia), it is called an accelerated junctional rhythm.
Question 4: Which characteristic best identifies atrial fibrillation on an ECG?
- Irregularly irregular rhythm with absent distinct P waves (Correct answer)
- Regular rhythm with inverted P waves
- Sawtooth flutter waves at 300 bpm
- Wide QRS complexes with AV dissociation
Correct answer: Irregularly irregular rhythm with absent distinct P waves
Atrial fibrillation is characterized by an irregularly irregular ventricular response and chaotic atrial activity replacing distinct P waves.
Question 5: A PVC that falls on the T wave of the preceding beat is clinically dangerous because it may:
- Trigger ventricular fibrillation (R-on-T phenomenon) (Correct answer)
- Cause sinus arrest
- Induce atrial flutter
- Produce a complete heart block
Correct answer: Trigger ventricular fibrillation (R-on-T phenomenon)
The R-on-T phenomenon occurs when a PVC fires during the vulnerable period of ventricular repolarization, potentially precipitating VF.
Question 6: Sawtooth-pattern flutter waves at approximately 300 bpm with a regular ventricular response at 75 bpm indicates atrial flutter with what conduction ratio?
- 4:1 block (Correct answer)
- 2:1 block
- 3:1 block
- 1:1 conduction
Correct answer: 4:1 block
At 300 atrial beats per minute with 75 ventricular beats per minute, every 4th atrial impulse conducts, giving a 4:1 ratio.
Question 7: Which of the following is TRUE about a left bundle branch block (LBBB) on ECG?
- QRS duration ≥120 ms with broad notched R in V5/V6 (Correct answer)
- RSR' pattern in V1 with wide S in V6
- QRS duration <100 ms
- Presence of delta waves
Correct answer: QRS duration ≥120 ms with broad notched R in V5/V6
LBBB features a wide QRS (≥120 ms), broad monophasic R waves in lateral leads (I, aVL, V5, V6), and absence of septal Q waves.
ECG Rhythm Analysis 3
What is the intrinsic escape rate of the ventricular (His-Purkinje) pacemaker cells?